Merge branch 'zahary' into araq2

25 files changed, 1017 insertions, 84 deletions

diff --git a/compiler/ast.nim b/compiler/ast.nim
index 5b68e9712..ee6391919 100644
--- a/compiler/ast.nim
+++ b/compiler/ast.nim
@@ -212,6 +212,8 @@ type
     nkIteratorTy,         # iterator type
     nkSharedTy,           # 'shared T'
                           # we use 'nkPostFix' for the 'not nil' addition
+    nkInTy,               # prefix `in` used to mark contravariant types
+    nkOutTy,              # prefix `out` used to mark covariant types
     nkEnumTy,             # enum body
     nkEnumFieldDef,       # `ident = expr` in an enumeration
     nkArgList,            # argument list
@@ -268,6 +270,9 @@ type
     sfDiscardable,    # returned value may be discarded implicitly
     sfOverriden,      # proc is overriden
     sfGenSym          # symbol is 'gensym'ed; do not add to symbol table
+    sfCovariant       # covariant generic param mimicing a ptr type
+    sfWeakCovariant   # covariant generic param mimicing a seq/array type
+    sfContravariant   # contravariant generic param
 
   TSymFlags* = set[TSymFlag]
 
@@ -1004,15 +1009,17 @@ proc add*(father, son: PNode) =
   if isNil(father.sons): father.sons = @[]
   add(father.sons, son)
 
-proc `[]`*(n: PNode, i: int): PNode {.inline.} =
-  result = n.sons[i]
+type Indexable = PNode | PType
+
+template `[]`*(n: Indexable, i: int): Indexable =
+  n.sons[i]
 
 template `-|`*(b, s: untyped): untyped =
   (if b >= 0: b else: s.len + b)
 
 # son access operators with support for negative indices
-template `{}`*(n: PNode, i: int): untyped = n[i -| n]
-template `{}=`*(n: PNode, i: int, s: PNode) =
+template `{}`*(n: Indexable, i: int): untyped = n[i -| n]
+template `{}=`*(n: Indexable, i: int, s: Indexable) =
   n.sons[i -| n] = s
 
 when defined(useNodeIds):
diff --git a/compiler/ccgexprs.nim b/compiler/ccgexprs.nim
index f981f9595..f2c6f076c 100644
--- a/compiler/ccgexprs.nim
+++ b/compiler/ccgexprs.nim
@@ -2203,7 +2203,7 @@ proc getNullValueAux(p: BProc; obj, cons: PNode, result: var Rope) =
     let field = obj.sym
     for i in 1..<cons.len:
       if cons[i].kind == nkExprColonExpr:
-        if cons[i][0].sym == field:
+        if cons[i][0].sym.name == field.name:
           result.add genConstExpr(p, cons[i][1])
           return
       elif i == field.position:
diff --git a/compiler/condsyms.nim b/compiler/condsyms.nim
index 4303fd6c8..a4f47ac72 100644
--- a/compiler/condsyms.nim
+++ b/compiler/condsyms.nim
@@ -103,3 +103,4 @@ proc initDefines*() =
   defineSymbol("nimNewShiftOps")
   defineSymbol("nimDistros")
   defineSymbol("nimHasCppDefine")
+  defineSymbol("nimGenericInOutFlags")
diff --git a/compiler/msgs.nim b/compiler/msgs.nim
index 3a97f1ed2..e416a3826 100644
--- a/compiler/msgs.nim
+++ b/compiler/msgs.nim
@@ -113,6 +113,7 @@ type
     errGenericLambdaNotAllowed,
     errProcHasNoConcreteType,
     errCompilerDoesntSupportTarget,
+    errInOutFlagNotExtern,
     errUser,
     warnCannotOpenFile,
     warnOctalEscape, warnXIsNeverRead, warnXmightNotBeenInit,
@@ -380,6 +381,7 @@ const
                                 "of the generic paramers can be inferred from the expected signature.",
     errProcHasNoConcreteType: "'$1' doesn't have a concrete type, due to unspecified generic parameters.",
     errCompilerDoesntSupportTarget: "The current compiler \'$1\' doesn't support the requested compilation target",
+    errInOutFlagNotExtern: "The `$1` modifier can be used only with imported types",
     errUser: "$1",
     warnCannotOpenFile: "cannot open \'$1\'",
     warnOctalEscape: "octal escape sequences do not exist; leading zero is ignored",
diff --git a/compiler/parser.nim b/compiler/parser.nim
index e2d17b455..788ad7cd7 100644
--- a/compiler/parser.nim
+++ b/compiler/parser.nim
@@ -1520,6 +1520,13 @@ proc parseGenericParam(p: var TParser): PNode =
   # progress guaranteed
   while true:
     case p.tok.tokType
+    of tkIn, tkOut:
+      let kind = if p.tok.tokType == tkIn: nkInTy
+                 else: nkOutTy
+      a = newNodeP(kind, p)
+      getTok(p)
+      expectIdent(p)
+      a.addSon(parseSymbol(p))
     of tkSymbol, tkAccent:
       a = parseSymbol(p)
       if a.kind == nkEmpty: return
@@ -1548,7 +1555,7 @@ proc parseGenericParamList(p: var TParser): PNode =
   getTok(p)
   optInd(p, result)
   # progress guaranteed
-  while p.tok.tokType in {tkSymbol, tkAccent}:
+  while p.tok.tokType in {tkSymbol, tkAccent, tkIn, tkOut}:
     var a = parseGenericParam(p)
     addSon(result, a)
     if p.tok.tokType notin {tkComma, tkSemiColon}: break
diff --git a/compiler/pbraces.nim b/compiler/pbraces.nim
index b59fbc6cf..fa4ccc139 100644
--- a/compiler/pbraces.nim
+++ b/compiler/pbraces.nim
@@ -1336,6 +1336,11 @@ proc parseGenericParam(p: var TParser): PNode =
   result = newNodeP(nkIdentDefs, p)
   while true:
     case p.tok.tokType
+    of tkIn, tkOut:
+      let t = p.tok.tokType
+      getTok(p)
+      expectIdent(p)
+      a = parseSymbol(p)
     of tkSymbol, tkAccent:
       a = parseSymbol(p)
       if a.kind == nkEmpty: return
diff --git a/compiler/semcall.nim b/compiler/semcall.nim
index 1089ab7db..4fa4f7f32 100644
--- a/compiler/semcall.nim
+++ b/compiler/semcall.nim
@@ -429,7 +429,7 @@ proc explicitGenericSym(c: PContext, n: PNode, s: PSym): PNode =
   for i in 1..sonsLen(n)-1:
     let formal = s.ast.sons[genericParamsPos].sons[i-1].typ
     let arg = n[i].typ
-    let tm = typeRel(m, formal, arg, true)
+    let tm = typeRel(m, formal, arg)
     if tm in {isNone, isConvertible}: return nil
   var newInst = generateInstance(c, s, m.bindings, n.info)
   newInst.typ.flags.excl tfUnresolved
diff --git a/compiler/semdata.nim b/compiler/semdata.nim
index 8f2c802de..5da2b70fa 100644
--- a/compiler/semdata.nim
+++ b/compiler/semdata.nim
@@ -277,6 +277,10 @@ proc makeTypeFromExpr*(c: PContext, n: PNode): PType =
   assert n != nil
   result.n = n
 
+proc newTypeWithSons2*(kind: TTypeKind, owner: PSym, sons: seq[PType]): PType =
+  result = newType(kind, owner)
+  result.sons = sons
+
 proc newTypeWithSons*(c: PContext, kind: TTypeKind,
                       sons: seq[PType]): PType =
   result = newType(kind, getCurrOwner(c))
diff --git a/compiler/semexprs.nim b/compiler/semexprs.nim
index 8ff5fdd9c..59fa208d2 100644
--- a/compiler/semexprs.nim
+++ b/compiler/semexprs.nim
@@ -683,27 +683,9 @@ proc bracketedMacro(n: PNode): PSym =
     if result.kind notin {skMacro, skTemplate}:
       result = nil
 
-proc semBracketedMacro(c: PContext; outer, inner: PNode; s: PSym;
-                       flags: TExprFlags): PNode =
-  # We received untransformed bracket expression coming from macroOrTmpl[].
-  # Transform it to macro or template call, where first come normal
-  # arguments, next come generic template arguments.
-  var sons = newSeq[PNode]()
-  sons.add inner.sons[0]
-  # Normal arguments:
-  for i in 1..<outer.len:
-    sons.add outer.sons[i]
-  # Generic template arguments from bracket expression:
-  for i in 1..<inner.len:
-    sons.add inner.sons[i]
-  shallowCopy(outer.sons, sons)
-  # FIXME: Shouldn't we check sfImmediate and call semDirectOp?
-  # However passing to semDirectOp doesn't work here.
-  case s.kind
-  of skMacro: result = semMacroExpr(c, outer, outer, s, flags)
-  of skTemplate: result = semTemplateExpr(c, outer, s, flags)
-  else: assert(false)
-  return
+proc setGenericParams(c: PContext, n: PNode) =
+  for i in 1 .. <n.len:
+    n[i].typ = semTypeNode(c, n[i], nil)
 
 proc afterCallActions(c: PContext; n, orig: PNode, flags: TExprFlags): PNode =
   result = n
@@ -745,7 +727,8 @@ proc semIndirectOp(c: PContext, n: PNode, flags: TExprFlags): PNode =
     elif n.sons[0].kind == nkBracketExpr:
       let s = bracketedMacro(n.sons[0])
       if s != nil:
-        return semBracketedMacro(c, n, n.sons[0], s, flags)
+        setGenericParams(c, n[0])
+        return semDirectOp(c, n, flags)
 
   let nOrig = n.copyTree
   semOpAux(c, n)
@@ -2164,10 +2147,6 @@ proc shouldBeBracketExpr(n: PNode): bool =
           n.sons[0] = be
           return true
 
-proc setGenericParams(c: PContext, n: PNode) =
-  for i in 1 .. <n.len:
-    n[i].typ = semTypeNode(c, n[i], nil)
-
 proc semExpr(c: PContext, n: PNode, flags: TExprFlags = {}): PNode =
   result = n
   if gCmd == cmdIdeTools: suggestExpr(c, n)
diff --git a/compiler/semmagic.nim b/compiler/semmagic.nim
index 3e1989eaf..eb6259df0 100644
--- a/compiler/semmagic.nim
+++ b/compiler/semmagic.nim
@@ -89,9 +89,9 @@ proc semInstantiationInfo(c: PContext, n: PNode): PNode =
 proc toNode(t: PType, i: TLineInfo): PNode =
   result = newNodeIT(nkType, i, t)
 
-const 
+const
   # these are types that use the bracket syntax for instantiation
-  # they can be subjected to the type traits `genericHead` and 
+  # they can be subjected to the type traits `genericHead` and
   # `Uninstantiated`
   tyUserDefinedGenerics* = {tyGenericInst, tyGenericInvocation,
                             tyUserTypeClassInst}
@@ -109,27 +109,43 @@ proc uninstantiate(t: PType): PType =
     of tyCompositeTypeClass: uninstantiate t.sons[1]
     else: t
 
-proc evalTypeTrait(trait: PNode, operand: PType, context: PSym): PNode =
-  var typ = operand.skipTypes({tyTypeDesc})
+proc evalTypeTrait(traitCall: PNode, operand: PType, context: PSym): PNode =
+  const skippedTypes = {tyTypeDesc}
+  let trait = traitCall[0]
+  internalAssert trait.kind == nkSym
+  var operand = operand.skipTypes(skippedTypes)
+
+  template operand2: PType =
+    traitCall.sons[2].typ.skipTypes({tyTypeDesc})
+
+  template typeWithSonsResult(kind, sons): PNode =
+    newTypeWithSons2(kind, context, sons).toNode(traitCall.info)
+
   case trait.sym.name.s
+  of "or", "|":
+    return typeWithSonsResult(tyOr, @[operand, operand2])
+  of "and":
+    return typeWithSonsResult(tyAnd, @[operand, operand2])
+  of "not":
+    return typeWithSonsResult(tyNot, @[operand])
   of "name":
-    result = newStrNode(nkStrLit, typ.typeToString(preferName))
+    result = newStrNode(nkStrLit, operand.typeToString(preferName))
     result.typ = newType(tyString, context)
-    result.info = trait.info
+    result.info = traitCall.info
   of "arity":
-    result = newIntNode(nkIntLit, typ.len - ord(typ.kind==tyProc))
+    result = newIntNode(nkIntLit, operand.len - ord(operand.kind==tyProc))
     result.typ = newType(tyInt, context)
-    result.info = trait.info
+    result.info = traitCall.info
   of "genericHead":
-    var res = uninstantiate(typ)
-    if res == typ and res.kind notin tyMagicGenerics:
-      localError(trait.info,
+    var res = uninstantiate(operand)
+    if res == operand and res.kind notin tyMagicGenerics:
+      localError(traitCall.info,
         "genericHead expects a generic type. The given type was " &
-        typeToString(typ))
-      return newType(tyError, context).toNode(trait.info)
-    result = res.base.toNode(trait.info)
+        typeToString(operand))
+      return newType(tyError, context).toNode(traitCall.info)
+    result = res.base.toNode(traitCall.info)
   of "stripGenericParams":
-    result = uninstantiate(typ).toNode(trait.info)
+    result = uninstantiate(operand).toNode(traitCall.info)
   else:
     internalAssert false
 
@@ -140,7 +156,7 @@ proc semTypeTraits(c: PContext, n: PNode): PNode =
   if t.sonsLen > 0:
     # This is either a type known to sem or a typedesc
     # param to a regular proc (again, known at instantiation)
-    result = evalTypeTrait(n[0], t, getCurrOwner(c))
+    result = evalTypeTrait(n, t, getCurrOwner(c))
   else:
     # a typedesc variable, pass unmodified to evals
     result = n
diff --git a/compiler/semstmts.nim b/compiler/semstmts.nim
index e52a1b5cc..f15cdddee 100644
--- a/compiler/semstmts.nim
+++ b/compiler/semstmts.nim
@@ -730,7 +730,9 @@ proc typeSectionLeftSidePass(c: PContext, n: PNode) =
     var s: PSym
     if name.kind == nkDotExpr:
       s = qualifiedLookUp(c, name, {checkUndeclared, checkModule})
-      if s.kind != skType or s.typ.skipTypes(abstractPtrs).kind != tyObject or tfPartial notin s.typ.skipTypes(abstractPtrs).flags:
+      if s.kind != skType or
+         s.typ.skipTypes(abstractPtrs).kind != tyObject or
+         tfPartial notin s.typ.skipTypes(abstractPtrs).flags:
         localError(name.info, "only .partial objects can be extended")
     else:
       s = semIdentDef(c, name, skType)
@@ -742,6 +744,87 @@ proc typeSectionLeftSidePass(c: PContext, n: PNode) =
       if sfGenSym notin s.flags: addInterfaceDecl(c, s)
     a.sons[0] = newSymNode(s)
 
+proc checkCovariantParamsUsages(genericType: PType) =
+  var body = genericType{-1}
+
+  proc traverseSubTypes(t: PType): bool =
+    template error(msg) = localError(genericType.sym.info, msg)
+
+    result = false
+
+    template subresult(r) =
+      let sub = r
+      result = result or sub
+
+    case t.kind
+    of tyGenericParam:
+      t.sym.flags.incl sfWeakCovariant
+      return true
+
+    of tyObject:
+      for field in t.n:
+        subresult traverseSubTypes(field.typ)
+
+    of tyArray:
+      return traverseSubTypes(t[1])
+
+    of tyProc:
+      for subType in t.sons:
+        if subType != nil:
+          subresult traverseSubTypes(subType)
+      if result:
+        error("non-invariant type param used in a proc type: " &  $t)
+
+    of tySequence:
+      return traverseSubTypes(t[0])
+
+    of tyGenericInvocation:
+      let targetBody = t[0]
+      for i in 1 .. <t.len:
+        let param = t[i]
+        if param.kind == tyGenericParam:
+          if sfCovariant in param.sym.flags:
+            let formalFlags = targetBody[i-1].sym.flags
+            if sfCovariant notin formalFlags:
+              error("covariant param '" & param.sym.name.s &
+                    "' used in a non-covariant position")
+            elif sfWeakCovariant in formalFlags:
+              param.sym.flags.incl sfWeakCovariant
+            result = true
+          elif sfContravariant in param.sym.flags:
+            let formalParam = targetBody[i-1].sym
+            if sfContravariant notin formalParam.flags:
+              error("contravariant param '" & param.sym.name.s &
+                    "' used in a non-contravariant position")
+            result = true
+        else:
+          subresult traverseSubTypes(param)
+
+    of tyAnd, tyOr, tyNot, tyStatic, tyBuiltInTypeClass, tyCompositeTypeClass:
+      error("non-invariant type parameters cannot be used with types such '" & $t & "'")
+
+    of tyUserTypeClass, tyUserTypeClassInst:
+      error("non-invariant type parameters are not supported in concepts")
+
+    of tyTuple:
+      for fieldType in t.sons:
+        subresult traverseSubTypes(fieldType)
+
+    of tyPtr, tyRef, tyVar:
+      if t.base.kind == tyGenericParam: return true
+      return traverseSubTypes(t.base)
+
+    of tyDistinct, tyAlias:
+      return traverseSubTypes(t.lastSon)
+
+    of tyGenericInst:
+      internalAssert false
+
+    else:
+      discard
+
+  discard traverseSubTypes(body)
+
 proc typeSectionRightSidePass(c: PContext, n: PNode) =
   for i in countup(0, sonsLen(n) - 1):
     var a = n.sons[i]
@@ -782,6 +865,22 @@ proc typeSectionRightSidePass(c: PContext, n: PNode) =
         body.sym = s
         body.size = -1 # could not be computed properly
         s.typ.sons[sonsLen(s.typ) - 1] = body
+        if sfCovariant in s.flags:
+          checkCovariantParamsUsages(s.typ)
+          # XXX: This is a temporary limitation:
+          # The codegen currently produces various failures with
+          # generic imported types that have fields, but we need
+          # the fields specified in order to detect weak covariance.
+          # The proper solution is to teach the codegen how to handle
+          # such types, because this would offer various interesting
+          # possibilities such as instantiating C++ generic types with
+          # garbage collected Nim types.
+          if sfImportc in s.flags:
+            var body = s.typ.lastSon
+            if body.kind == tyObject:
+              # erases all declared fields
+              body.n.sons = nil
+
       popOwner(c)
       closeScope(c)
     elif a.sons[2].kind != nkEmpty:
diff --git a/compiler/semtypes.nim b/compiler/semtypes.nim
index 16066da91..e0dedf135 100644
--- a/compiler/semtypes.nim
+++ b/compiler/semtypes.nim
@@ -1584,10 +1584,25 @@ proc semGenericParamList(c: PContext, n: PNode, father: PType = nil): PNode =
                       # type for each generic param. the index
                       # of the parameter will be stored in the
                       # attached symbol.
+      var paramName = a.sons[j]
+      var covarianceFlag = sfPure
+
+      if paramName.kind in {nkInTy, nkOutTy}:
+        if not nimEnableCovariance or paramName.kind == nkInTy:
+          if father == nil or sfImportc notin father.sym.flags:
+            localError(paramName.info, errInOutFlagNotExtern,
+                       if paramName.kind == nkInTy: "in" else: "out")
+        covarianceFlag = if paramName.kind == nkInTy: sfContravariant
+                         else: sfCovariant
+        if father != nil: father.sym.flags.incl sfCovariant
+        paramName = paramName[0]
+
       var s = if finalType.kind == tyStatic or tfWildcard in typ.flags:
-          newSymG(skGenericParam, a.sons[j], c).linkTo(finalType)
+          newSymG(skGenericParam, paramName, c).linkTo(finalType)
         else:
-          newSymG(skType, a.sons[j], c).linkTo(finalType)
+          newSymG(skType, paramName, c).linkTo(finalType)
+
+      if covarianceFlag != sfPure: s.flags.incl(covarianceFlag)
       if def.kind != nkEmpty: s.ast = def
       if father != nil: addSonSkipIntLit(father, s.typ)
       s.position = result.len
diff --git a/compiler/sigmatch.nim b/compiler/sigmatch.nim
index d6a0c6382..36975c3db 100644
--- a/compiler/sigmatch.nim
+++ b/compiler/sigmatch.nim
@@ -69,6 +69,12 @@ type
     mutabilityProblem*: uint8 # tyVar mismatch
     inheritancePenalty: int  # to prefer closest father object type
 
+  TTypeRelFlag* = enum
+    trDontBind
+    trNoCovariance
+
+  TTypeRelFlags* = set[TTypeRelFlag]
+
   TTypeRelation* = enum      # order is important!
     isNone, isConvertible,
     isIntConv,
@@ -296,7 +302,9 @@ proc describeArgs*(c: PContext, n: PNode, startIdx = 1;
     add(result, argTypeToString(arg, prefer))
     if i != sonsLen(n) - 1: add(result, ", ")
 
-proc typeRel*(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation
+proc typeRel*(c: var TCandidate, f, aOrig: PType,
+              flags: TTypeRelFlags = {}): TTypeRelation
+
 proc concreteType(c: TCandidate, t: PType): PType =
   case t.kind
   of tyNil:
@@ -860,7 +868,28 @@ template subtypeCheck() =
   if result <= isSubrange and f.lastSon.skipTypes(abstractInst).kind in {tyRef, tyPtr, tyVar}:
     result = isNone
 
-proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
+proc isCovariantPtr(c: var TCandidate, f, a: PType): bool =
+  # this proc is always called for a pair of matching types
+  assert f.kind == a.kind
+
+  template baseTypesCheck(lhs, rhs: PType): bool =
+    lhs.kind notin {tyPtr, tyRef, tyVar} and
+      typeRel(c, lhs, rhs, {trNoCovariance}) == isSubtype
+
+  case f.kind
+  of tyRef, tyPtr:
+    return baseTypesCheck(f.base, a.base)
+  of tyGenericInst:
+    let body = f.base
+    return body == a.base and
+           a.sonsLen == 3 and
+           sfWeakCovariant notin body.sons[0].sym.flags and
+           baseTypesCheck(f.sons[1], a.sons[1])
+  else:
+    return false
+
+proc typeRel(c: var TCandidate, f, aOrig: PType,
+             flags: TTypeRelFlags = {}): TTypeRelation =
   # typeRel can be used to establish various relationships between types:
   #
   # 1) When used with concrete types, it will check for type equivalence
@@ -927,6 +956,8 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
         result = isEqual
       return
 
+  template doBind: bool = trDontBind notin flags
+
   # var and static arguments match regular modifier-free types
   var a = aOrig.skipTypes({tyStatic, tyVar}).maybeSkipDistinct(c.calleeSym)
   # XXX: Theoretically, maybeSkipDistinct could be called before we even
@@ -956,23 +987,28 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
 
   case a.kind
   of tyOr:
+    # XXX: deal with the current dual meaning of tyGenericParam
+    c.typedescMatched = true
     # seq[int|string] vs seq[number]
     # both int and string must match against number
     # but ensure that '[T: A|A]' matches as good as '[T: A]' (bug #2219):
     result = isGeneric
     for branch in a.sons:
-      let x = typeRel(c, f, branch, false)
+      let x = typeRel(c, f, branch, flags + {trDontBind})
       if x == isNone: return isNone
       if x < result: result = x
+    return
 
   of tyAnd:
+    # XXX: deal with the current dual meaning of tyGenericParam
+    c.typedescMatched = true
     # seq[Sortable and Iterable] vs seq[Sortable]
     # only one match is enough
     for branch in a.sons:
-      let x = typeRel(c, f, branch, false)
+      let x = typeRel(c, f, branch, flags + {trDontBind})
       if x != isNone:
         return if x >= isGeneric: isGeneric else: x
-    result = isNone
+    return isNone
 
   of tyNot:
     case f.kind
@@ -996,7 +1032,7 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
   of tyUserTypeClass, tyUserTypeClassInst:
     # consider this: 'var g: Node' *within* a concept where 'Node'
     # is a concept too (tgraph)
-    let x = typeRel(c, a, f, false)
+    let x = typeRel(c, a, f, flags + {trDontBind})
     if x >= isGeneric:
       return isGeneric
   else: discard
@@ -1042,7 +1078,7 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
   of tyFloat128: result = handleFloatRange(f, a)
   of tyVar:
     if aOrig.kind == tyVar: result = typeRel(c, f.base, aOrig.base)
-    else: result = typeRel(c, f.base, aOrig)
+    else: result = typeRel(c, f.base, aOrig, flags + {trNoCovariance})
     subtypeCheck()
   of tyArray:
     case a.kind
@@ -1056,9 +1092,17 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
           fRange = a
         else:
           fRange = prev
-      result = typeRel(c, f.sons[1].skipTypes({tyTypeDesc}),
-                          a.sons[1].skipTypes({tyTypeDesc}))
-      if result < isGeneric: return isNone
+      let ff = f.sons[1].skipTypes({tyTypeDesc})
+      let aa = a.sons[1].skipTypes({tyTypeDesc})
+      result = typeRel(c, ff, aa)
+      if result < isGeneric:
+        if nimEnableCovariance and
+           trNoCovariance notin flags and
+           ff.kind == aa.kind and
+           isCovariantPtr(c, ff, aa):
+          result = isSubtype
+        else:
+          return isNone
 
       if fRange.rangeHasUnresolvedStatic:
         return inferStaticsInRange(c, fRange, a)
@@ -1077,20 +1121,31 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
       if tfOldSchoolExprStmt in f.sons[0].flags:
         if f.sons[0].kind == tyExpr: return
       elif f.sons[0].kind == tyStmt: return
+
+    template matchArrayOrSeq(aBase: PType) =
+      let ff = f.base
+      let aa = aBase
+      let baseRel = typeRel(c, ff, aa)
+      if baseRel >= isGeneric:
+        result = isConvertible
+      elif nimEnableCovariance and
+           trNoCovariance notin flags and
+           ff.kind == aa.kind and
+           isCovariantPtr(c, ff, aa):
+        result = isConvertible
+
     case a.kind
     of tyOpenArray, tyVarargs:
       result = typeRel(c, base(f), base(a))
       if result < isGeneric: result = isNone
     of tyArray:
       if (f.sons[0].kind != tyGenericParam) and (a.sons[1].kind == tyEmpty):
-        result = isSubtype
-      elif typeRel(c, base(f), a.sons[1]) >= isGeneric:
-        result = isConvertible
+        return isSubtype
+      matchArrayOrSeq(a.sons[1])
     of tySequence:
       if (f.sons[0].kind != tyGenericParam) and (a.sons[0].kind == tyEmpty):
-        result = isConvertible
-      elif typeRel(c, base(f), a.sons[0]) >= isGeneric:
-        result = isConvertible
+        return isConvertible
+      matchArrayOrSeq(a.sons[0])
     of tyString:
       if f.kind == tyOpenArray:
         if f.sons[0].kind == tyChar:
@@ -1105,8 +1160,17 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
       if (f.sons[0].kind != tyGenericParam) and (a.sons[0].kind == tyEmpty):
         result = isSubtype
       else:
-        result = typeRel(c, f.sons[0], a.sons[0])
-        if result < isGeneric: result = isNone
+        let ff = f.sons[0]
+        let aa = a.sons[0]
+        result = typeRel(c, ff, aa)
+        if result < isGeneric:
+          if nimEnableCovariance and
+             trNoCovariance notin flags and
+             ff.kind == aa.kind and
+             isCovariantPtr(c, ff, aa):
+            result = isSubtype
+          else:
+            result = isNone
         elif tfNotNil in f.flags and tfNotNil notin a.flags:
           result = isNilConversion
     of tyNil: result = f.allowsNil
@@ -1158,7 +1222,7 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
       if a.len < f.len: return isNone
       for i in 0..f.len-2:
         if typeRel(c, f.sons[i], a.sons[i]) == isNone: return isNone
-      result = typeRel(c, f.lastSon, a.lastSon)
+      result = typeRel(c, f.lastSon, a.lastSon, flags + {trNoCovariance})
       subtypeCheck()
       if result <= isConvertible: result = isNone
       elif tfNotNil in f.flags and tfNotNil notin a.flags:
@@ -1230,15 +1294,30 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
     var m = c
     if a.kind == tyGenericInst:
       if roota.base == rootf.base:
+        let nextFlags = flags + {trNoCovariance}
+        var hasCovariance = false
         for i in 1 .. rootf.sonsLen-2:
           let ff = rootf.sons[i]
           let aa = roota.sons[i]
-          result = typeRel(c, ff, aa)
-          if result notin {isEqual, isGeneric}: return isNone
-          # if ff.kind == tyRange and result != isEqual: return isNone
+          result = typeRel(c, ff, aa, nextFlags)
+          if result notin {isEqual, isGeneric}:
+            if trNoCovariance notin flags and ff.kind == aa.kind:
+              let paramFlags = rootf.base.sons[i-1].sym.flags
+              hasCovariance =
+                if sfCovariant in paramFlags:
+                  if sfWeakCovariant in paramFlags:
+                    isCovariantPtr(c, ff, aa)
+                  else:
+                    ff.kind notin {tyRef, tyPtr} and result == isSubtype
+                else:
+                  sfContravariant in paramFlags and
+                    typeRel(c, aa, ff) == isSubtype
+              if hasCovariance:
+                continue
 
+            return isNone
         if prev == nil: put(c, f, a)
-        result = isGeneric
+        result = if hasCovariance: isGeneric else: isGeneric
       else:
         let fKind = rootf.lastSon.kind
         if fKind in {tyAnd, tyOr}:
@@ -1450,7 +1529,7 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
           result = isNone
       else:
         if f.sonsLen > 0 and f.sons[0].kind != tyNone:
-          result = typeRel(c, f.lastSon, a, false)
+          result = typeRel(c, f.lastSon, a, flags + {trDontBind})
           if doBind and result notin {isNone, isGeneric}:
             let concrete = concreteType(c, a)
             if concrete == nil: return isNone
diff --git a/compiler/vm.nim b/compiler/vm.nim
index 043506e62..e201e98dc 100644
--- a/compiler/vm.nim
+++ b/compiler/vm.nim
@@ -1609,8 +1609,9 @@ proc setupMacroParam(x: PNode, typ: PType): TFullReg =
 iterator genericParamsInMacroCall*(macroSym: PSym, call: PNode): (PSym, PNode) =
   let gp = macroSym.ast[genericParamsPos]
   for i in 0 .. <gp.len:
-    let idx = macroSym.typ.len + i
-    yield (gp[i].sym, call.sons[idx])
+    let genericParam = gp[i].sym
+    let posInCall = macroSym.typ.len + i
+    yield (genericParam, call[posInCall])
 
 var evalMacroCounter: int
 
diff --git a/doc/manual/type_rel.txt b/doc/manual/type_rel.txt
index 5b68f73aa..1d1425934 100644
--- a/doc/manual/type_rel.txt
+++ b/doc/manual/type_rel.txt
@@ -111,6 +111,105 @@ relation is extended to the types ``var``, ``ref``, ``ptr``:
 .. XXX nil is a special value!
 
 
+Covariance
+----------
+
+Covariance in Nim can be introduced only though pointer-like types such
+as ``ptr`` and ``ref``. Sequence, Array and OpenArray types, instantiated
+with pointer-like types will be considered covariant if and only if they
+are also immutable. The introduction of a ``var`` modifier or additional
+``ptr`` or ``ref`` indirections would result in invariant treatment of
+these types.
+
+``proc`` types are currently always invariant, but future versions of Nim
+may relax this rule.
+
+User-defined generic types may also be covariant with respect to some of
+their parameters. By default, all generic params are considered invariant,
+but you may choose the apply the prefix modifier ``in`` to a parameter to
+make it contravariant or ``out`` to make it covariant:
+
+.. code-block:: nim
+  type
+    AnnotatedPtr[out T] =
+      metadata: MyTypeInfo
+      p: ref T
+
+    RingBuffer[out T] =
+      startPos: int
+      data: seq[T]
+
+    Action {.importcpp: "std::function<void ('0)>".} [in T] = object
+
+When the designated generic parameter is used to instantiate a pointer-like
+type as in the case of `AnnotatedPtr` above, the resulting generic type will
+also have pointer-like covariance:
+
+.. code-block:: nim
+  type
+    GuiWidget = object of RootObj
+    Button = object of GuiWidget
+    ComboBox = object of GuiWidget
+
+  var
+    widgetPtr: AnnotatedPtr[GuiWidget]
+    buttonPtr: AnnotatedPtr[Button]
+
+  ...
+
+  proc drawWidget[T](x: AnnotatedPtr[GuiWidget]) = ...
+
+  # you can call procs expecting base types by supplying a derived type
+  drawWidget(buttonPtr)
+
+  # and you can convert more-specific pointer types to more general ones
+  widgetPtr = buttonPtr
+
+Just like with regular pointers, covariance will be enabled only for immutable
+values:
+
+.. code-block:: nim
+  proc makeComboBox[T](x: var AnnotatedPtr[GuiWidget]) =
+    x.p = new(ComboBox)
+
+  makeComboBox(buttonPtr) # Error, AnnotatedPtr[Button] cannot be modified
+                          # to point to a ComboBox
+
+On the other hand, in the `RingBuffer` example above, the designated generic
+param is used to instantiate the non-pointer ``seq`` type, which means that
+the resulting generic type will have covariance that mimics an array or
+sequence (i.e. it will be covariant only when instantiated with ``ptr`` and
+``ref`` types):
+
+.. code-block:: nim
+
+  type
+    Base = object of RootObj
+    Derived = object of Base
+
+  proc consumeBaseValues(b: RingBuffer[Base]) = ...
+
+  var derivedValues: RingBuffer[Derived]
+
+  consumeBaseValues(derivedValues) # Error, Base and Derived values may differ
+                                   # in size
+
+  proc consumeBasePointers(b: RingBuffer[ptr Base]) = ...
+
+  var derivedPointers: RingBuffer[ptr Derived]
+
+  consumeBaseValues(derivedPointers) # This is legal
+
+Please note that Nim will treat the user-defined pointer-like types as
+proper alternatives to the built-in pointer types. That is, types such
+as `seq[AnnotatedPtr[T]]` or `RingBuffer[AnnotatedPtr[T]]` will also be
+considered covariant and you can create new pointer-like types by instantiating
+other user-defined pointer-like types.
+
+The contravariant parameters introduced with the ``in`` modifier are currently
+useful only when interfacing with imported types having such semantics.
+
+
 Convertible relation
 --------------------
 A type ``a`` is **implicitly** convertible to type ``b`` iff the following
@@ -119,6 +218,8 @@ algorithm returns true:
 .. code-block:: nim
   # XXX range types?
   proc isImplicitlyConvertible(a, b: PType): bool =
+    if isSubtype(a, b) or isCovariant(a, b):
+      return true
     case a.kind
     of int:     result = b in {int8, int16, int32, int64, uint, uint8, uint16,
                                uint32, uint64, float, float32, float64}
diff --git a/lib/core/macros.nim b/lib/core/macros.nim
index 83776f16b..5e0d4030d 100644
--- a/lib/core/macros.nim
+++ b/lib/core/macros.nim
@@ -68,6 +68,8 @@ type
     nnkProcTy,
     nnkIteratorTy,         # iterator type
     nnkSharedTy,           # 'shared T'
+    nnkInTy,
+    nnkOutTy,
     nnkEnumTy,
     nnkEnumFieldDef,
     nnkArglist, nnkPattern
diff --git a/lib/system.nim b/lib/system.nim
index 9b41253cc..05b4b88f2 100644
--- a/lib/system.nim
+++ b/lib/system.nim
@@ -49,6 +49,9 @@ type
   cstring* {.magic: Cstring.} ## built-in cstring (*compatible string*) type
   pointer* {.magic: Pointer.} ## built-in pointer type, use the ``addr``
                               ## operator to get a pointer to a variable
+
+  typedesc* {.magic: TypeDesc.} ## meta type to denote a type description
+
 const
   on* = true    ## alias for ``true``
   off* = false  ## alias for ``false``
@@ -56,6 +59,15 @@ const
 {.push warning[GcMem]: off, warning[Uninit]: off.}
 {.push hints: off.}
 
+proc `or` *(a, b: typedesc): typedesc {.magic: "TypeTrait", noSideEffect.}
+  ## Constructs an `or` meta class
+
+proc `and` *(a, b: typedesc): typedesc {.magic: "TypeTrait", noSideEffect.}
+  ## Constructs an `and` meta class
+
+proc `not` *(a: typedesc): typedesc {.magic: "TypeTrait", noSideEffect.}
+  ## Constructs an `not` meta class
+
 type
   Ordinal* {.magic: Ordinal.}[T] ## Generic ordinal type. Includes integer,
                                  ## bool, character, and enumeration types
@@ -66,11 +78,11 @@ type
   `ref`* {.magic: Pointer.}[T] ## built-in generic traced pointer type
 
   `nil` {.magic: "Nil".}
+
   expr* {.magic: Expr, deprecated.} ## meta type to denote an expression (for templates)
-                        ## **Deprecated** since version 0.15. Use ``untyped`` instead.
+    ## **Deprecated** since version 0.15. Use ``untyped`` instead.
   stmt* {.magic: Stmt, deprecated.} ## meta type to denote a statement (for templates)
     ## **Deprecated** since version 0.15. Use ``typed`` instead.
-  typedesc* {.magic: TypeDesc.} ## meta type to denote a type description
   void* {.magic: "VoidType".}   ## meta type to denote the absence of any type
   auto* {.magic: Expr.} ## meta type for automatic type determination
   any* = distinct auto ## meta type for any supported type
@@ -1332,6 +1344,7 @@ const
   hasThreadSupport = compileOption("threads") and not defined(nimscript)
   hasSharedHeap = defined(boehmgc) or defined(gogc) # don't share heaps; every thread has its own
   taintMode = compileOption("taintmode")
+  nimEnableCovariance* = defined(nimEnableCovariance) # or true
 
 when hasThreadSupport and defined(tcc) and not compileOption("tlsEmulation"):
   # tcc doesn't support TLS
diff --git a/tests/cpp/tcovariancerules.nim b/tests/cpp/tcovariancerules.nim
new file mode 100644
index 000000000..dfe4cb941
--- /dev/null
+++ b/tests/cpp/tcovariancerules.nim
@@ -0,0 +1,424 @@
+discard """
+cmd: "nim cpp $file"
+output: '''
+cat
+cat
+dog
+dog
+cat
+cat
+dog
+dog X
+cat
+cat
+dog
+dog
+dog value
+cat value
+dog value
+cat value
+dog
+dog
+dog value
+cat value
+dog 1
+dog 2
+'''
+"""
+
+template accept(x) =
+  static: assert(compiles(x))
+
+template reject(x) =
+  static: assert(not compiles(x))
+
+import macros
+
+macro skipElse(n: untyped): typed = n[0]
+
+template acceptWithCovariance(x, otherwise): typed =
+  when nimEnableCovariance:
+    x
+  else:
+    reject(x)
+    skipElse(otherwise)
+
+type
+  Animal = object of RootObj
+    x: string
+
+  Dog = object of Animal
+    y: int
+
+  Cat = object of Animal
+    z: int
+
+  AnimalRef = ref Animal
+  AnimalPtr = ptr Animal
+
+  RefAlias[T] = ref T
+
+var dog = new(Dog)
+dog.x = "dog"
+
+var cat = new(Cat)
+cat.x = "cat"
+
+proc makeDerivedRef(x: string): ref Dog =
+  new(result)
+  result.x = x
+
+proc makeDerived(x: string): Dog =
+  result.x = x
+
+var covariantSeq = @[makeDerivedRef("dog 1"), makeDerivedRef("dog 2")]
+var nonCovariantSeq = @[makeDerived("dog 1"), makeDerived("dog 2")]
+var covariantArr = [makeDerivedRef("dog 1"), makeDerivedRef("dog 2")]
+var nonCovariantArr = [makeDerived("dog 1"), makeDerived("dog 2")]
+
+proc wantsCovariantSeq1(s: seq[ref Animal]) =
+  for a in s: echo a.x
+
+proc wantsCovariantSeq2(s: seq[AnimalRef]) =
+  for a in s: echo a.x
+
+proc wantsCovariantSeq3(s: seq[RefAlias[Animal]]) =
+  for a in s: echo a.x
+
+proc wantsCovariantOperArray(s: openarray[ref Animal]) =
+  for a in s: echo a.x
+
+proc modifiesCovariantOperArray(s: var openarray[ref Animal]) =
+  for a in s: echo a.x
+
+proc modifiesDerivedOperArray(s: var openarray[ref Dog]) =
+  for a in s: echo a.x
+
+proc wantsNonCovariantOperArray(s: openarray[Animal]) =
+  for a in s: echo a.x
+
+proc wantsCovariantArray(s: array[2, ref Animal]) =
+  for a in s: echo a.x
+
+proc wantsNonCovariantSeq(s: seq[Animal]) =
+  for a in s: echo a.x
+
+proc wantsNonCovariantArray(s: array[2, Animal]) =
+  for a in s: echo a.x
+
+proc modifiesCovariantSeq(s: var seq[ref Animal]) =
+  for a in s: echo a.x
+
+proc modifiesCovariantArray(s: var array[2, ref Animal]) =
+  for a in s: echo a.x
+
+proc modifiesCovariantSeq(s: ptr seq[ref Animal]) =
+  for a in s[]: echo a.x
+
+proc modifiesCovariantArray(s: ptr array[2, ref Animal]) =
+  for a in s[]: echo a.x
+
+proc modifiesDerivedSeq(s: var seq[ref Dog]) =
+  for a in s: echo a.x
+
+proc modifiesDerivedArray(s: var array[2, ref Dog]) =
+  for a in s: echo a.x
+
+proc modifiesDerivedSeq(s: ptr seq[ref Dog]) =
+  for a in s[]: echo a.x
+
+proc modifiesDerivedArray(s: ptr array[2, ref Dog]) =
+  for a in s[]: echo a.x
+
+accept:
+  wantsCovariantArray([AnimalRef(dog), AnimalRef(dog)])
+  wantsCovariantArray([AnimalRef(cat), AnimalRef(dog)])
+  wantsCovariantArray([AnimalRef(cat), dog])
+
+  # there is a special rule that detects the base
+  # type of polymorphic arrays
+  wantsCovariantArray([cat, dog])
+
+acceptWithCovariance:
+  wantsCovariantArray([cat, cat])
+else:
+  echo "cat"
+  echo "cat"
+
+var animalRefArray: array[2, ref Animal]
+
+accept:
+  animalRefArray = [AnimalRef(dog), AnimalRef(dog)]
+  animalRefArray = [AnimalRef(cat), dog]
+
+acceptWithCovariance:
+  animalRefArray = [dog, dog]
+  wantsCovariantArray animalRefArray
+else:
+  echo "dog"
+  echo "dog"
+
+accept:
+  var animal: AnimalRef = dog
+  animal = cat
+
+var vdog: Dog
+vdog.x = "dog value"
+var vcat: Cat
+vcat.x = "cat value"
+
+reject:
+  vcat = vdog
+
+# XXX: The next two cases seem incosistent, perhaps we should change the rules
+accept:
+  # truncating copies are allowed
+  var vanimal: Animal = vdog
+  vanimal = vdog
+
+reject:
+  # truncating copies are not allowed with arrays
+  var vanimalArray: array[2, Animal]
+  var vdogArray = [vdog, vdog]
+  vanimalArray = vdogArray
+
+accept:
+  # a more explicit version of a truncating copy that
+  # should probably always remain allowed
+  var vnextnimal: Animal = Animal(vdog)
+
+proc wantsRefSeq(x: seq[AnimalRef]) = discard
+
+accept:
+  wantsCovariantSeq1(@[AnimalRef(dog), AnimalRef(dog)])
+  wantsCovariantSeq1(@[AnimalRef(cat), AnimalRef(dog)])
+  wantsCovariantSeq1(@[AnimalRef(cat), dog])
+  wantsCovariantSeq1(@[cat, dog])
+
+  wantsCovariantSeq2(@[AnimalRef(dog), AnimalRef(dog)])
+  wantsCovariantSeq2(@[AnimalRef(cat), AnimalRef(dog)])
+  wantsCovariantSeq2(@[AnimalRef(cat), dog])
+  wantsCovariantSeq2(@[cat, dog])
+
+  wantsCovariantSeq3(@[AnimalRef(dog), AnimalRef(dog)])
+  wantsCovariantSeq3(@[AnimalRef(cat), AnimalRef(dog)])
+  wantsCovariantSeq3(@[AnimalRef(cat), dog])
+  wantsCovariantSeq3(@[cat, dog])
+
+  wantsCovariantOperArray([cat, dog])
+
+acceptWithCovariance:
+  wantsCovariantSeq1(@[cat, cat])
+  wantsCovariantSeq2(@[dog, makeDerivedRef("dog X")])
+  # XXX: wantsCovariantSeq3(@[cat, cat])
+
+  wantsCovariantOperArray(@[cat, cat])
+  wantsCovariantOperArray([dog, dog])
+else:
+  echo "cat"
+  echo "cat"
+  echo "dog"
+  echo "dog X"
+  echo "cat"
+  echo "cat"
+  echo "dog"
+  echo "dog"
+
+var dogRefs = @[dog, dog]
+var dogRefsArray = [dog, dog]
+var animalRefs = @[dog, cat]
+
+accept:
+  modifiesDerivedArray(dogRefsArray)
+  modifiesDerivedSeq(dogRefs)
+
+reject modifiesCovariantSeq(dogRefs)
+reject modifiesCovariantSeq(addr(dogRefs))
+reject modifiesCovariantSeq(dogRefs.addr)
+
+reject modifiesCovariantArray([dog, dog])
+reject modifiesCovariantArray(dogRefsArray)
+reject modifiesCovariantArray(addr(dogRefsArray))
+reject modifiesCovariantArray(dogRefsArray.addr)
+
+var dogValues = @[vdog, vdog]
+var dogValuesArray = [vdog, vdog]
+var animalValues = @[Animal(vdog), Animal(vcat)]
+var animalValuesArray = [Animal(vdog), Animal(vcat)]
+
+wantsNonCovariantSeq animalValues
+wantsNonCovariantArray animalValuesArray
+
+reject wantsNonCovariantSeq(dogRefs)
+reject modifiesCovariantOperArray(dogRefs)
+reject wantsNonCovariantArray(dogRefsArray)
+reject wantsNonCovariantSeq(dogValues)
+reject wantsNonCovariantArray(dogValuesArray)
+reject modifiesValueArray()
+
+modifiesDerivedOperArray dogRefs
+reject modifiesDerivedOperArray(dogValues)
+reject modifiesDerivedOperArray(animalRefs)
+
+wantsNonCovariantOperArray animalValues
+reject wantsNonCovariantOperArray(animalRefs)
+reject wantsNonCovariantOperArray(dogRefs)
+reject wantsNonCovariantOperArray(dogValues)
+
+var animalRefSeq: seq[ref Animal]
+
+accept:
+  animalRefSeq = @[AnimalRef(dog), AnimalRef(dog)]
+  animalRefSeq = @[AnimalRef(cat), dog]
+
+acceptWithCovariance:
+  animalRefSeq = @[makeDerivedRef("dog 1"), makeDerivedRef("dog 2")]
+  wantsCovariantSeq1(animalRefSeq)
+else:
+  echo "dog 1"
+  echo "dog 2"
+
+var pdog: ptr Dog
+var pcat: ptr Cat
+
+proc wantsPointer(x: ptr Animal) =
+  discard
+
+accept:
+  wantsPointer pdog
+  wantsPointer pcat
+
+# covariance should be disabled when var is involved
+proc wantsVarPointer1(x: var ptr Animal) =
+  discard
+
+proc wantsVarPointer2(x: var AnimalPtr) =
+  discard
+
+reject wantsVarPointer1(pdog)
+reject wantsVarPointer2(pcat)
+
+# covariance may be allowed for certain extern types
+
+{.emit: """
+template <class T> struct FN { typedef void (*type)(T); };
+template <class T> struct ARR { typedef T DataType[2]; DataType data; };
+""".}
+
+type
+  MyPtr {.importcpp: "'0 *"} [out T] = object
+
+  MySeq {.importcpp: "ARR<'0>", nodecl} [out T] = object
+    data: array[2, T]
+
+  MyAction {.importcpp: "FN<'0>::type"} [in T] = object
+
+var
+  cAnimal: MyPtr[Animal]
+  cDog: MyPtr[Dog]
+  cCat: MyPtr[Cat]
+
+  cAnimalFn: MyAction[Animal]
+  cCatFn: MyAction[Cat]
+  cDogFn: MyAction[Dog]
+
+  cRefAnimalFn: MyAction[ref Animal]
+  cRefCatFn: MyAction[ref Cat]
+  cRefDogFn: MyAction[ref Dog]
+
+accept:
+  cAnimal = cDog
+  cAnimal = cCat
+
+  cDogFn = cAnimalFn
+  cCatFn = cAnimalFn
+
+  cRefDogFn = cRefAnimalFn
+  cRefCatFn = cRefAnimalFn
+
+reject: cDogFn = cRefAnimalFn
+reject: cCatFn = cRefAnimalFn
+
+reject: cCat = cDog
+reject: cAnimalFn = cDogFn
+reject: cAnimalFn = cCatFn
+reject: cRefAnimalFn = cRefDogFn
+reject: cRefAnimalFn = cRefCatFn
+reject: cRefAnimalFn = cDogFn
+
+var
+  ptrPtrDog: ptr ptr Dog
+  ptrPtrAnimal: ptr ptr Animal
+
+reject: ptrPtrDog = ptrPtrAnimal
+
+# Try to break the rules by introducing some tricky
+# double indirection types:
+var
+  cPtrRefAnimal: MyPtr[ref Animal]
+  cPtrRefDog: MyPtr[ref Dog]
+
+  cPtrAliasRefAnimal: MyPtr[RefAlias[Animal]]
+  cPtrAliasRefDog: MyPtr[RefAlias[Dog]]
+
+  cDoublePtrAnimal: MyPtr[MyPtr[Animal]]
+  cDoublePtrDog: MyPtr[MyPtr[Dog]]
+
+reject: cPtrRefAnimal = cPtrRefDog
+reject: cDoublePtrAnimal = cDoublePtrDog
+reject: cRefAliasPtrAnimal = cRefAliasPtrDog
+reject: cPtrRefAnimal = cRefAliasPtrDog
+reject: cPtrAliasRefAnimal = cPtrRefDog
+
+var
+  # Array and Sequence types are covariant only
+  # when instantiated with ref or ptr types:
+  cAnimals: MySeq[ref Animal]
+  cDogs: MySeq[ref Dog]
+
+  # "User-defined" pointer types should be OK too:
+  cAnimalPtrSeq: MySeq[MyPtr[Animal]]
+  cDogPtrSeq: MySeq[MyPtr[Dog]]
+
+  # Value types shouldn't work:
+  cAnimalValues: MySeq[Animal]
+  cDogValues: MySeq[Dog]
+
+  # Double pointer types should not work either:
+  cAnimalRefPtrSeq: MySeq[ref MyPtr[Animal]]
+  cDogRefPtrSeq: MySeq[ref MyPtr[Dog]]
+  cAnimalPtrPtrSeq: MySeq[ptr ptr Animal]
+  cDogPtrPtrSeq: MySeq[ptr ptr Dog]
+
+accept:
+  cAnimals = cDogs
+  cAnimalPtrSeq = cDogPtrSeq
+
+reject: cAnimalValues = cDogValues
+reject: cAnimalRefPtrSeq = cDogRefPtrSeq
+reject: cAnimalPtrPtrSeq = cDogPtrPtrSeq
+
+proc wantsAnimalSeq(x: MySeq[Animal]) = discard
+proc wantsAnimalRefSeq(x: MySeq[ref Animal]) = discard
+proc modifiesAnimalRefSeq(x: var MySeq[ref Animal]) = discard
+proc usesAddressOfAnimalRefSeq(x: ptr MySeq[ref Animal]) = discard
+
+accept wantsAnimalSeq(cAnimalValues)
+reject wantsAnimalSeq(cDogValues)
+reject wantsAnimalSeq(cAnimals)
+
+reject wantsAnimalRefSeq(cAnimalValues)
+reject wantsAnimalRefSeq(cDogValues)
+accept wantsAnimalRefSeq(cAnimals)
+accept wantsAnimalRefSeq(cDogs)
+
+reject modifiesAnimalRefSeq(cAnimalValues)
+reject modifiesAnimalRefSeq(cDogValues)
+accept modifiesAnimalRefSeq(cAnimals)
+reject modifiesAnimalRefSeq(cDogs)
+
+reject usesAddressOfAnimalRefSeq(addr cAnimalValues)
+reject usesAddressOfAnimalRefSeq(addr cDogValues)
+accept usesAddressOfAnimalRefSeq(addr cAnimals)
+reject usesAddressOfAnimalRefSeq(addr cDogs)
+
diff --git a/tests/errmsgs/tinvalidinout.nim b/tests/errmsgs/tinvalidinout.nim
new file mode 100644
index 000000000..ce7eb6022
--- /dev/null
+++ b/tests/errmsgs/tinvalidinout.nim
@@ -0,0 +1,26 @@
+discard """
+cmd: "nim check $file"
+errormsg: "The `in` modifier can be used only with imported types"
+nimout: '''
+tinvalidinout.nim(14, 7) Error: The `out` modifier can be used only with imported types
+tinvalidinout.nim(17, 9) Error: The `in` modifier can be used only with imported types
+tinvalidinout.nim(18, 9) Error: The `in` modifier can be used only with imported types
+'''
+"""
+
+type
+  Foo {.header: "foo.h", importcpp.} [in T] = object
+
+  Bar[out X] = object
+    x: int
+
+proc f1[in T](x: T) = discard
+proc f2[in T](x: T) {.importc: "f", header: "foo.h"}
+
+var
+  f: Foo[int]
+  b: Bar[string]
+
+f1 f
+f2 b
+
diff --git a/tests/generics/tfakecovariance.nim b/tests/generics/tfakecovariance.nim
new file mode 100644
index 000000000..0920cb504
--- /dev/null
+++ b/tests/generics/tfakecovariance.nim
@@ -0,0 +1,78 @@
+template accept(x) =
+  static: assert(compiles(x))
+
+template reject(x) =
+  static: assert(not compiles(x))
+
+type
+  BaseObj = object of RootObj
+  DerivedObj = object of BaseObj
+  NonDerivedObj = object
+
+  Container[T] = object
+
+var base: BaseObj
+var derived: DerivedObj
+var nonDerived: NonDerivedObj
+
+var baseContainer: Container[BaseObj]
+var derivedContainer: Container[DerivedObj]
+var nonDerivedContainer: Container[NonDerivedObj]
+
+# We can fake covariance by listing some specific derived types that
+# will be allowed with our overload. This is not a real covariance,
+# because there will be multiple instantiations of the proc, but for
+# many purposes, it will suffice:
+
+proc wantsSpecificContainers(c: Container[BaseObj or DerivedObj]) = discard
+
+accept wantsSpecificContainers(baseContainer)
+accept wantsSpecificContainers(derivedContainer)
+
+reject wantsSpecificContainers(nonDerivedContainer)
+reject wantsSpecificContainers(derived)
+
+# Now, let's make a more general solution able to catch all derived types:
+
+type
+  DerivedFrom[T] = concept type D
+    var derived: ref D
+    var base: ref T = derived
+
+proc wantsDerived(x: DerivedFrom[BaseObj]) = discard
+
+accept wantsDerived(base)
+accept wantsDerived(derived)
+
+reject wantsDerived(nonDerived)
+reject wantsDerived(baseContainer)
+
+proc wantsDerivedContainer(c: Container[DerivedFrom[BaseObj]]) = discard
+
+accept wantsDerivedContainer(baseContainer)
+accept wantsDerivedContainer(derivedContainer)
+
+reject wantsDerivedContainer(nonDerivedContainer)
+
+# The previous solutions were solving the problem for a single overload.
+# Let's solve it for multiple overloads by introducing a converter:
+
+type
+  OtherContainer[T] = object
+
+proc wantsBaseContainer1(c: OtherContainer[BaseObj]) = discard
+proc wantsBaseContainer2(c: OtherContainer[BaseObj]) = discard
+
+converter derivedToBase(c: OtherContainer[DerivedFrom[BaseObj]]): OtherContainer[BaseObj] = discard
+
+block:
+  var baseContainer: OtherContainer[BaseObj]
+  var derivedContainer: OtherContainer[DerivedObj]
+  var nonDerivedContainer: OtherContainer[NonDerivedObj]
+
+  accept wantsBaseContainer1(derivedContainer)
+  reject wantsBaseContainer1(nonDerivedContainer)
+
+  accept wantsBaseContainer2(derivedContainer)
+  reject wantsBaseContainer2(nonDerivedContainer)
+
diff --git a/tests/generics/tgenericconst.nim b/tests/generics/tgenericconst.nim
new file mode 100644
index 000000000..3c86888df
--- /dev/null
+++ b/tests/generics/tgenericconst.nim
@@ -0,0 +1,39 @@
+discard """
+output: '''
+@[1, 2]
+@[3, 4]
+1
+'''
+"""
+
+# https://github.com/nim-lang/Nim/issues/5756
+
+type
+  Vec*[N : static[int]] = object
+    x: int
+    arr*: array[N, int32]
+
+  Mat*[M,N: static[int]] = object
+    x: int
+    arr*: array[M, Vec[N]]
+
+proc vec2*(x,y:int32) : Vec[2] =
+  result.arr = [x,y]
+  result.x = 10
+
+proc mat2*(a,b: Vec[2]): Mat[2,2] =
+  result.arr = [a,b]
+  result.x = 20
+
+const M = mat2(vec2(1, 2), vec2(3, 4))
+
+let m1 = M
+echo @(m1.arr[0].arr)
+echo @(m1.arr[1].arr)
+
+proc foo =
+  let m2 = M
+  echo m1.arr[0].arr[0]
+
+foo()
+
diff --git a/tests/generics/tptrinheritance.nim b/tests/generics/tptrinheritance.nim
index 1e1115fa5..221b8777b 100644
--- a/tests/generics/tptrinheritance.nim
+++ b/tests/generics/tptrinheritance.nim
@@ -10,7 +10,7 @@ proc newMutableArrayAbstract*(): NSMutableArrayAbstract = discard
 template newMutableArray*(T: typedesc): NSMutableArray[T] =
   cast[NSMutableArray[T]](newMutableArrayAbstract())
 
-proc writeObjects*(p: NSPasteboard, o: ptr NSArray[NSPasteboardItem]) = discard
+proc writeObjects*(p: NSPasteboard, o: NSArray[NSPasteboardItem]) = discard
 
 let a = newMutableArray NSPasteboardItem
 var x: NSMutableArray[NSPasteboardItem]
diff --git a/tests/stdlib/tmarshal.nim b/tests/stdlib/tmarshal.nim
index b05cb5a10..6a53a2964 100644
--- a/tests/stdlib/tmarshal.nim
+++ b/tests/stdlib/tmarshal.nim
@@ -1,7 +1,10 @@
 discard """
   output: '''{"age": 12, "bio": "\u042F Cletus", "blob": [65, 66, 67, 128], "name": "Cletus"}
 true
-true'''
+true
+alpha 100
+omega 200
+'''
 """
 
 import marshal
@@ -83,3 +86,22 @@ var instance1 = Person(name: "Cletus", age: 12,
 echo($$instance1)
 echo(to[Person]($$instance1).bio == instance1.bio)
 echo(to[Person]($$instance1).blob == instance1.blob)
+
+# bug 5757
+
+type
+  Something = object
+    x: string
+    y: int
+
+var data1 = """{"x": "alpha", "y": 100}"""
+var data2 = """{"x": "omega", "y": 200}"""
+
+var r = to[Something](data1)
+
+echo r.x, " ", r.y
+
+r = to[Something](data2)
+
+echo r.x, " ", r.y
+
diff --git a/tests/template/tgenerictemplates.nim b/tests/template/tgenerictemplates.nim
new file mode 100644
index 000000000..2c83bc0ec
--- /dev/null
+++ b/tests/template/tgenerictemplates.nim
@@ -0,0 +1,13 @@
+type
+  SomeObj = object of RootObj
+
+  Foo[T, U] = object
+    x: T
+    y: U
+
+template someTemplate[T](): tuple[id: int32, obj: T] =
+  var result: tuple[id: int32, obj: T] = (0'i32, T())
+  result
+
+let ret = someTemplate[SomeObj]()
+
diff --git a/web/bountysource.nim b/web/bountysource.nim
index 1d47cea56..5dfdb4497 100644
--- a/web/bountysource.nim
+++ b/web/bountysource.nim
@@ -33,13 +33,13 @@ proc getSupporters(self: BountySource): Future[JsonNode] {.async.} =
   let response = await self.client.get(apiUrl &
     "/supporters?order=monthly&per_page=200&team_slug=" & self.team)
   doAssert response.status.startsWith($Http200)
-  return parseJson(response.body)
+  return parseJson(await response.body)
 
 proc getGithubUser(username: string): Future[JsonNode] {.async.} =
   let client = newAsyncHttpClient()
   let response = await client.get(githubApiUrl & "/users/" & username)
   if response.status.startsWith($Http200):
-    return parseJson(response.body)
+    return parseJson(await response.body)
   else:
     echo("Could not get Github user: ", username, ". ", response.status)
     return nil