path: root/compiler/semexprs.nim

#
#
#           The Nimrod Compiler
#        (c) Copyright 2011 Andreas Rumpf
#
#    See the file "copying.txt", included in this
#    distribution, for details about the copyright.
#

# this module does the semantic checking for expressions
const 
  ConstAbstractTypes = {tyNil, tyChar, tyInt..tyInt64, tyFloat..tyFloat128, 
    tyArrayConstr, tyTuple, tySet}

proc semTemplateExpr(c: PContext, n: PNode, s: PSym, semCheck = true): PNode = 
  markUsed(n, s)
  pushInfoContext(n.info)
  result = evalTemplate(c, n, s)
  if semCheck: result = semAfterMacroCall(c, result, s)
  popInfoContext()

proc semFieldAccess(c: PContext, n: PNode, flags: TExprFlags = {}): PNode

proc semExprWithType(c: PContext, n: PNode, flags: TExprFlags = {}): PNode = 
  result = semExpr(c, n, flags)
  if result.kind == nkEmpty: 
    # do not produce another redundant error message:
    raiseRecoverableError()
  if result.typ != nil: 
    if result.typ.kind == tyVar: 
      var d = newNodeIT(nkHiddenDeref, result.info, result.typ.sons[0])
      addSon(d, result)
      result = d
  else:
    GlobalError(n.info, errExprXHasNoType, 
                renderTree(result, {renderNoComments}))

proc semSymGenericInstantiation(c: PContext, n: PNode, s: PSym): PNode =
  result = symChoice(c, n, s)
  
proc semSym(c: PContext, n: PNode, s: PSym, flags: TExprFlags): PNode = 
  case s.kind
  of skProc, skMethod, skIterator, skConverter: 
    if not (sfProcVar in s.flags) and (s.typ.callConv == ccDefault) and
        (getModule(s).id != c.module.id): 
      LocalError(n.info, errXCannotBePassedToProcVar, s.name.s)
    result = symChoice(c, n, s)
  of skConst: 
    #
    # Consider::
    #     const x = []
    #     proc p(a: openarray[int])
    #     proc q(a: openarray[char])
    #     p(x)
    #     q(x)
    #
    # It is clear that ``[]`` means two totally different things. Thus, we
    # copy `x`'s AST into each context, so that the type fixup phase can
    # deal with two different ``[]``.
    #
    markUsed(n, s)
    if s.typ.kind in ConstAbstractTypes:
      result = copyTree(s.ast)
      result.typ = s.typ
      result.info = n.info
    else:
      result = newSymNode(s, n.info)
  of skMacro: result = semMacroExpr(c, n, s)
  of skTemplate: result = semTemplateExpr(c, n, s)
  of skVar:
    markUsed(n, s)
    # if a proc accesses a global variable, it is not side effect free:
    if sfGlobal in s.flags: incl(c.p.owner.flags, sfSideEffect)
    result = newSymNode(s, n.info)
  of skGenericParam:
    if s.ast == nil: InternalError(n.info, "no default for")
    result = semExpr(c, s.ast)
  of skType:
    if efAllowType notin flags: GlobalError(n.info, errATypeHasNoValue)
    markUsed(n, s)
    result = newSymNode(s, n.info)
  else:
    markUsed(n, s)
    result = newSymNode(s, n.info)

proc checkConversionBetweenObjects(info: TLineInfo, castDest, src: PType) =
  var diff = inheritanceDiff(castDest, src)
  if diff == high(int):
    GlobalError(info, errGenerated, MsgKindToString(errIllegalConvFromXtoY) % [
        src.typeToString, castDest.typeToString])
  
proc checkConvertible(info: TLineInfo, castDest, src: PType) = 
  const 
    IntegralTypes = {tyBool, tyEnum, tyChar, tyInt..tyFloat128}
  if sameType(castDest, src): 
    # don't annoy conversions that may be needed on another processor:
    if not (castDest.kind in {tyInt..tyFloat128, tyNil}): 
      Message(info, hintConvFromXtoItselfNotNeeded, typeToString(castDest))
    return 
  var d = skipTypes(castDest, abstractVar)
  var s = skipTypes(src, abstractVar)
  while (d != nil) and (d.Kind in {tyPtr, tyRef}) and (d.Kind == s.Kind): 
    d = base(d)
    s = base(s)
  if d == nil: 
    GlobalError(info, errGenerated, `%`(msgKindToString(errIllegalConvFromXtoY), [
        typeToString(src), typeToString(castDest)]))
  elif d.Kind == tyObject and s.Kind == tyObject: 
    checkConversionBetweenObjects(info, d, s)
  elif (skipTypes(castDest, abstractVarRange).Kind in IntegralTypes) and
      (skipTypes(src, abstractVarRange).Kind in IntegralTypes): 
    # accept conversion between integral types
  else: 
    # we use d, s here to speed up that operation a bit:
    case cmpTypes(d, s)
    of isNone, isGeneric: 
      if not equalOrDistinctOf(castDest, src) and
          not equalOrDistinctOf(src, castDest): 
        GlobalError(info, errGenerated, `%`(
            MsgKindToString(errIllegalConvFromXtoY), 
            [typeToString(src), typeToString(castDest)]))
    else: 
      nil

proc isCastable(dst, src: PType): bool = 
  #const
  #  castableTypeKinds = {tyInt, tyPtr, tyRef, tyCstring, tyString, 
  #                       tySequence, tyPointer, tyNil, tyOpenArray,
  #                       tyProc, tySet, tyEnum, tyBool, tyChar}
  var ds, ss: biggestInt
  # this is very unrestrictive; cast is allowed if castDest.size >= src.size
  ds = computeSize(dst)
  ss = computeSize(src)
  if ds < 0: 
    result = false
  elif ss < 0: 
    result = false
  else: 
    result = (ds >= ss) or
        (skipTypes(dst, abstractInst).kind in {tyInt..tyFloat128}) or
        (skipTypes(src, abstractInst).kind in {tyInt..tyFloat128})
  
proc semConv(c: PContext, n: PNode, s: PSym): PNode = 
  if sonsLen(n) != 2: GlobalError(n.info, errConvNeedsOneArg)
  result = newNodeI(nkConv, n.info)
  result.typ = semTypeNode(c, n.sons[0], nil)
  addSon(result, copyTree(n.sons[0]))
  addSon(result, semExprWithType(c, n.sons[1]))
  var op = result.sons[1]
  if op.kind != nkSymChoice: 
    checkConvertible(result.info, result.typ, op.typ)
  else: 
    for i in countup(0, sonsLen(op) - 1): 
      if sameType(result.typ, op.sons[i].typ): 
        markUsed(n, op.sons[i].sym)
        return op.sons[i]
    localError(n.info, errUseQualifier, op.sons[0].sym.name.s)

proc semCast(c: PContext, n: PNode): PNode = 
  if optSafeCode in gGlobalOptions: localError(n.info, errCastNotInSafeMode)
  incl(c.p.owner.flags, sfSideEffect)
  checkSonsLen(n, 2)
  result = newNodeI(nkCast, n.info)
  result.typ = semTypeNode(c, n.sons[0], nil)
  addSon(result, copyTree(n.sons[0]))
  addSon(result, semExprWithType(c, n.sons[1]))
  if not isCastable(result.typ, result.sons[1].Typ): 
    GlobalError(result.info, errExprCannotBeCastedToX, typeToString(result.Typ))
  
proc semLowHigh(c: PContext, n: PNode, m: TMagic): PNode = 
  const 
    opToStr: array[mLow..mHigh, string] = ["low", "high"]
  if sonsLen(n) != 2: 
    GlobalError(n.info, errXExpectsTypeOrValue, opToStr[m])
  else: 
    n.sons[1] = semExprWithType(c, n.sons[1], {efAllowType})
    var typ = skipTypes(n.sons[1].typ, abstractVarRange)
    case typ.Kind
    of tySequence, tyString, tyOpenArray: 
      n.typ = getSysType(tyInt)
    of tyArrayConstr, tyArray: 
      n.typ = n.sons[1].typ.sons[0] # indextype
    of tyInt..tyInt64, tyChar, tyBool, tyEnum: 
      n.typ = n.sons[1].typ
    else: GlobalError(n.info, errInvalidArgForX, opToStr[m])
  result = n

proc semSizeof(c: PContext, n: PNode): PNode = 
  if sonsLen(n) != 2: GlobalError(n.info, errXExpectsTypeOrValue, "sizeof")
  else: n.sons[1] = semExprWithType(c, n.sons[1], {efAllowType})
  n.typ = getSysType(tyInt)
  result = n

proc semIs(c: PContext, n: PNode): PNode = 
  if sonsLen(n) == 3: 
    n.sons[1] = semExprWithType(c, n.sons[1], {efAllowType})
    n.sons[2] = semExprWithType(c, n.sons[2], {efAllowType})
    var a = n.sons[1].typ
    var b = n.sons[2].typ
    # a and b can be nil in case of an error:
    if a != nil and b != nil:
      if (b.kind != tyObject) or (a.kind != tyObject): 
        GlobalError(n.info, errIsExpectsObjectTypes)
      while (b != nil) and (b.id != a.id): b = b.sons[0]
      if b == nil: 
        GlobalError(n.info, errXcanNeverBeOfThisSubtype, typeToString(a))
    n.typ = getSysType(tyBool)
  else: 
    GlobalError(n.info, errIsExpectsTwoArguments)
  result = n

proc semOpAux(c: PContext, n: PNode) = 
  for i in countup(1, sonsLen(n) - 1): 
    var a = n.sons[i]
    if a.kind == nkExprEqExpr and sonsLen(a) == 2: 
      var info = a.sons[0].info
      a.sons[0] = newIdentNode(considerAcc(a.sons[0]), info)
      a.sons[1] = semExprWithType(c, a.sons[1])
      a.typ = a.sons[1].typ
    else: 
      n.sons[i] = semExprWithType(c, a)
  
proc overloadedCallOpr(c: PContext, n: PNode): PNode = 
  # quick check if there is *any* () operator overloaded:
  var par = getIdent("()")
  if SymtabGet(c.Tab, par) == nil: 
    result = nil
  else: 
    result = newNodeI(nkCall, n.info)
    addSon(result, newIdentNode(par, n.info))
    for i in countup(0, sonsLen(n) - 1): addSon(result, n.sons[i])
    result = semExpr(c, result)

proc changeType(n: PNode, newType: PType) = 
  case n.kind
  of nkCurly, nkBracket: 
    for i in countup(0, sonsLen(n) - 1): 
      changeType(n.sons[i], elemType(newType))
  of nkPar: 
    if newType.kind != tyTuple: 
      InternalError(n.info, "changeType: no tuple type for constructor")
    if newType.n == nil: InternalError(n.info, "changeType: no tuple fields")
    if (sonsLen(n) > 0) and (n.sons[0].kind == nkExprColonExpr): 
      for i in countup(0, sonsLen(n) - 1): 
        var m = n.sons[i].sons[0]
        if m.kind != nkSym: 
          internalError(m.info, "changeType(): invalid tuple constr")
        var f = getSymFromList(newType.n, m.sym.name)
        if f == nil: internalError(m.info, "changeType(): invalid identifier")
        changeType(n.sons[i].sons[1], f.typ)
    else: 
      for i in countup(0, sonsLen(n) - 1): 
        var m = n.sons[i]
        var a = newNodeIT(nkExprColonExpr, m.info, newType.sons[i])
        addSon(a, newSymNode(newType.n.sons[i].sym))
        addSon(a, m)
        changeType(m, newType.sons[i])
        n.sons[i] = a
  else: nil
  n.typ = newType

proc semArrayConstr(c: PContext, n: PNode): PNode = 
  result = newNodeI(nkBracket, n.info)
  result.typ = newTypeS(tyArrayConstr, c)
  addSon(result.typ, nil)     # index type
  if sonsLen(n) == 0: 
    addSon(result.typ, newTypeS(tyEmpty, c)) # needs an empty basetype!
  else: 
    var x = n.sons[0]
    var lastIndex: biggestInt = 0
    var indexType = getSysType(tyInt)
    if x.kind == nkExprColonExpr and sonsLen(x) == 2: 
      var idx = semConstExpr(c, x.sons[0])
      lastIndex = getOrdValue(idx)
      indexType = idx.typ
      x = x.sons[1]
    
    addSon(result, semExprWithType(c, x))
    var typ = skipTypes(result.sons[0].typ, {tyGenericInst, tyVar, tyOrdinal})
    for i in countup(1, sonsLen(n) - 1): 
      x = n.sons[i]
      if x.kind == nkExprColonExpr and sonsLen(x) == 2: 
        var idx = semConstExpr(c, x.sons[0])
        idx = fitNode(c, indexType, idx)
        if lastIndex+1 != getOrdValue(idx):
          localError(x.info, errInvalidOrderInArrayConstructor)
        x = x.sons[1]
      
      n.sons[i] = semExprWithType(c, x)
      addSon(result, fitNode(c, typ, n.sons[i]))
      inc(lastIndex)
    addSon(result.typ, typ)
  result.typ.sons[0] = makeRangeType(c, 0, sonsLen(result) - 1, n.info)

proc fixAbstractType(c: PContext, n: PNode) = 
  for i in countup(1, sonsLen(n) - 1): 
    var it = n.sons[i]
    case it.kind
    of nkHiddenStdConv, nkHiddenSubConv: 
      if it.sons[1].kind == nkBracket: 
        it.sons[1] = semArrayConstr(c, it.sons[1])
      if skipTypes(it.typ, abstractVar).kind == tyOpenArray: 
        var s = skipTypes(it.sons[1].typ, abstractVar)
        if (s.kind == tyArrayConstr) and (s.sons[1].kind == tyEmpty): 
          s = copyType(s, getCurrOwner(), false)
          skipTypes(s, abstractVar).sons[1] = elemType(
              skipTypes(it.typ, abstractVar))
          it.sons[1].typ = s
      elif skipTypes(it.sons[1].typ, abstractVar).kind in
          {tyNil, tyArrayConstr, tyTuple, tySet}: 
        var s = skipTypes(it.typ, abstractVar)
        changeType(it.sons[1], s)
        n.sons[i] = it.sons[1]
    of nkBracket: 
      # an implicitely constructed array (passed to an open array):
      n.sons[i] = semArrayConstr(c, it)
    else: 
      if (it.typ == nil): 
        InternalError(it.info, "fixAbstractType: " & renderTree(it))
  
proc skipObjConv(n: PNode): PNode = 
  case n.kind
  of nkHiddenStdConv, nkHiddenSubConv, nkConv: 
    if skipTypes(n.sons[1].typ, abstractPtrs).kind in {tyTuple, tyObject}: 
      result = n.sons[1]
    else: 
      result = n
  of nkObjUpConv, nkObjDownConv: result = n.sons[0]
  else: result = n
  
type 
  TAssignableResult = enum 
    arNone,                   # no l-value and no discriminant
    arLValue,                 # is an l-value
    arDiscriminant            # is a discriminant

proc isAssignable(n: PNode): TAssignableResult = 
  result = arNone
  case n.kind
  of nkSym: 
    if (n.sym.kind in {skVar, skTemp}): result = arLValue
  of nkDotExpr: 
    if skipTypes(n.sons[0].typ, abstractInst).kind in {tyVar, tyPtr, tyRef}: 
      result = arLValue
    else: 
      result = isAssignable(n.sons[0])
    if (result == arLValue) and (sfDiscriminant in n.sons[1].sym.flags): 
      result = arDiscriminant
  of nkBracketExpr: 
    if skipTypes(n.sons[0].typ, abstractInst).kind in {tyVar, tyPtr, tyRef}: 
      result = arLValue
    else: 
      result = isAssignable(n.sons[0])
  of nkHiddenStdConv, nkHiddenSubConv, nkConv: 
    # Object and tuple conversions are still addressable, so we skip them
    if skipTypes(n.typ, abstractPtrs).kind in {tyOpenArray, tyTuple, tyObject}: 
      result = isAssignable(n.sons[1])
  of nkHiddenDeref, nkDerefExpr: 
    result = arLValue
  of nkObjUpConv, nkObjDownConv, nkCheckedFieldExpr: 
    result = isAssignable(n.sons[0])
  else: 
    nil

proc newHiddenAddrTaken(c: PContext, n: PNode): PNode = 
  if n.kind == nkHiddenDeref: 
    checkSonsLen(n, 1)
    result = n.sons[0]
  else: 
    result = newNodeIT(nkHiddenAddr, n.info, makeVarType(c, n.typ))
    addSon(result, n)
    if isAssignable(n) != arLValue: 
      localError(n.info, errVarForOutParamNeeded)

proc analyseIfAddressTaken(c: PContext, n: PNode): PNode = 
  result = n
  case n.kind
  of nkSym: 
    if skipTypes(n.sym.typ, abstractInst).kind != tyVar: 
      incl(n.sym.flags, sfAddrTaken)
      result = newHiddenAddrTaken(c, n)
  of nkDotExpr: 
    checkSonsLen(n, 2)
    if n.sons[1].kind != nkSym: internalError(n.info, "analyseIfAddressTaken")
    if skipTypes(n.sons[1].sym.typ, abstractInst).kind != tyVar: 
      incl(n.sons[1].sym.flags, sfAddrTaken)
      result = newHiddenAddrTaken(c, n)
  of nkBracketExpr: 
    checkMinSonsLen(n, 1)
    if skipTypes(n.sons[0].typ, abstractInst).kind != tyVar: 
      if n.sons[0].kind == nkSym: incl(n.sons[0].sym.flags, sfAddrTaken)
      result = newHiddenAddrTaken(c, n)
  else: 
    result = newHiddenAddrTaken(c, n) # BUGFIX!
  
proc analyseIfAddressTakenInCall(c: PContext, n: PNode) = 
  const 
    FakeVarParams = {mNew, mNewFinalize, mInc, ast.mDec, mIncl, mExcl, 
      mSetLengthStr, mSetLengthSeq, mAppendStrCh, mAppendStrStr, mSwap, 
      mAppendSeqElem, mNewSeq, mReset}
  checkMinSonsLen(n, 1)
  var t = n.sons[0].typ
  if (n.sons[0].kind == nkSym) and (n.sons[0].sym.magic in FakeVarParams): 
    # BUGFIX: check for L-Value still needs to be done for the arguments!
    for i in countup(1, sonsLen(n) - 1): 
      if i < sonsLen(t) and t.sons[i] != nil and
          skipTypes(t.sons[i], abstractInst).kind == tyVar: 
        if isAssignable(n.sons[i]) != arLValue: 
          LocalError(n.sons[i].info, errVarForOutParamNeeded)
    return
  for i in countup(1, sonsLen(n) - 1): 
    if (i < sonsLen(t)) and
        (skipTypes(t.sons[i], abstractInst).kind == tyVar): 
      n.sons[i] = analyseIfAddressTaken(c, n.sons[i])
  
proc semDirectCallAnalyseEffects(c: PContext, n: PNode,
                                 flags: TExprFlags): PNode = 
  var symflags = {skProc, skMethod, skConverter}
  if efWantIterator in flags:
    # for ``type countup(1,3)``, see ``tests/ttoseq``.
    symflags = {skIterator}
  result = semDirectCall(c, n, symflags)
  if result != nil: 
    if result.sons[0].kind != nkSym: 
      InternalError("semDirectCallAnalyseEffects")
    var callee = result.sons[0].sym
    if (callee.kind == skIterator) and (callee.id == c.p.owner.id): 
      GlobalError(n.info, errRecursiveDependencyX, callee.name.s)
    if not (sfNoSideEffect in callee.flags): 
      if (sfForward in callee.flags) or
          ({sfImportc, sfSideEffect} * callee.flags != {}): 
        incl(c.p.owner.flags, sfSideEffect)
  
proc semIndirectOp(c: PContext, n: PNode, flags: TExprFlags): PNode = 
  result = nil
  var prc = n.sons[0]
  checkMinSonsLen(n, 1)
  if n.sons[0].kind == nkDotExpr: 
    checkSonsLen(n.sons[0], 2)
    n.sons[0] = semFieldAccess(c, n.sons[0])
    if n.sons[0].kind == nkDotCall: 
      # it is a static call!
      result = n.sons[0]
      result.kind = nkCall
      for i in countup(1, sonsLen(n) - 1): addSon(result, n.sons[i])
      return semExpr(c, result, flags)
  else: 
    n.sons[0] = semExpr(c, n.sons[0])
  semOpAux(c, n)
  var t: PType = nil
  if (n.sons[0].typ != nil): t = skipTypes(n.sons[0].typ, abstractInst)
  if (t != nil) and (t.kind == tyProc): 
    var m: TCandidate
    initCandidate(m, t)
    matches(c, n, m)
    if m.state != csMatch: 
      var msg = msgKindToString(errTypeMismatch)
      for i in countup(1, sonsLen(n) - 1): 
        if i > 1: add(msg, ", ")
        add(msg, typeToString(n.sons[i].typ))
      add(msg, ")\n" & msgKindToString(errButExpected) & "\n" &
          typeToString(n.sons[0].typ))
      GlobalError(n.Info, errGenerated, msg)
      result = nil
    else: 
      result = m.call
    # we assume that a procedure that calls something indirectly 
    # has side-effects:
    if not (tfNoSideEffect in t.flags): incl(c.p.owner.flags, sfSideEffect)
  else: 
    result = overloadedCallOpr(c, n)
    # Now that nkSym does not imply an iteration over the proc/iterator space,
    # the old ``prc`` (which is likely an nkIdent) has to be restored:
    if result == nil: 
      n.sons[0] = prc
      result = semDirectCallAnalyseEffects(c, n, flags)
    if result == nil: 
      GlobalError(n.info, errExprXCannotBeCalled, 
                  renderTree(n, {renderNoComments}))
  fixAbstractType(c, result)
  analyseIfAddressTakenInCall(c, result)

proc semDirectOp(c: PContext, n: PNode, flags: TExprFlags): PNode = 
  # this seems to be a hotspot in the compiler!
  semOpAux(c, n)
  result = semDirectCallAnalyseEffects(c, n, flags)
  if result == nil: 
    result = overloadedCallOpr(c, n)
    if result == nil: GlobalError(n.Info, errGenerated, getNotFoundError(c, n))
  fixAbstractType(c, result)
  analyseIfAddressTakenInCall(c, result)

proc buildStringify(c: PContext, arg: PNode): PNode = 
  if arg.typ != nil and skipTypes(arg.typ, abstractInst).kind == tyString:
    result = arg
  else:
    result = newNodeI(nkCall, arg.info)
    addSon(result, newIdentNode(getIdent"$", arg.info))
    addSon(result, arg)

proc semEcho(c: PContext, n: PNode): PNode = 
  # this really is a macro
  checkMinSonsLen(n, 1)
  for i in countup(1, sonsLen(n) - 1): 
    var arg = semExprWithType(c, n.sons[i])
    n.sons[i] = semExpr(c, buildStringify(c, arg))
  result = n

proc lookUpForDefined(c: PContext, i: PIdent, onlyCurrentScope: bool): PSym =
  if onlyCurrentScope: 
    result = SymtabLocalGet(c.tab, i)
  else: 
    result = SymtabGet(c.Tab, i) # no need for stub loading

proc LookUpForDefined(c: PContext, n: PNode, onlyCurrentScope: bool): PSym = 
  case n.kind
  of nkIdent: 
    result = LookupForDefined(c, n.ident, onlyCurrentScope)
  of nkDotExpr:
    result = nil
    if onlyCurrentScope: return 
    checkSonsLen(n, 2)
    var m = LookupForDefined(c, n.sons[0], onlyCurrentScope)
    if (m != nil) and (m.kind == skModule): 
      if (n.sons[1].kind == nkIdent): 
        var ident = n.sons[1].ident
        if m == c.module: 
          result = StrTableGet(c.tab.stack[ModuleTablePos], ident)
        else: 
          result = StrTableGet(m.tab, ident)
      else: 
        GlobalError(n.sons[1].info, errIdentifierExpected, "")
  of nkAccQuoted:
    result = lookupForDefined(c, considerAcc(n), onlyCurrentScope)
  of nkSym:
    result = n.sym
  else: 
    GlobalError(n.info, errIdentifierExpected, renderTree(n))
    result = nil

proc semDefined(c: PContext, n: PNode, onlyCurrentScope: bool): PNode = 
  checkSonsLen(n, 2)
  # we replace this node by a 'true' or 'false' node:
  result = newIntNode(nkIntLit, 0)
  if LookUpForDefined(c, n.sons[1], onlyCurrentScope) != nil: 
    result.intVal = 1
  elif not onlyCurrentScope and (n.sons[1].kind == nkIdent) and
      condsyms.isDefined(n.sons[1].ident): 
    result.intVal = 1
  result.info = n.info
  result.typ = getSysType(tyBool)

proc setMs(n: PNode, s: PSym): PNode = 
  result = n
  n.sons[0] = newSymNode(s)
  n.sons[0].info = n.info

proc semMagic(c: PContext, n: PNode, s: PSym, flags: TExprFlags): PNode = 
  # this is a hotspot in the compiler!
  result = n
  case s.magic # magics that need special treatment
  of mDefined: result = semDefined(c, setMs(n, s), false)
  of mDefinedInScope: result = semDefined(c, setMs(n, s), true)
  of mLow: result = semLowHigh(c, setMs(n, s), mLow)
  of mHigh: result = semLowHigh(c, setMs(n, s), mHigh)
  of mSizeOf: result = semSizeof(c, setMs(n, s))
  of mIs: result = semIs(c, setMs(n, s))
  of mEcho: result = semEcho(c, setMs(n, s))
  of mCreateThread: 
    result = semDirectOp(c, n, flags)
    if semthreads.needsGlobalAnalysis():
      c.threadEntries.add(result)
  else: result = semDirectOp(c, n, flags)
  
proc isTypeExpr(n: PNode): bool = 
  case n.kind
  of nkType, nkTypeOfExpr: result = true
  of nkSym: result = n.sym.kind == skType
  else: result = false
  
proc lookupInRecordAndBuildCheck(c: PContext, n, r: PNode, field: PIdent, 
                                 check: var PNode): PSym = 
  # transform in a node that contains the runtime check for the
  # field, if it is in a case-part...
  result = nil
  case r.kind
  of nkRecList: 
    for i in countup(0, sonsLen(r) - 1): 
      result = lookupInRecordAndBuildCheck(c, n, r.sons[i], field, check)
      if result != nil: return 
  of nkRecCase: 
    checkMinSonsLen(r, 2)
    if (r.sons[0].kind != nkSym): IllFormedAst(r)
    result = lookupInRecordAndBuildCheck(c, n, r.sons[0], field, check)
    if result != nil: return 
    var s = newNodeI(nkCurly, r.info)
    for i in countup(1, sonsLen(r) - 1): 
      var it = r.sons[i]
      case it.kind
      of nkOfBranch: 
        result = lookupInRecordAndBuildCheck(c, n, lastSon(it), field, check)
        if result == nil: 
          for j in 0..sonsLen(it)-2: addSon(s, copyTree(it.sons[j]))
        else: 
          if check == nil: 
            check = newNodeI(nkCheckedFieldExpr, n.info)
            addSon(check, ast.emptyNode) # make space for access node
          s = newNodeI(nkCurly, n.info)
          for j in countup(0, sonsLen(it) - 2): addSon(s, copyTree(it.sons[j]))
          var inExpr = newNodeI(nkCall, n.info)
          addSon(inExpr, newIdentNode(getIdent("in"), n.info))
          addSon(inExpr, copyTree(r.sons[0]))
          addSon(inExpr, s)   #writeln(output, renderTree(inExpr));
          addSon(check, semExpr(c, inExpr))
          return 
      of nkElse: 
        result = lookupInRecordAndBuildCheck(c, n, lastSon(it), field, check)
        if result != nil: 
          if check == nil: 
            check = newNodeI(nkCheckedFieldExpr, n.info)
            addSon(check, ast.emptyNode) # make space for access node
          var inExpr = newNodeI(nkCall, n.info)
          addSon(inExpr, newIdentNode(getIdent("in"), n.info))
          addSon(inExpr, copyTree(r.sons[0]))
          addSon(inExpr, s)
          var notExpr = newNodeI(nkCall, n.info)
          addSon(notExpr, newIdentNode(getIdent("not"), n.info))
          addSon(notExpr, inExpr)
          addSon(check, semExpr(c, notExpr))
          return 
      else: illFormedAst(it)
  of nkSym: 
    if r.sym.name.id == field.id: result = r.sym
  else: illFormedAst(n)
  
proc makeDeref(n: PNode): PNode = 
  var t = skipTypes(n.typ, {tyGenericInst})
  result = n
  if t.kind == tyVar: 
    result = newNodeIT(nkHiddenDeref, n.info, t.sons[0])
    addSon(result, n)
    t = skipTypes(t.sons[0], {tyGenericInst})
  if t.kind in {tyPtr, tyRef}: 
    var a = result
    result = newNodeIT(nkHiddenDeref, n.info, t.sons[0])
    addSon(result, a)

proc builtinFieldAccess(c: PContext, n: PNode, flags: TExprFlags): PNode =
  ## returns nil if it's not a built-in field access
  var s = qualifiedLookup(c, n, {checkAmbiguity, checkUndeclared})
  if s != nil:
    return semSym(c, n, s, flags)

  checkSonsLen(n, 2)
  n.sons[0] = semExprWithType(c, n.sons[0], {efAllowType} + flags)
  var i = considerAcc(n.sons[1])
  var ty = n.sons[0].Typ
  var f: PSym = nil
  result = nil
  if isTypeExpr(n.sons[0]):
    if ty.kind == tyEnum: 
      # look up if the identifier belongs to the enum:
      while ty != nil: 
        f = getSymFromList(ty.n, i)
        if f != nil: break 
        ty = ty.sons[0]         # enum inheritance
      if f != nil: 
        result = newSymNode(f)
        result.info = n.info
        result.typ = ty
        markUsed(n, f)
        return 
    elif efAllowType notin flags: 
      GlobalError(n.sons[0].info, errATypeHasNoValue)
      return
  ty = skipTypes(ty, {tyGenericInst, tyVar, tyPtr, tyRef})
  var check: PNode = nil
  if ty.kind == tyObject: 
    while true: 
      check = nil
      f = lookupInRecordAndBuildCheck(c, n, ty.n, i, check)
      if f != nil: break 
      if ty.sons[0] == nil: break 
      ty = skipTypes(ty.sons[0], {tyGenericInst})
    if f != nil: 
      if {sfStar, sfMinus} * f.flags != {} or getModule(f).id == c.module.id: 
        # is the access to a public field or in the same module?
        n.sons[0] = makeDeref(n.sons[0])
        n.sons[1] = newSymNode(f) # we now have the correct field
        n.typ = f.typ
        markUsed(n, f)
        if check == nil: 
          result = n
        else: 
          check.sons[0] = n
          check.typ = n.typ
          result = check
  elif ty.kind == tyTuple and ty.n != nil: 
    f = getSymFromList(ty.n, i)
    if f != nil: 
      n.sons[0] = makeDeref(n.sons[0])
      n.sons[1] = newSymNode(f)
      n.typ = f.typ
      result = n
      markUsed(n, f)

proc semFieldAccess(c: PContext, n: PNode, flags: TExprFlags): PNode = 
  # this is difficult, because the '.' is used in many different contexts
  # in Nimrod. We first allow types in the semantic checking.
  result = builtinFieldAccess(c, n, flags)
  if result == nil:
    var i = considerAcc(n.sons[1])
    var f = SymTabGet(c.tab, i)
    # if f != nil and f.kind == skStub: loadStub(f)
    # ``loadStub`` is not correct here as we don't care for ``f`` really
    if f != nil: 
      # BUGFIX: do not check for (f.kind in [skProc, skMethod, skIterator]) here
      # This special node kind is to merge with the call handler in `semExpr`.
      result = newNodeI(nkDotCall, n.info)
      addSon(result, newIdentNode(i, n.info))
      addSon(result, copyTree(n[0]))
    else: 
      GlobalError(n.Info, errUndeclaredFieldX, i.s)

proc buildOverloadedSubscripts(n: PNode, inAsgn: bool): PNode =
  result = newNodeI(nkCall, n.info)
  result.add(newIdentNode(
    if inAsgn: getIdent"[]=" else: getIdent"[]", n.info))
  for i in 0 .. n.len-1: result.add(n[i])
  
proc semDeref(c: PContext, n: PNode): PNode =
  checkSonsLen(n, 1)
  n.sons[0] = semExprWithType(c, n.sons[0])
  result = n
  var t = skipTypes(n.sons[0].typ, {tyGenericInst, tyVar})
  case t.kind
  of tyRef, tyPtr: n.typ = t.sons[0]
  else: result = nil
  #GlobalError(n.sons[0].info, errCircumNeedsPointer) 
  
proc semSubscript(c: PContext, n: PNode, flags: TExprFlags): PNode =
  ## returns nil if not a built-in subscript operator; also called for the
  ## checking of assignments
  if sonsLen(n) == 1: 
    var x = semDeref(c, n)
    if x == nil: return nil
    result = newNodeIT(nkDerefExpr, x.info, x.typ)
    result.add(x[0])
    return
  checkMinSonsLen(n, 2)
  n.sons[0] = semExprWithType(c, n.sons[0], flags - {efAllowType})
  var arr = skipTypes(n.sons[0].typ, {tyGenericInst, tyVar, tyPtr, tyRef})
  case arr.kind
  of tyArray, tyOpenArray, tyArrayConstr, tySequence, tyString, tyCString: 
    checkSonsLen(n, 2)
    n.sons[0] = makeDeref(n.sons[0])
    for i in countup(1, sonsLen(n) - 1): 
      n.sons[i] = semExprWithType(c, n.sons[i], flags - {efAllowType})
    var indexType = if arr.kind == tyArray: arr.sons[0] else: getSysType(tyInt)
    var arg = IndexTypesMatch(c, indexType, n.sons[1].typ, n.sons[1])
    if arg != nil: 
      n.sons[1] = arg
      result = n
      result.typ = elemType(arr)
    #GlobalError(n.info, errIndexTypesDoNotMatch)
  of tyTuple: 
    checkSonsLen(n, 2)
    n.sons[0] = makeDeref(n.sons[0])
    # [] operator for tuples requires constant expression:
    n.sons[1] = semConstExpr(c, n.sons[1])
    if skipTypes(n.sons[1].typ, {tyGenericInst, tyRange, tyOrdinal}).kind in
        {tyInt..tyInt64}: 
      var idx = getOrdValue(n.sons[1])
      if (idx >= 0) and (idx < sonsLen(arr)): n.typ = arr.sons[int(idx)]
      else: GlobalError(n.info, errInvalidIndexValueForTuple)
    else: 
      GlobalError(n.info, errIndexTypesDoNotMatch)
    result = n
  else: nil
  
proc semArrayAccess(c: PContext, n: PNode, flags: TExprFlags): PNode = 
  result = semSubscript(c, n, flags)
  if result == nil:
    # overloaded [] operator:
    result = semExpr(c, buildOverloadedSubscripts(n, inAsgn=false))

proc semIfExpr(c: PContext, n: PNode): PNode = 
  result = n
  checkMinSonsLen(n, 2)
  var typ: PType = nil
  for i in countup(0, sonsLen(n) - 1): 
    var it = n.sons[i]
    case it.kind
    of nkElifExpr: 
      checkSonsLen(it, 2)
      it.sons[0] = forceBool(c, semExprWithType(c, it.sons[0]))
      it.sons[1] = semExprWithType(c, it.sons[1])
      if typ == nil: typ = it.sons[1].typ
      else: it.sons[1] = fitNode(c, typ, it.sons[1])
    of nkElseExpr: 
      checkSonsLen(it, 1)
      it.sons[0] = semExprWithType(c, it.sons[0])
      if typ == nil: InternalError(it.info, "semIfExpr")
      it.sons[0] = fitNode(c, typ, it.sons[0])
    else: illFormedAst(n)
  result.typ = typ

proc semSetConstr(c: PContext, n: PNode): PNode = 
  result = newNodeI(nkCurly, n.info)
  result.typ = newTypeS(tySet, c)
  if sonsLen(n) == 0: 
    addSon(result.typ, newTypeS(tyEmpty, c))
  else: 
    # only semantic checking for all elements, later type checking:
    var typ: PType = nil
    for i in countup(0, sonsLen(n) - 1): 
      if isRange(n.sons[i]): 
        checkSonsLen(n.sons[i], 3)
        n.sons[i].sons[1] = semExprWithType(c, n.sons[i].sons[1])
        n.sons[i].sons[2] = semExprWithType(c, n.sons[i].sons[2])
        if typ == nil: 
          typ = skipTypes(n.sons[i].sons[1].typ, 
                          {tyGenericInst, tyVar, tyOrdinal})
        n.sons[i].typ = n.sons[i].sons[2].typ # range node needs type too
      elif n.sons[i].kind == nkRange:
        # already semchecked
        if typ == nil:
          typ = skipTypes(n.sons[i].sons[0].typ, 
                          {tyGenericInst, tyVar, tyOrdinal})
      else:
        n.sons[i] = semExprWithType(c, n.sons[i])
        if typ == nil: 
          typ = skipTypes(n.sons[i].typ, {tyGenericInst, tyVar, tyOrdinal})
    if not isOrdinalType(typ): 
      GlobalError(n.info, errOrdinalTypeExpected)
      return 
    if lengthOrd(typ) > MaxSetElements: 
      typ = makeRangeType(c, 0, MaxSetElements - 1, n.info)
    addSon(result.typ, typ)
    for i in countup(0, sonsLen(n) - 1): 
      var m: PNode
      if isRange(n.sons[i]):
        m = newNodeI(nkRange, n.sons[i].info)
        addSon(m, fitNode(c, typ, n.sons[i].sons[1]))
        addSon(m, fitNode(c, typ, n.sons[i].sons[2]))
      elif n.sons[i].kind == nkRange: m = n.sons[i] # already semchecked
      else:
        m = fitNode(c, typ, n.sons[i])
      addSon(result, m)

proc semTableConstr(c: PContext, n: PNode): PNode = 
  # we simply transform ``{key: value, key2: value}`` to 
  # ``[(key, value), (key2, value2)]``
  result = newNodeI(nkBracket, n.info)
  for i in 0..n.len-1:
    var x = n.sons[i]
    if x.kind == nkExprColonExpr and sonsLen(x) == 2:
      var pair = newNodeI(nkPar, x.info)
      pair.add(x[0])
      pair.add(x[1])
      result.add(pair)
    else:
      illFormedAst(x)
  result = semExpr(c, result)

type 
  TParKind = enum 
    paNone, paSingle, paTupleFields, paTuplePositions

proc checkPar(n: PNode): TParKind = 
  var length = sonsLen(n)
  if length == 0: 
    result = paTuplePositions # ()
  elif length == 1: 
    result = paSingle         # (expr)
  else: 
    if n.sons[0].kind == nkExprColonExpr: result = paTupleFields
    else: result = paTuplePositions
    for i in countup(0, length - 1): 
      if result == paTupleFields: 
        if (n.sons[i].kind != nkExprColonExpr) or
            not (n.sons[i].sons[0].kind in {nkSym, nkIdent}): 
          GlobalError(n.sons[i].info, errNamedExprExpected)
          return paNone
      else: 
        if n.sons[i].kind == nkExprColonExpr: 
          GlobalError(n.sons[i].info, errNamedExprNotAllowed)
          return paNone

proc semTupleFieldsConstr(c: PContext, n: PNode): PNode = 
  result = newNodeI(nkPar, n.info)
  var typ = newTypeS(tyTuple, c)
  typ.n = newNodeI(nkRecList, n.info) # nkIdentDefs
  var ids = initIntSet()
  for i in countup(0, sonsLen(n) - 1): 
    if (n.sons[i].kind != nkExprColonExpr) or
        not (n.sons[i].sons[0].kind in {nkSym, nkIdent}): 
      illFormedAst(n.sons[i])
    var id: PIdent
    if n.sons[i].sons[0].kind == nkIdent: id = n.sons[i].sons[0].ident
    else: id = n.sons[i].sons[0].sym.name
    if ContainsOrIncl(ids, id.id): 
      localError(n.sons[i].info, errFieldInitTwice, id.s)
    n.sons[i].sons[1] = semExprWithType(c, n.sons[i].sons[1])
    var f = newSymS(skField, n.sons[i].sons[0], c)
    f.typ = n.sons[i].sons[1].typ
    addSon(typ, f.typ)
    addSon(typ.n, newSymNode(f))
    n.sons[i].sons[0] = newSymNode(f)
    addSon(result, n.sons[i])
  result.typ = typ

proc semTuplePositionsConstr(c: PContext, n: PNode): PNode = 
  result = n                  # we don't modify n, but compute the type:
  var typ = newTypeS(tyTuple, c)  # leave typ.n nil!
  for i in countup(0, sonsLen(n) - 1): 
    n.sons[i] = semExprWithType(c, n.sons[i])
    addSon(typ, n.sons[i].typ)
  result.typ = typ

proc semStmtListExpr(c: PContext, n: PNode): PNode = 
  result = n
  checkMinSonsLen(n, 1)
  var length = sonsLen(n)
  for i in countup(0, length - 2): 
    n.sons[i] = semStmt(c, n.sons[i])
  if length > 0: 
    n.sons[length - 1] = semExprWithType(c, n.sons[length - 1])
    n.typ = n.sons[length - 1].typ

proc semBlockExpr(c: PContext, n: PNode): PNode = 
  result = n
  Inc(c.p.nestedBlockCounter)
  checkSonsLen(n, 2)
  openScope(c.tab)            # BUGFIX: label is in the scope of block!
  if n.sons[0].kind != nkEmpty: addDecl(c, newSymS(skLabel, n.sons[0], c))
  n.sons[1] = semStmtListExpr(c, n.sons[1])
  n.typ = n.sons[1].typ
  closeScope(c.tab)
  Dec(c.p.nestedBlockCounter)

proc isCallExpr(n: PNode): bool = 
  result = n.kind in {nkCall, nkInfix, nkPrefix, nkPostfix, nkCommand,
                      nkCallStrLit}

proc semMacroStmt(c: PContext, n: PNode, semCheck = true): PNode = 
  checkMinSonsLen(n, 2)
  var a: PNode
  if isCallExpr(n.sons[0]): a = n.sons[0].sons[0]
  else: a = n.sons[0]
  var s = qualifiedLookup(c, a, {checkUndeclared})
  if s != nil: 
    case s.kind
    of skMacro: 
      result = semMacroExpr(c, n, s, semCheck)
    of skTemplate: 
      # transform
      # nkMacroStmt(nkCall(a...), stmt, b...)
      # to
      # nkCall(a..., stmt, b...)
      result = newNodeI(nkCall, n.info)
      addSon(result, a)
      if isCallExpr(n.sons[0]): 
        for i in countup(1, sonsLen(n.sons[0]) - 1): 
          addSon(result, n.sons[0].sons[i])
      for i in countup(1, sonsLen(n) - 1): addSon(result, n.sons[i])
      result = semTemplateExpr(c, result, s, semCheck)
    else: GlobalError(n.info, errXisNoMacroOrTemplate, s.name.s)
  else: 
    GlobalError(n.info, errInvalidExpressionX, 
                renderTree(a, {renderNoComments}))

proc semExpr(c: PContext, n: PNode, flags: TExprFlags = {}): PNode = 
  result = n
  if gCmd == cmdIdeTools: suggestExpr(c, n)
  if nfSem in n.flags: return 
  case n.kind
  of nkIdent, nkAccQuoted:
    var s = lookUp(c, n)
    result = semSym(c, n, s, flags)
  of nkSym:
    # because of the changed symbol binding, this does not mean that we
    # don't have to check the symbol for semantics here again!
    result = semSym(c, n, n.sym, flags)
  of nkEmpty, nkNone: 
    nil
  of nkNilLit: 
    result.typ = getSysType(tyNil)
  of nkType: 
    if not (efAllowType in flags): GlobalError(n.info, errATypeHasNoValue)
    n.typ = semTypeNode(c, n, nil)
  of nkIntLit: 
    if result.typ == nil: result.typ = getSysType(tyInt)
  of nkInt8Lit: 
    if result.typ == nil: result.typ = getSysType(tyInt8)
  of nkInt16Lit: 
    if result.typ == nil: result.typ = getSysType(tyInt16)
  of nkInt32Lit: 
    if result.typ == nil: result.typ = getSysType(tyInt32)
  of nkInt64Lit: 
    if result.typ == nil: result.typ = getSysType(tyInt64)
  of nkFloatLit: 
    if result.typ == nil: result.typ = getSysType(tyFloat)
  of nkFloat32Lit: 
    if result.typ == nil: result.typ = getSysType(tyFloat32)
  of nkFloat64Lit: 
    if result.typ == nil: result.typ = getSysType(tyFloat64)
  of nkStrLit..nkTripleStrLit: 
    if result.typ == nil: result.typ = getSysType(tyString)
  of nkCharLit: 
    if result.typ == nil: result.typ = getSysType(tyChar)
  of nkDotExpr: 
    result = semFieldAccess(c, n, flags)
    if result.kind == nkDotCall: 
      result.kind = nkCall
      result = semExpr(c, result, flags)
  of nkBind: 
    result = semExpr(c, n.sons[0], flags)
  of nkCall, nkInfix, nkPrefix, nkPostfix, nkCommand, nkCallStrLit: 
    # check if it is an expression macro:
    checkMinSonsLen(n, 1)
    var s = qualifiedLookup(c, n.sons[0], {checkUndeclared})
    if s != nil: 
      case s.kind
      of skMacro: result = semMacroExpr(c, n, s)
      of skTemplate: result = semTemplateExpr(c, n, s)
      of skType: 
        if n.kind != nkCall: GlobalError(n.info, errXisNotCallable, s.name.s)
        # XXX does this check make any sense?
        result = semConv(c, n, s)
      of skProc, skMethod, skConverter, skIterator: 
        if s.magic == mNone: result = semDirectOp(c, n, flags)
        else: result = semMagic(c, n, s, flags)
      else: 
        #liMessage(n.info, warnUser, renderTree(n));
        result = semIndirectOp(c, n, flags)
    elif n.sons[0].kind == nkSymChoice: 
      result = semDirectOp(c, n, flags)
    else: 
      result = semIndirectOp(c, n, flags)
  of nkMacroStmt: 
    result = semMacroStmt(c, n)
  of nkBracketExpr: 
    checkMinSonsLen(n, 1)
    var s = qualifiedLookup(c, n.sons[0], {checkUndeclared})
    if s != nil and s.kind in {skProc, skMethod, skConverter, skIterator}: 
      # type parameters: partial generic specialization
      n.sons[0] = semSymGenericInstantiation(c, n.sons[0], s)
      result = explicitGenericInstantiation(c, n, s)
    else: 
      result = semArrayAccess(c, n, flags)
  of nkPragmaExpr: 
    # which pragmas are allowed for expressions? `likely`, `unlikely`
    internalError(n.info, "semExpr() to implement") # XXX: to implement
  of nkPar: 
    case checkPar(n)
    of paNone: result = nil
    of paTuplePositions: result = semTuplePositionsConstr(c, n)
    of paTupleFields: result = semTupleFieldsConstr(c, n)
    of paSingle: result = semExpr(c, n.sons[0], flags)
  of nkCurly: result = semSetConstr(c, n)
  of nkBracket: result = semArrayConstr(c, n)
  of nkLambda: result = semLambda(c, n)
  of nkDerefExpr: 
    Message(n.info, warnDerefDeprecated)
    result = semDeref(c, n)
  of nkAddr: 
    result = n
    checkSonsLen(n, 1)
    n.sons[0] = semExprWithType(c, n.sons[0])
    if isAssignable(n.sons[0]) != arLValue: 
      GlobalError(n.info, errExprHasNoAddress)
    n.typ = makePtrType(c, n.sons[0].typ)
  of nkHiddenAddr, nkHiddenDeref:
    checkSonsLen(n, 1)
    n.sons[0] = semExpr(c, n.sons[0], flags)
  of nkCast: result = semCast(c, n)
  of nkIfExpr: result = semIfExpr(c, n)
  of nkStmtListExpr: result = semStmtListExpr(c, n)
  of nkBlockExpr: result = semBlockExpr(c, n)
  of nkHiddenStdConv, nkHiddenSubConv, nkConv, nkHiddenCallConv: 
    checkSonsLen(n, 2)
  of nkStringToCString, nkCStringToString, nkPassAsOpenArray, nkObjDownConv, 
     nkObjUpConv: 
    checkSonsLen(n, 1)
  of nkChckRangeF, nkChckRange64, nkChckRange: 
    checkSonsLen(n, 3)
  of nkCheckedFieldExpr: 
    checkMinSonsLen(n, 2)
  of nkTableConstr:
    result = semTableConstr(c, n)
  of nkSymChoice:
    GlobalError(n.info, errExprXAmbiguous, renderTree(n, {renderNoComments}))
  else:
    GlobalError(n.info, errInvalidExpressionX, 
                renderTree(n, {renderNoComments}))
  incl(result.flags, nfSem)