fixes #6989

3 files changed, 116 insertions, 7 deletions

diff --git a/compiler/semfold.nim b/compiler/semfold.nim
index f690615ac..55cdc334c 100644
--- a/compiler/semfold.nim
+++ b/compiler/semfold.nim
@@ -30,7 +30,12 @@ proc newIntNodeT(intVal: BiggestInt, n: PNode): PNode =
   case skipTypes(n.typ, abstractVarRange).kind
   of tyInt:
     result = newIntNode(nkIntLit, intVal)
-    result.typ = getIntLitType(result)
+    # See bug #6989. 'pred' et al only produce an int literal type if the
+    # original type was 'int', not a distinct int etc.
+    if n.typ.kind == tyInt:
+      result.typ = getIntLitType(result)
+    else:
+      result.typ = n.typ
     # hrm, this is not correct: 1 + high(int) shouldn't produce tyInt64 ...
     #setIntLitType(result)
   of tyChar:
diff --git a/lib/system.nim b/lib/system.nim
index 2c0617e4d..4e071e802 100644
--- a/lib/system.nim
+++ b/lib/system.nim
@@ -318,7 +318,7 @@ type
   Slice*[T] = HSlice[T, T] ## an alias for ``HSlice[T, T]``
 
 proc `..`*[T, U](a: T, b: U): HSlice[T, U] {.noSideEffect, inline, magic: "DotDot".} =
-  ## `slice`:idx: operator that constructs an interval ``[a, b]``, both `a`
+  ## binary `slice`:idx: operator that constructs an interval ``[a, b]``, both `a`
   ## and `b` are inclusive. Slices can also be used in the set constructor
   ## and in ordinal case statements, but then they are special-cased by the
   ## compiler.
@@ -326,7 +326,7 @@ proc `..`*[T, U](a: T, b: U): HSlice[T, U] {.noSideEffect, inline, magic: "DotDo
   result.b = b
 
 proc `..`*[T](b: T): HSlice[int, T] {.noSideEffect, inline, magic: "DotDot".} =
-  ## `slice`:idx: operator that constructs an interval ``[default(int), b]``
+  ## unary `slice`:idx: operator that constructs an interval ``[default(int), b]``
   result.b = b
 
 when not defined(niminheritable):
@@ -677,12 +677,12 @@ proc `<`*[T](x: Ordinal[T]): T {.magic: "UnaryLt", noSideEffect, deprecated.}
   ## write ``0 ..< 10`` instead of ``0 .. < 10`` (look at the spacing).
   ## For ``<x`` write ``pred(x)``.
 
-proc succ*[T](x: Ordinal[T], y = 1): T {.magic: "Succ", noSideEffect.}
+proc succ*[T: Ordinal](x: T, y = 1): T {.magic: "Succ", noSideEffect.}
   ## returns the ``y``-th successor of the value ``x``. ``T`` has to be
   ## an ordinal type. If such a value does not exist, ``EOutOfRange`` is raised
   ## or a compile time error occurs.
 
-proc pred*[T](x: Ordinal[T], y = 1): T {.magic: "Pred", noSideEffect.}
+proc pred*[T: Ordinal](x: T, y = 1): T {.magic: "Pred", noSideEffect.}
   ## returns the ``y``-th predecessor of the value ``x``. ``T`` has to be
   ## an ordinal type. If such a value does not exist, ``EOutOfRange`` is raised
   ## or a compile time error occurs.
@@ -3505,8 +3505,8 @@ template `..^`*(a, b: untyped): untyped =
   a .. ^b
 
 template `..<`*(a, b: untyped): untyped =
-  ## a shortcut for 'a..pred(b)'.
-  a .. pred(b)
+  ## a shortcut for 'a .. (when b is BackwardsIndex: succ(b) else: pred(b))'.
+  a .. (when b is BackwardsIndex: succ(b) else: pred(b))
 
 when defined(nimNewRoof):
   iterator `..<`*[T](a, b: T): T =
diff --git a/tests/array/troofregression2.nim b/tests/array/troofregression2.nim
new file mode 100644
index 000000000..78d9b9718
--- /dev/null
+++ b/tests/array/troofregression2.nim
@@ -0,0 +1,104 @@
+discard """
+  output: '''OK
+OK
+OK
+OK
+OK
+dflfdjkl__abcdefgasfsgdfgsgdfggsdfasdfsafewfkljdsfajs
+dflfdjkl__abcdefgasfsgdfgsgdfggsdfasdfsafewfkljdsfajsdf
+kgdchlfniambejop
+fjpmholcibdgeakn
+'''
+"""
+
+import strutils, sequtils
+# bug #6989
+
+import typetraits
+
+type Dist = distinct int
+
+proc mypred[T: Ordinal](x: T): T = T(int(x)-1)
+proc cons(x: int): Dist = Dist(x)
+
+var d: Dist
+
+template `^+`(s, i: untyped): untyped =
+  (when i is BackwardsIndex: s.len - int(i) else: int(i))
+
+proc `...`*[T, U](a: T, b: U): HSlice[T, U] =
+  result.a = a
+  result.b = b
+
+proc `...`*[T](b: T): HSlice[int, T] =
+  result.b = b
+
+template `...<`*(a, b: untyped): untyped =
+  ## a shortcut for 'a..pred(b)'.
+  a ... pred(b)
+
+template check(a, b) =
+  if $a == b: echo "OK"
+  else: echo "Failure ", a, " != ", b
+
+check type(4 ...< 1), "HSlice[system.int, system.int]"
+
+check type(4 ...< ^1), "HSlice[system.int, system.BackwardsIndex]"
+check type(4 ... pred(^1)), "HSlice[system.int, system.BackwardsIndex]"
+
+check type(4 ... mypred(8)), "HSlice[system.int, system.int]"
+
+check type(4 ... mypred(^1)), "HSlice[system.int, system.BackwardsIndex]"
+
+var rot = 8
+
+proc bug(s: string): string =
+  result = s
+  result = result[result.len - rot .. ^1] & "__" & result[0 ..< ^rot]
+
+const testStr = "abcdefgasfsgdfgsgdfggsdfasdfsafewfkljdsfajsdflfdjkl"
+
+echo bug(testStr)
+echo testStr[testStr.len - 8 .. testStr.len - 1] & "__" & testStr[0 .. testStr.len - pred(rot)]
+
+
+
+const
+  instructions = readFile("./inputs/16.txt").split(',')
+  programs = "abcdefghijklmnop"
+
+proc dance(dancers: string): string =
+  result = dancers
+  for instr in instructions:
+    let rem = instr[1 .. instr.high]
+    case instr[0]
+    of 's':
+      let rot = rem.parseInt
+      result = result[result.len - rot .. ^1] & result[0 ..< ^rot]
+    of 'x':
+      let
+        x = rem.split('/')
+        a = x[0].parseInt
+        b = x[1].parseInt
+      swap(result[a], result[b])
+    of 'p':
+      let
+        a = result.find(rem[0])
+        b = result.find(rem[^1])
+      result[a] = rem[^1]
+      result[b] = rem[0]
+    else: discard
+
+proc longDance(dancers: string, iterations = 1_000_000_000): string =
+  var
+    dancers = dancers
+    seen = @[dancers]
+  for i in 1 .. iterations:
+    dancers = dancers.dance()
+    if dancers in seen:
+      return seen[iterations mod i]
+    seen.add(dancers)
+
+
+echo dance(programs)
+echo longDance(programs)