A certain combination of iterators and procs crashes the compiler

[EyeCon]

Code to reproduce

import std / sequtils

iterator combinations*[T](s: seq[T], r: Positive): seq[T] =
  yield @[s[0], s[1]]

iterator pairwise*[T](s: openArray[T]): seq[T] =
  yield @[s[0], s[1]]

proc checkSpecialSubset5(s: seq[int]): bool =
  toSeq(
    toSeq(
      s.combinations(2)
    ).map(proc(a: auto): int = a[0]).pairwise()
  ).any(proc(a: auto): bool = true)

echo checkSpecialSubset5 @[1, 2]

Current Output

> nim c -r "d:\[REDACTED].nim"
Hint: used config file 'C:\Users\EyeCon\.choosenim\toolchains\nim-#devel\config\nim.cfg' [Conf]
Hint: used config file 'C:\Users\EyeCon\.choosenim\toolchains\nim-#devel\config\config.nims' [Conf]
Hint: used config file 'd:\prog\Nim\pe\config.nims' [Conf]
...............................................................
[REDACTED](1, 12) Warning: imported and not used: 'algorithm' [UnusedImport]
CC: pe0103.nim
C:\Users\EyeCon\nimcache\pe0103_d\@mpe0103.nim.c: In function 'checkSpecialSubset5__pe48494851_13':
C:\Users\EyeCon\nimcache\pe0103_d\@mpe0103.nim.c:629:55: error: redeclaration of 'resultX60gensym6_' with no linkage
  629 |                 tySequence__PSP8snSsRoFs9cDiya9bd7UQ* resultX60gensym6_;
      |                                                       ^~~~~~~~~~~~~~~~~
C:\Users\EyeCon\nimcache\pe0103_d\@mpe0103.nim.c:626:55: note: previous declaration of 'resultX60gensym6_' with type 'tySequence__PSP8snSsRoFs9cDiya9bd7UQ *'      
  626 |                 tySequence__PSP8snSsRoFs9cDiya9bd7UQ* resultX60gensym6_;
      |                                                       ^~~~~~~~~~~~~~~~~
C:\Users\EyeCon\nimcache\pe0103_d\@mpe0103.nim.c:633:55: error: redeclaration of 'resultX60gensym6_' with no linkage
  633 |                 tySequence__PSP8snSsRoFs9cDiya9bd7UQ* resultX60gensym6_;
      |                                                       ^~~~~~~~~~~~~~~~~
C:\Users\EyeCon\nimcache\pe0103_d\@mpe0103.nim.c:629:55: note: previous declaration of 'resultX60gensym6_' with type 'tySequence__PSP8snSsRoFs9cDiya9bd7UQ *'      
  629 |                 tySequence__PSP8snSsRoFs9cDiya9bd7UQ* resultX60gensym6_;
      |                                                       ^~~~~~~~~~~~~~~~~
C:\Users\EyeCon\nimcache\pe0103_d\@mpe0103.nim.c:636:55: error: redeclaration of 'resultX60gensym6_' with no linkage
  636 |                 tySequence__PSP8snSsRoFs9cDiya9bd7UQ* resultX60gensym6_;
      |                                                       ^~~~~~~~~~~~~~~~~
C:\Users\EyeCon\nimcache\pe0103_d\@mpe0103.nim.c:633:55: note: previous declaration of 'resultX60gensym6_' with type 'tySequence__PSP8snSsRoFs9cDiya9bd7UQ *'      
  633 |                 tySequence__PSP8snSsRoFs9cDiya9bd7UQ* resultX60gensym6_;
      |                                                       ^~~~~~~~~~~~~~~~~
Error: execution of an external compiler program 'gcc.exe -c  -w -fmax-errors=3 -mno-ms-bitfields -O3 -fno-strict-aliasing -fno-ident   -IC:\Users\EyeCon\.choosenim\toolchains\nim-#devel\lib -Id:\prog\Nim\pe -o C:\Users\EyeCon\nimcache\pe0103_d\@mpe0103.nim.c.o C:\Users\EyeCon\nimcache\pe0103_d\@mpe0103.nim.c' failed with exit code: 1

Expected Output

That the file is compiled without problems

Possible Solution

From the discussion in Discord:

Solitude: i can change inner iterator to seq and it will still break
Solitude: but pairwise needs to be openArray to not work

Additional Information

>nim -v
Nim Compiler Version 1.7.1 [Windows: amd64]
Compiled at 2022-03-27
Copyright (c) 2006-2022 by Andreas Rumpf

active boot switches: -d:release

# make sure to include the git hash if not using a tagged release

For easier testing, here a slightly different version with templates:

import std / [algorithm, sequtils, sugar]

template bar(): untyped =
    toSeq(toSeq(s.combinations(2)).map(a => a).pairwise()).any(a => true) # Exhibits the bug
    # toSeq(toSeq(s.combinations(2)).map(proc(p: auto): auto = p).pairwise()).any(a => true) # Exhibits the bug
    # toSeq(toSeq(s.combinations(2)).pairwise()).any(a => true) # Everything OK

iterator combinations*[T](s: openArray[T], r: Positive): seq[T] =
  yield @[s[0], s[1]]
  yield @[s[0], s[1]]

iterator pairwise*[T](s: openArray[T]): seq[T] =
  yield @[s[0], s[1]]
  yield @[s[0], s[1]]

proc checkSpecialSubset5(s: seq[int]): bool = bar()

when isMainModule:
    let s = @[3,4,5,6]
    echo bar() # Works
    if bar(): # Works
        echo "Foo"
    echo checkSpecialSubset5(s) # With the bug: doesn't work

[metagn]

The transformed AST is:

proc checkSpecialSubset5(s: seq[int]): bool =
  result = any(
    var result`gensym0: seq[typeof(map(
      var result`gensym4: seq[typeof(s.combinations(2))] = @[]
      for x`gensym4 in combinations(s, 2):
        add(result`gensym4, x`gensym4)
      result`gensym4, proc (a: auto): int = result = a[0]).pairwise())] = []
    block :tmp:
      var x`gensym0
      x`gensym0 = @[map(
        var result`gensym5: seq[typeof(s.combinations(2))] = []
        block :tmp:
          var x`gensym5
          x`gensym5 = @[s[0], s[1]]
          add(result`gensym5, x`gensym5)
        result`gensym5, (proc (a: auto): int = result = a[0], nil))[0], map(
        var result`gensym5: seq[typeof(s.combinations(2))] = []
        block :tmp:
          var x`gensym5
          x`gensym5 = @[s[0], s[1]]
          add(result`gensym5, x`gensym5)
        result`gensym5,
          (proc (a: auto): int = result = a[0], nil))[1]]
      add(result`gensym0, x`gensym0)
    result`gensym0,
               (proc (a: auto): bool = result = true, nil))

So a case of #24023 triggered by iterators duplicating the expression of openarray values (#13417).