pass varargs[seq[T]] to iterator give empty seq

[slangmgh]

When pass varargs[seq[T]] to iterator, only the first arg is valid, the following is always empty.

Example

iterator ff(sq: varargs[seq[int]]): int =
   for x in sq:
      echo x

for x in ff(@[1,2], @[1,2,3]):
   echo x

Current Output

@[1,2]
@[]

Expected Output

@[1,2]
@[1,2,3]

$ nim -v
Nim Compiler Version 1.1.0 [Windows: i386]
Compiled at 2019-11-02
Copyright (c) 2006-2019 by Andreas Rumpf

active boot switches: -d:release

[slangmgh]

The program run fine with v0.20.0 and breaks with v0.20.2.

[slangmgh]

The following code in transf.nim casue the problem, maybe paFastAsgnTakeTypeFromArg type doesn't be handled correctly in proc transformFor:

proc putArgInto(arg: PNode, formal: PType): TPutArgInto =
  # This analyses how to treat the mapping "formal <-> arg" in an
  # inline context.
  if formal.kind == tyTypeDesc: return paDirectMapping
  if skipTypes(formal, abstractInst).kind in {tyOpenArray, tyVarargs}:
    case arg.kind
    of nkStmtListExpr:
      return paComplexOpenarray
    of nkBracket:
      return paFastAsgnTakeTypeFromArg    # oops
    else:
      return paDirectMapping    # XXX really correct?
                                # what if ``arg`` has side-effects?

[slangmgh]

It is caused by fixing issue #8316.