Cannot reuse typed argument which has lambda with auto return type

[metagn]

If a macro takes in a typed argument with a lambda in it, then uses that typed argument as a result; if it has to be checked again (in this case, because the return type is auto); it will give an "illformed AST" error. The error is most likely because the name of the lambda node is set to Sym ":anonymous".

Replace auto with void, int in the example, it will work. Whether or not there are parameters does not change the result, neither does where you use the lambda.

This breaks expandMacros for macros like capture.

Example

import macros

expandMacros:
  (proc: auto = discard)()
  # or discard (proc: auto = 1)()
  # or let _ = proc: auto = discard
  # as long as the return type is auto

Current Output

proc (): auto = discard ()
/usercode/in.nim(3, 1) template/generic instantiation of `expandMacros` from here
/usercode/in.nim(4, 4) Error: illformed AST: proc (): auto = discard

Expected Output

proc (): auto = discard ()

(this render is incorrect, but that's unrelated)

More basic example:

macro foo(x: typed): untyped =
  result = x

foo:
  (proc: auto = discard)()

This fixes it:

import macros

macro foo(x: typed): untyped =
  result = copy x # first copy
  result[0] = newEmptyNode() # then delete name

foo:
  (proc: auto = discard)()

$ nim -v
Nim Compiler Version 1.5.1 [Windows: amd64]
Compiled at 2021-02-09
Copyright (c) 2006-2021 by Andreas Rumpf

git hash: 4576cf20af3df84a077427faf69d6bfb47821897
active boot switches:

[saem]

I have a partial fix for this in this branch (https://github.com/nim-lang/Nim/compare/devel...saem:saem-look-for-formal-param-sem-issues?expand=1), but getting a code gen issue because something else isn't being handled.

[saem]

I believe this and #16906 are effectively the same issue.

[metagn]

Closing as duplicate or subproblem of #16906