Symbols in when false section affect lookup

[arnetheduck]

Description

A follow-up on #23091 - real code is here: arnetheduck/nim-results#35 (error happens with resultsGenericBindingWorkaround=false).

The error symbol inside when false should obviously have no effect on compiling the sample.

testit.nim:

type Result*[E] = object
  e*: E

proc error*[E](v: Result[E]): E = discard

template valueOr*[E](self: Result[E], def: untyped): int =
  when E isnot void:
    when false:
      # Comment line below to make it work
      template error(): E {.used, gensym.} = s.e
      discard
    else:
      template error(): E {.used, inject.} =
        self.e

      def
  else:
    def


block:
  let rErr = Result[string](e: "a")
  let rErrV = rErr.valueOr:
    ord(error[0])

Nim Version

nim -v
Nim Compiler Version 2.1.1 [Linux: amd64]
Compiled at 2024-02-20
Copyright (c) 2006-2024 by Andreas Rumpf

git hash: 773c066
active boot switches: -d:release

(same error on 1.6)

Current Output

$ nim c /tmp/testit
.....................................................................
/tmp/testit.nim(23, 19) template/generic instantiation of `valueOr` from here
/tmp/testit.nim(24, 8) Error: type mismatch
Expression: ord(error)
  [1] error: proc (v: Result[system.int]): int{.noSideEffect, gcsafe.}

Expected one of (first mismatch at [position]):
[1] func ord[T: Ordinal | enum](x: T): int

Expected Output

No response

Possible Solution

No response

Additional Information

No response

[litlighilit]

A clearer demo:

template valueOr*(def: untyped): int =
  when false:
    # Comment line below to make it work
    template error(): int {.gensym.} = discard
    discard
  else:
    template error(): int {.inject.} = 1
  def

let rErrV = valueOr error

It seems the symbol capture doesn't cover edge cases, when it comes to gensym

[litlighilit]

In addition,

maybe there is another factor:

template valueOr*(def: untyped): int =
  template error(): int {.gensym.} = 1
  def

let rErrV = valueOr error

gensym is not considered when lookup

[metagn]

Culprit is maybe this?

Nim/compiler/semtempl.nim

Line 264 in 94c5996

if s.owner == c.owner and (s.kind == skParam or sfGenSym in s.flags):

The gensym template is added to scope here and the injected template declaration finds it and reuses the symbol? Presumably to allow overloading the gensym'd symbol since adding gensym again defines a different symbol, even though omitting gensym implies inject for routines. Then again why does gensym again define a different symbol when we only get to use one at a time in the template?

We could move the gensym reuse logic inside the logic for when gensym is given, if this breaks code we could allow opting out of the symbol reuse on an explicit inject.

This is all letting alone the logic of actually using error inside the template after the when block, also I'm speculating about all this from my phone so I might be wrong.