[PATCH] D70524: Support DebugInfo generation for auto return type for C++ functions.

[David Blaikie via Phabricator via cfe-commits]

dblaikie added a comment.

In D70524#1787566 <https://reviews.llvm.org/D70524#1787566>, @SouraVX wrote:

> > It looks like this implementation is a bit buggy in one way and incomplete in another:
> > 
> > 1. even if the auto-returning function is defined, that function definition doesn't describe the concrete return type. Compare GCC and Clang's output for your example and note that... oh.
>
> I think that's correct behavior, consider this for a moment --
>
>   struct foo {
>   auto foo_func();
>   };
>   int foo::foo_func(){return 0;}
>   clang error: ->
>   error: return type of out-of-line definition of 'foo::foo_func' differs from that in the declaration
>
>
> So this seems fair to me, regardless of the concrete return type{assuming this is what this patch is doing}. We should be emitting `auto` in declaration. AKA in unspecified_type. GCC(trunk) also seems fair here.


Sorry, the example I had in mind was this:

  struct type {
    auto func();
  };
  auto type::func() {
    return 3;
  }

Which GCC produces:

  DW_TAG_structure_type
    DW_AT_name ("type")
    DW_TAG_subprogram
      DW_AT_name ("func")
      DW_AT_type (DW_TAG_unspecified_type "auto")
      ...
  DW_TAG_subprogram
    DW_AT_specification (... ^)
    DW_AT_type (DW_TAG_base_type "int")
    ...

(this should be the same debug info even if the function is defined inline in the class, rather than defined out of line (assuming the function's called - which produces a definition))

>> Hmm, maybe this feature/suggestion is broken or at least not exactly awesome when it comes to auto-returning functions that are eventually void-returning functions? Now the function definition has no DW_AT_type to override the unspecified_type in the declaration... :/ that's unfortunate (@probinson - thoughts?)
> 
> I'm a bit confused here, regardless of the concrete return type{void/int/float} GCC(trunk) is emitting 
>  `DW_TAG_unspecified_type`
>  `DW_AT_name "auto"` 
>  Are you trying to say we should be emitting `DW_AT_type void/int/float` ?? That's what functionality of clang is right now, and if we entertain that, then their is no point of emitting `DW_TAG_unspecifed_type auto` at first place.

As @probinson said - for the definition DIE/DISubprogram the type should be the concrete, deduced type. For the declaration DIE/DISubprogram, the return type should be "auto".


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https://reviews.llvm.org/D70524