Re: [PATCH] c++/modules: deduced return type merging [PR114795]

[Jason Merrill <jason@redhat.com>]

On 4/23/24 09:41, Patrick Palka wrote:
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
> 
> -- >8 --
> 
> When merging an imported function template specialization with an
> existing one, if the existing one has an undeduced return type and the
> imported one's is already deduced, we need to propagate the deduced type
> since once we install the imported definition we won't get a chance to
> deduce it by normal means.
> 
> This patch makes is_matching_decl propagate the deduced return type
> alongside the existing propagate of the existing specification.  I
> suppose could instead propagate it later when installing the imported
> definition from read_definition, but it seems best to propagate it
> sooner rather than later.
> 
> 	PR c++/114795
> 
> gcc/cp/ChangeLog:
> 
> 	* module.cc (trees_in::is_matching_decl): Propagate deduced
> 	function return type.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/modules/auto-4_a.H: New test.
> 	* g++.dg/modules/auto-4_b.C: New test.
> ---
>   gcc/cp/module.cc                        |  5 +++++
>   gcc/testsuite/g++.dg/modules/auto-4_a.H | 14 ++++++++++++++
>   gcc/testsuite/g++.dg/modules/auto-4_b.C | 15 +++++++++++++++
>   3 files changed, 34 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/modules/auto-4_a.H
>   create mode 100644 gcc/testsuite/g++.dg/modules/auto-4_b.C
> 
> diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
> index d94d8ff4df9..e10e19ac9f7 100644
> --- a/gcc/cp/module.cc
> +++ b/gcc/cp/module.cc
> @@ -11537,6 +11537,11 @@ trees_in::is_matching_decl (tree existing, tree decl, bool is_typedef)
>         else if (!DEFERRED_NOEXCEPT_SPEC_P (d_spec)
>   	       && !comp_except_specs (d_spec, e_spec, ce_type))
>   	goto mismatch;
> +
> +      /* Similarly if EXISTING has an undeduced return type, but DECL's
> +	 is already deduced.  */
> +      if (undeduced_auto_decl (existing) && !undeduced_auto_decl (decl))
> +	TREE_TYPE (existing) = change_return_type (TREE_TYPE (d_type), e_type);

Perhaps this should dump a note like the noexcept merge does?  OK either 
way.

Jason