[PATCH] c++: abbreviated function template friend matching [PR96106]

[Jason Merrill]

On 7/20/20 4:18 PM, Patrick Palka wrote:
> In the below testcase, duplicate_decls wasn't merging the tsubsted
> friend declaration for 'void add(auto)' with its definition, because
> reduce_template_parm_level (during tsubst_friend_function) lost the
> DECL_VIRTUAL_P flag on the invented 'auto' template parameter, which
> made template_heads_equivalent_p deem the two template heads as not
> equivalent in C++20 mode.
> 
> This patch makes reduce_template_parm_level carry over the
> DECL_VIRTUAL_P from the original TEMPLATE_PARM_DECL.
> 
> Passes 'make check-c++' and the cmcstl2 testsuite.  Does this look OK
> for trunk after a full bootstrap and regtest?

OK.

> gcc/cp/ChangeLog:
> 
> 	PR c++/96106
> 	* pt.c (reduce_template_parm_level): Carry over DECL_VIRTUAL_P
> 	from the original TEMPLATE_PARM_DECL.
> 
> gcc/testsuite/ChangeLog:
> 
> 	PR c++/96106
> 	* g++.dg/concepts/abbrev7.C: New test.
> ---
>   gcc/cp/pt.c                             |  1 +
>   gcc/testsuite/g++.dg/concepts/abbrev7.C | 14 ++++++++++++++
>   2 files changed, 15 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/concepts/abbrev7.C
> 
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index defc2a9abd8..3f7b89141b6 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -4453,6 +4453,7 @@ reduce_template_parm_level (tree index, tree type, int levels, tree args,
>   			      type);
>         TREE_CONSTANT (decl) = TREE_CONSTANT (orig_decl);
>         TREE_READONLY (decl) = TREE_READONLY (orig_decl);
> +      DECL_VIRTUAL_P (decl) = DECL_VIRTUAL_P (orig_decl);
>         DECL_ARTIFICIAL (decl) = 1;
>         SET_DECL_TEMPLATE_PARM_P (decl);
>   
> diff --git a/gcc/testsuite/g++.dg/concepts/abbrev7.C b/gcc/testsuite/g++.dg/concepts/abbrev7.C
> new file mode 100644
> index 00000000000..443c1b7871b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/concepts/abbrev7.C
> @@ -0,0 +1,14 @@
> +// PR c++/96106
> +// { dg-do compile { target concepts } }
> +
> +template<typename>
> +struct number {
> +  friend void add(auto);
> +};
> +
> +void add(auto) { }
> +
> +void foo() {
> +  number<int> n;
> +  add(n); // { dg-bogus "ambiguous" }
> +}
>