[pushed] c++: implicit operator== with previous decl [PR94583]

[Marek Polacek]

On Sat, Apr 25, 2020 at 12:17:18AM -0400, Jason Merrill via Gcc-patches wrote:
> P2085 clarified that a defaulted comparison operator must be the first
> declaration of the function.  Rejecting that avoids the ICE trying to
> compare the noexcept-specifications.
> 
> Tested x86_64-pc-linux-gnu, applying to trunk.
> 
> gcc/cp/ChangeLog
> 2020-04-24  Jason Merrill  <jason@redhat.com>
> 
> 	PR c++/94583
> 	* decl.c (redeclaration_error_message): Reject defaulted comparison
> 	operator that has been previously declared.
> ---
>  gcc/cp/decl.c                                 |  8 ++++++++
>  gcc/testsuite/g++.dg/cpp2a/spaceship-synth6.C | 11 +++++++++++
>  2 files changed, 19 insertions(+)
>  create mode 100644 gcc/testsuite/g++.dg/cpp2a/spaceship-synth6.C
> 
> diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
> index c8c2f080763..31b5884ca3a 100644
> --- a/gcc/cp/decl.c
> +++ b/gcc/cp/decl.c
> @@ -2972,6 +2972,14 @@ redeclaration_error_message (tree newdecl, tree olddecl)
>  	    }
>  	}
>  
> +      /* [class.compare.default]: A definition of a comparison operator as
> +	 defaulted that appears in a class shall be the first declaration of
> +	 that function.  */
> +      special_function_kind sfk = special_function_p (olddecl);
> +      if (sfk == sfk_comparison && DECL_DEFAULTED_FN (newdecl))
> +	return G_("comparison operator %q+D defaulted after "
> +		  "its first declaration");
> +

This one still ICEs:

namespace std { struct strong_ordering { }; }

struct Q {
  friend std::strong_ordering operator<=>(const Q&, const Q&) = default;
};
bool operator==(const Q&, const Q&) noexcept;

Either we need to reject redeclaring the implicit operator==, or teach
get_defaulted_eh_spec how to handle its DEFERRED_NOEXCEPT, before
build_comparison_op/~comp_info resolves it, because otherwise we couldn't
check that the noexcept-specifications match.  Thoughts?

Marek