Re: [PATCH] c++: fix truncated diagnostic in C++23 [PR111272]

[Jason Merrill <jason@redhat.com>]

On 10/13/23 18:15, Marek Polacek wrote:
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK.

> -- >8 --
> In C++23, since P2448, a constexpr function F that calls a non-constexpr
> function N is OK as long as we don't actually call F in a constexpr
> context.  So instead of giving an error in maybe_save_constexpr_fundef,
> we only give an error when evaluating the call.  Unfortunately, as shown
> in this PR, the diagnostic can be truncated:
> 
> z.C:10:13: note: 'constexpr Jam::Jam()' is not usable as a 'constexpr' function because:
>     10 |   constexpr Jam() { ft(); }
>        |             ^~~
> 
> ...because what?  With this patch, we say:
> 
> z.C:10:13: note: 'constexpr Jam::Jam()' is not usable as a 'constexpr' function because:
>     10 |   constexpr Jam() { ft(); }
>        |             ^~~
> z.C:10:23: error: call to non-'constexpr' function 'int Jam::ft()'
>     10 |   constexpr Jam() { ft(); }
>        |                     ~~^~
> z.C:8:7: note: 'int Jam::ft()' declared here
>      8 |   int ft() { return 42; }
>        |       ^~
> 
> Like maybe_save_constexpr_fundef, explain_invalid_constexpr_fn should
> also check the body of a constructor, not just the mem-initializer.
> 
> 	PR c++/111272
> 
> gcc/cp/ChangeLog:
> 
> 	* constexpr.cc (explain_invalid_constexpr_fn): Also check the body of
> 	a constructor in C++14 and up.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/constexpr-diag1.C: New test.
> ---
>   gcc/cp/constexpr.cc                          | 10 +++++++++-
>   gcc/testsuite/g++.dg/cpp1y/constexpr-diag1.C | 21 ++++++++++++++++++++
>   2 files changed, 30 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-diag1.C
> 
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index 0f948db7c2d..dde4fec4a44 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -1098,7 +1098,15 @@ explain_invalid_constexpr_fn (tree fun)
>   	  body = massage_constexpr_body (fun, body);
>   	  require_potential_rvalue_constant_expression (body);
>   	  if (DECL_CONSTRUCTOR_P (fun))
> -	    cx_check_missing_mem_inits (DECL_CONTEXT (fun), body, true);
> +	    {
> +	      cx_check_missing_mem_inits (DECL_CONTEXT (fun), body, true);
> +	      if (cxx_dialect > cxx11)
> +		{
> +		  /* Also check the body, not just the ctor-initializer.  */
> +		  body = DECL_SAVED_TREE (fun);
> +		  require_potential_rvalue_constant_expression (body);
> +		}
> +	    }
>   	}
>       }
>   }
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-diag1.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-diag1.C
> new file mode 100644
> index 00000000000..0e2909e83ef
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-diag1.C
> @@ -0,0 +1,21 @@
> +// PR c++/111272
> +// { dg-do compile { target c++14 } }
> +// { dg-options "-Werror=invalid-constexpr" }
> +// { dg-prune-output "some warnings being treated as errors" }
> +
> +struct Jam
> +{
> +  // constexpr  // n.b.
> +  int ft() { return 42; } // { dg-message "declared here" }
> +
> +  constexpr Jam() { ft(); } // { dg-error "call to non-.constexpr. function" }
> +// { dg-message "declared here" "" { target c++20_down } .-1 }
> +};
> +
> +constexpr bool test()
> +{
> +  Jam j; // { dg-error "called in a constant expression" }
> +  return true;
> +}
> +
> +static_assert(test(), ""); // { dg-error "non-constant condition" }
> 
> base-commit: d78fef5371759849944966dec65d9e987efba509