Re: [PATCH] c++: constexpr error with defaulted virtual dtor [PR93413]

[Jason Merrill <jason@redhat.com>]

On 11/3/22 08:33, Patrick Palka wrote:
> We're rejecting the below testcase with
> 
>    error: 'virtual constexpr Base::~Base()' used before its definition
>    error: 'virtual constexpr Derived::~Derived()' used before its definition
> 
> due to an early exit added to mark_used by r181272 to defer synthesis of
> virtual destructors until EOF where we can set their linkage.  This makes
> them effectively unusable for constexpr evaluation since that needs to
> be done immediately and can't also be deferred.
> 
> Fortunately, this early exit seems to no longer be necessary ever since
> r208030 enabled us to tentatively set linkage of all defaulted virtual
> destructors, including templated ones.  So this patch just gets rid of
> this early exit.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?

OK.

> 	PR c++/93413
> 
> gcc/cp/ChangeLog:
> 
> 	* decl2.cc (mark_used): Don't defer synthesis of virtual
> 	functions.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp2a/constexpr-virtual21.C: New test.
> ---
>   gcc/cp/decl2.cc                                  |  8 --------
>   gcc/testsuite/g++.dg/cpp2a/constexpr-virtual21.C | 10 ++++++++++
>   2 files changed, 10 insertions(+), 8 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-virtual21.C
> 
> diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
> index e6779268ad4..eeb59eae64f 100644
> --- a/gcc/cp/decl2.cc
> +++ b/gcc/cp/decl2.cc
> @@ -5788,14 +5788,6 @@ mark_used (tree decl, tsubst_flags_t complain /* = tf_warning_or_error */)
>         && !DECL_DEFAULTED_OUTSIDE_CLASS_P (decl)
>         && ! DECL_INITIAL (decl))
>       {
> -      /* Defer virtual destructors so that thunks get the right
> -	 linkage.  */
> -      if (DECL_VIRTUAL_P (decl) && !at_eof)
> -	{
> -	  note_vague_linkage_fn (decl);
> -	  return true;
> -	}
> -
>         /* Remember the current location for a function we will end up
>   	 synthesizing.  Then we can inform the user where it was
>   	 required in the case of error.  */
> diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual21.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual21.C
> new file mode 100644
> index 00000000000..8b70c5f538b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual21.C
> @@ -0,0 +1,10 @@
> +// PR c++/93413
> +// { dg-do compile { target c++20 } }
> +
> +struct Base {
> +  virtual ~Base() = default;
> +};
> +constexpr Base b;
> +
> +struct Derived : Base { };
> +constexpr Derived d;