Re: [PATCH] c++: excessive satisfaction in check_methods [PR108579]

[Jason Merrill <jason@redhat.com>]

On 1/30/23 14:10, Patrick Palka wrote:
> In check_methods we're unnecessarily checking satisfaction for all
> constructors and assignment operators, even those that don't look like
> copy/move special members.  In the testcase below this manifests as an
> unstable satisfaction error because the satisfaction result is first
> determined to be false during check_methods (since A<int> is incomplete
> at this point) and later true after completion of A<int>.
> 
> This patch fixes this simply by swapping the order of the
> constraint_satisfied_p and copy_fn_p / move_fn_p tests.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
> OK for trunk?  This doesn't fix the regression completely, since
> we get a similar unstable satisfaction error if one of the constrained
> members is actually a copy/move special member.  I suppose we need to
> rearrange things in finish_struct_1 so that check_methods gets called in
> a complete class context?

I think the way to make that work, if indeed that's desirable, would be 
to determine those properties lazily instead of at finish_struct time.

The patch is OK.

> 	PR c++/108579
> 
> gcc/cp/ChangeLog:
> 
> 	* class.cc (check_methods): Test constraints_satisfied_p after
> 	testing copy_fn_p / move_fn_p instead of beforehand.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp2a/concepts-pr108579.C: New test.
> ---
>   gcc/cp/class.cc                                | 16 ++++++++--------
>   gcc/testsuite/g++.dg/cpp2a/concepts-pr108579.C | 14 ++++++++++++++
>   2 files changed, 22 insertions(+), 8 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-pr108579.C
> 
> diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
> index 351de6c5419..d3ce8532d56 100644
> --- a/gcc/cp/class.cc
> +++ b/gcc/cp/class.cc
> @@ -4822,11 +4822,11 @@ check_methods (tree t)
>   	/* Might be trivial.  */;
>         else if (TREE_CODE (fn) == TEMPLATE_DECL)
>   	/* Templates are never special members.  */;
> -      else if (!constraints_satisfied_p (fn))
> -	/* Not eligible.  */;
> -      else if (copy_fn_p (fn))
> +      else if (copy_fn_p (fn)
> +	       && constraints_satisfied_p (fn))
>   	TYPE_HAS_COMPLEX_COPY_CTOR (t) = true;
> -      else if (move_fn_p (fn))
> +      else if (move_fn_p (fn)
> +	       && constraints_satisfied_p (fn))
>   	TYPE_HAS_COMPLEX_MOVE_CTOR (t) = true;
>       }
>   
> @@ -4836,11 +4836,11 @@ check_methods (tree t)
>   	/* Might be trivial.  */;
>         else if (TREE_CODE (fn) == TEMPLATE_DECL)
>   	/* Templates are never special members.  */;
> -      else if (!constraints_satisfied_p (fn))
> -	/* Not eligible.  */;
> -      else if (copy_fn_p (fn))
> +      else if (copy_fn_p (fn)
> +	       && constraints_satisfied_p (fn))
>   	TYPE_HAS_COMPLEX_COPY_ASSIGN (t) = true;
> -      else if (move_fn_p (fn))
> +      else if (move_fn_p (fn)
> +	       && constraints_satisfied_p (fn))
>   	TYPE_HAS_COMPLEX_MOVE_ASSIGN (t) = true;
>       }
>   }
> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-pr108579.C b/gcc/testsuite/g++.dg/cpp2a/concepts-pr108579.C
> new file mode 100644
> index 00000000000..bc7d709f889
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-pr108579.C
> @@ -0,0 +1,14 @@
> +// PR c++/108579
> +// { dg-do compile { target c++20 } }
> +
> +template<class T>
> +struct A {
> +  A(double, char);
> +  A(int) requires requires { A(0.0, 'c'); };
> +  A& operator=(int) requires requires { A(1.0, 'd'); };
> +};
> +
> +int main() {
> +  A<int> x(3);
> +  x = 5;
> +}