Re: [PATCH 2/2] c++: redeclaring member of constrained class template [PR96830]

[Jason Merrill <jason@redhat.com>]

On 3/14/23 12:41, Patrick Palka wrote:
> When redeclaring a member of a constrained class template, we need to
> repeat the class's constraints, but it turns out we don't verify
> anywhere that the repeated constraints match the class's constraints.
> A safe place to do so seems to be in push_template_decl, nearby a
> similar consistency check for the template parameter list length of a
> redeclaration.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk/12?

Both patches are OK for trunk, but not 12; we tend not to backport fixes 
for accepts-invalid bugs, especially if they aren't regressions.  We 
don't want to add new diagnostics to an active release branch.

> 	PR c++/96830
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.cc (push_inline_template_parms_recursive): Set
> 	TEMPLATE_PARMS_CONSTRAINTS.
> 	(push_template_decl): For a redeclaration, verify constraints
> 	for each enclosing template scope match those of the original
> 	template declaratation.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp2a/concepts-class5.C: New test.
> 	* g++.dg/cpp2a/concepts-class5a.C: New test.
> ---
>   gcc/cp/pt.cc                                  | 26 +++++++++++++
>   gcc/testsuite/g++.dg/cpp2a/concepts-class5.C  | 37 ++++++++++++++++++
>   gcc/testsuite/g++.dg/cpp2a/concepts-class5a.C | 38 +++++++++++++++++++
>   3 files changed, 101 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-class5.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-class5a.C
> 
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 053cc52b285..370a4f70f08 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -471,6 +471,8 @@ push_inline_template_parms_recursive (tree parmlist, int levels)
>     current_template_parms
>       = tree_cons (size_int (current_template_depth + 1),
>   		 parms, current_template_parms);
> +  TEMPLATE_PARMS_CONSTRAINTS (current_template_parms)
> +    = TEMPLATE_PARMS_CONSTRAINTS (parmlist);
>     TEMPLATE_PARMS_FOR_INLINE (current_template_parms) = 1;
>   
>     begin_scope (TREE_VEC_LENGTH (parms) ? sk_template_parms : sk_template_spec,
> @@ -6134,6 +6136,30 @@ push_template_decl (tree decl, bool is_friend)
>   	  DECL_INTERFACE_KNOWN (decl) = 1;
>   	  return error_mark_node;
>   	}
> +
> +      /* Check that the constraints for each enclosing template scope are
> +	 consistent with the original template declaration.  */
> +      if (flag_concepts)
> +	{
> +	  tree decl_parms = DECL_TEMPLATE_PARMS (tmpl);
> +	  tree scope_parms = current_template_parms;
> +	  if (PRIMARY_TEMPLATE_P (tmpl))
> +	    {
> +	      decl_parms = TREE_CHAIN (decl_parms);
> +	      scope_parms = TREE_CHAIN (scope_parms);
> +	    }
> +	  while (decl_parms)
> +	    {
> +	      if (!template_requirements_equivalent_p (decl_parms, scope_parms))
> +		{
> +		  error ("redeclaration of %qD with different constraints",
> +			 TPARMS_PRIMARY_TEMPLATE (TREE_VALUE (decl_parms)));
> +		  break;
> +		}
> +	      decl_parms = TREE_CHAIN (decl_parms);
> +	      scope_parms = TREE_CHAIN (scope_parms);
> +	    }
> +	}
>       }
>   
>     gcc_checking_assert (!tmpl || DECL_TEMPLATE_RESULT (tmpl) == decl);
> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-class5.C b/gcc/testsuite/g++.dg/cpp2a/concepts-class5.C
> new file mode 100644
> index 00000000000..5d893d9f6a7
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-class5.C
> @@ -0,0 +1,37 @@
> +// PR c++/96830
> +// { dg-do compile { target c++20 } }
> +
> +template<class T> concept C = true;
> +
> +template<C T> requires true
> +struct A {
> +  void f();
> +  template<class U> void g();
> +
> +  struct B;
> +  template<class U> struct C;
> +
> +  static int D;
> +  template<class U> static int E;
> +};
> +
> +template<C T>
> +void A<T>::f() { }  // { dg-error "different constraints" }
> +
> +template<class T> requires true
> +template<class U>
> +void A<T>::g() { }  // { dg-error "different constraints" }
> +
> +template<C T> requires (!!true)
> +struct A<T>::B { }; // { dg-error "different constraints" }
> +
> +template<class T> requires true
> +template<class U>
> +struct A<T>::C { }; // { dg-error "different constraints" }
> +
> +template<C T>
> +int A<T>::D = 0;    // { dg-error "different constraints" }
> +
> +template<class T> requires true
> +template<class U>
> +int A<T>::E = 0;    // { dg-error "different constraints" }
> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-class5a.C b/gcc/testsuite/g++.dg/cpp2a/concepts-class5a.C
> new file mode 100644
> index 00000000000..a6a3684c258
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-class5a.C
> @@ -0,0 +1,38 @@
> +// PR c++/96830
> +// A valid version of concepts-class5.C.
> +// { dg-do compile { target c++20 } }
> +
> +template<class T> concept C = true;
> +
> +template<C T> requires true
> +struct A {
> +  void f();
> +  template<class U> void g();
> +
> +  struct B;
> +  template<class U> struct C;
> +
> +  static int D;
> +  template<class U> static int E;
> +};
> +
> +template<C T> requires true
> +void A<T>::f() { }
> +
> +template<C T> requires true
> +template<class U>
> +void A<T>::g() { }
> +
> +template<C T> requires true
> +struct A<T>::B { };
> +
> +template<C T> requires true
> +template<class U>
> +struct A<T>::C { };
> +
> +template<C T> requires true
> +int A<T>::D = 0;
> +
> +template<C T> requires true
> +template<class U>
> +int A<T>::E = 0;