Re: [PATCH] c++: diagnostic ICE b/c of empty TPARMS_PRIMARY_TEMPLATE [PR109655]

[Jason Merrill <jason@redhat.com>]

On 6/9/23 11:19, Patrick Palka wrote:
> When defining a previously declared class template, we neglect to set
> TPARMS_PRIMARY_TEMPLATE for the in-scope template parameters, which the
> class members go on to inherit, and so the members' DECL_TEMPLATE_PARMS
> will have empty TPARMS_PRIMARY_TEMPLATE at those levels as well.  This
> causes us to crash when diagnosing a constraint mismatch for an
> out-of-line declaration of a member of a constrained class template.
> 
> This patch fixes this by walking the context to get at the corresponding
> primary template instead.  I spent a while trying to get us to set
> TPARMS_PRIMARY_TEMPLATE for templated class definitions that are
> redeclarations, but it proved to be hairy in particular for partial
> specializations and nested templates.

Maybe set it a bit earlier in push_template_decl, if you're going to 
figure out here what it should be anyway?

> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk/13?
> 
> 	PR c++/109655
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.cc (push_template_decl): Handle TPARMS_PRIMARY_TEMPLATE
> 	being empty when diagnosing a constraint mismatch for an
> 	enclosing template scope.  Don't bother checking constraints
> 	if DECL_PARMS and SCOPE_PARMS are the same.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp2a/concepts-class6.C: New test.
> 	* g++.dg/cpp2a/concepts-class6a.C: New test.
> ---
>   gcc/cp/pt.cc                                  | 19 +++++++--
>   gcc/testsuite/g++.dg/cpp2a/concepts-class6.C  | 30 ++++++++++++++
>   gcc/testsuite/g++.dg/cpp2a/concepts-class6a.C | 40 +++++++++++++++++++
>   3 files changed, 86 insertions(+), 3 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-class6.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-class6a.C
> 
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 17bf4d24151..f913b248345 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -6155,12 +6155,25 @@ push_template_decl (tree decl, bool is_friend)
>   	      decl_parms = TREE_CHAIN (decl_parms);
>   	      scope_parms = TREE_CHAIN (scope_parms);
>   	    }
> -	  while (decl_parms)
> +	  while (decl_parms && decl_parms != scope_parms)
>   	    {
>   	      if (!template_requirements_equivalent_p (decl_parms, scope_parms))
>   		{
> -		  error ("redeclaration of %qD with different constraints",
> -			 TPARMS_PRIMARY_TEMPLATE (TREE_VALUE (decl_parms)));
> +		  tree td = TPARMS_PRIMARY_TEMPLATE (TREE_VALUE (decl_parms));
> +		  if (!td)
> +		    {
> +		      /* FIXME: TPARMS_PRIMARY_TEMPLATE doesn't always get
> +			 set for enclosing template scopes.  Work around
> +			 this by walking the context to obtain the relevant
> +			 (primary) template whose constraints we mismatch.  */
> +		      int level = TMPL_PARMS_DEPTH (decl_parms);
> +		      td = TYPE_TI_TEMPLATE (ctx);
> +		      while (!PRIMARY_TEMPLATE_P (td)
> +			     || (TMPL_PARMS_DEPTH (DECL_TEMPLATE_PARMS (td))
> +				 != level))
> +			td = TYPE_TI_TEMPLATE (DECL_CONTEXT (td));
> +		    }
> +		  error ("redeclaration of %qD with different constraints", td);
>   		  break;
>   		}
>   	      decl_parms = TREE_CHAIN (decl_parms);
> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-class6.C b/gcc/testsuite/g++.dg/cpp2a/concepts-class6.C
> new file mode 100644
> index 00000000000..dcef6a2c9d4
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-class6.C
> @@ -0,0 +1,30 @@
> +// PR c++/109655
> +// { dg-do compile { target c++20 } }
> +
> +class C {
> +  template<class>
> +  requires true
> +  friend class D;
> +
> +  template<class>
> +  requires true
> +  class E;
> +};
> +
> +template<class>
> +requires true
> +class D {
> +  void f();
> +};
> +
> +template<class T>
> +void D<T>::f() { } // { dg-error "class D' with different constraints" }
> +
> +template<class>
> +requires true
> +class C::E {
> +  void f();
> +};
> +
> +template<class T>
> +void C::E<T>::f() { } // { dg-error "class C::E' with different constraints" }
> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-class6a.C b/gcc/testsuite/g++.dg/cpp2a/concepts-class6a.C
> new file mode 100644
> index 00000000000..751d13cdf6c
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-class6a.C
> @@ -0,0 +1,40 @@
> +// PR c++/109655
> +// { dg-do compile { target c++20 } }
> +
> +template<class T>
> +requires true
> +class C {
> +  class D;
> +
> +  template<class>
> +  requires (!!true)
> +  class E;
> +};
> +
> +template<class T>
> +requires true
> +class C<T>::D {
> +  void f();
> +};
> +
> +template<class T>  // missing "requires true"
> +void C<T>::D::f() { } // { dg-error "class C' with different constraints" }
> +
> +template<class T>
> +requires true
> +template<class U>
> +requires (!!true)
> +class C<T>::E {
> +  void f();
> +  void g();
> +};
> +
> +template<class T>
> +requires true
> +template<class U>
> +void C<T>::E<U>::f() { } // { dg-error "class C<T>::E' with different constraints" }
> +
> +template<class T>
> +template<class U>
> +requires (!!true)
> +void C<T>::E<U>::g() { } // { dg-error "class C' with different constraints" }