Re: [PATCH] c++: constraint matching, TEMPLATE_ID_EXPR, current inst

[Jason Merrill <jason@redhat.com>]

On 9/15/22 11:58, Patrick Palka wrote:
> Here we're crashing during constraint matching for the instantiated
> hidden friends due to two issues with dependent substitution into a
> TEMPLATE_ID_EXPR naming a template from the current instantiation
> (as performed from maybe_substitute_reqs_for for C<3> with T=T):
> 
>    * tsubst_copy substitutes into such a TEMPLATE_DECL by looking it
>      up from the substituted class scope.  But for this to not fail when
>      the args are dependent, we need to pass entering_scope=true for the
>      class scope substitution so that we obtain the primary template type
>      A<T> (which has TYPE_BINFO) instead of the implicit instantiation
>      A<T> (which doesn't).
>    * lookup_and_finish_template_variable shouldn't instantiate a
>      TEMPLATE_ID_EXPR that names a TEMPLATE_DECL which has more than
>      one level of (unsubstituted) parameters (such as A<T>::C).
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.cc (lookup_and_finish_template_variable): Don't
> 	instantiate if the template's scope is dependent.
> 	(tsubst_copy) <case TEMPLATE_DECL>: Pass entering_scope=true
> 	when substituting the class scope.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp2a/concepts-friend10.C: New test.
> ---
>   gcc/cp/pt.cc                                  | 14 +++++++------
>   .../g++.dg/cpp2a/concepts-friend10.C          | 21 +++++++++++++++++++
>   2 files changed, 29 insertions(+), 6 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-friend10.C
> 
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index db4e808adec..bfcbe0b8670 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -10475,14 +10475,15 @@ tree
>   lookup_and_finish_template_variable (tree templ, tree targs,
>   				     tsubst_flags_t complain)
>   {
> -  templ = lookup_template_variable (templ, targs);
> -  if (!any_dependent_template_arguments_p (targs))
> +  tree var = lookup_template_variable (templ, targs);
> +  if (TMPL_PARMS_DEPTH (DECL_TEMPLATE_PARMS (templ)) == 1
> +      && !any_dependent_template_arguments_p (targs))

I notice that finish_id_expression_1 uses the equivalent of 
type_dependent_expression_p (var).  Does that work here?

>       {
> -      templ = finish_template_variable (templ, complain);
> -      mark_used (templ);
> +      var = finish_template_variable (var, complain);
> +      mark_used (var);
>       }
>   
> -  return convert_from_reference (templ);
> +  return convert_from_reference (var);
>   }
>   
>   /* If the set of template parameters PARMS contains a template parameter
> @@ -17282,7 +17283,8 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
>   	     TEMPLATE_DECL with `D<T>' as its DECL_CONTEXT.  Now we
>   	     have to substitute this with one having context `D<int>'.  */
>   
> -	  tree context = tsubst (DECL_CONTEXT (t), args, complain, in_decl);
> +	  tree context = tsubst_aggr_type (DECL_CONTEXT (t), args, complain,
> +					   in_decl, /*entering_scope=*/true);
>   	  return lookup_field (context, DECL_NAME(t), 0, false);
>   	}
>         else

This hunk is OK.

> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-friend10.C b/gcc/testsuite/g++.dg/cpp2a/concepts-friend10.C
> new file mode 100644
> index 00000000000..4b21a379f59
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-friend10.C
> @@ -0,0 +1,21 @@
> +// { dg-do compile { target c++20 } }
> +// Verify we don't crash during constraint matching containing
> +// a TEMPLATE_ID_EXPR referring to a template from the current
> +// instantiation.
> +
> +template<class T>
> +struct A {
> +  template<int N> static constexpr bool C = sizeof(T) > N;
> +  friend constexpr void f(A) requires C<3> { }
> +  friend constexpr void f(A) requires C<3> || true { }
> +};
> +
> +template<class T>
> +struct A<T*> {
> +  template<int N> static constexpr bool C = sizeof(T) > N;
> +  friend constexpr void g(A) requires C<3> { }
> +  friend constexpr void g(A) requires C<3> || true { }
> +};
> +
> +template struct A<int>;
> +template struct A<int*>;