Re: [PATCH 1/2] c++: requires-exprs and partial constraint subst [PR112769]

[Jason Merrill <jason@redhat.com>]

On 2/2/24 14:41, Patrick Palka wrote:
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this
> look OK for trunk?

OK.

> -- >8 --
> 
> In r11-3261-gb28b621ac67bee we made tsubst_requires_expr never partially
> substitute into a requires-expression so as to avoid checking its
> requirements out of order during e.g. generic lambda regeneration.
> 
> Unfortunately we still do need to partially substitute into a
> requires-expression in rare cases, in particular when it's used in
> associated constraints that we are directly substituting for sake of
> declaration matching or dguide constraint rewriting.  We can identify
> this situation by checking processing_constraint_expression_p, so this
> patch uses this predicate to control whether we defer substitution or
> partially substitute.  The entering_scope=true change in tsubst_baselink
> is needed to avoid ICEing from tsubst_baselink during name lookup when
> rewriting std::ranges::ref_view's dguide constraints.
> 
> 	PR c++/112769
> 	PR c++/110006
> 
> gcc/cp/ChangeLog:
> 
> 	* constraint.cc (tsubst_simple_requirement): Return a
> 	substituted _REQ node when processing_template_decl.
> 	(tsubst_type_requirement): Likewise.
> 	(tsubst_compound_requirement): Likewise.
> 	(tsubst_nested_requirement): Likewise.
> 	(tsubst_requires_expr): Don't defer partial substitution
> 	when processing_constraint_expression_p is true, in which
> 	case return a substituted REQUIRES_EXPR.
> 	* pt.cc (tsubst_baselink): Use tsubst_aggr_type with
> 	entring_scope=true instead of tsubst to substitute
> 	qualifying_scope.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp2a/class-deduction-alias18.C: New test.
> 	* g++.dg/cpp2a/concepts-friend16.C: New test.
> ---
>   gcc/cp/constraint.cc                          | 38 +++++++++++++++++--
>   gcc/cp/pt.cc                                  |  3 +-
>   .../g++.dg/cpp2a/class-deduction-alias18.C    | 13 +++++++
>   .../g++.dg/cpp2a/concepts-friend16.C          | 25 ++++++++++++
>   4 files changed, 74 insertions(+), 5 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/class-deduction-alias18.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-friend16.C
> 
> diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
> index fef68cf7ab2..450ae548f9a 100644
> --- a/gcc/cp/constraint.cc
> +++ b/gcc/cp/constraint.cc
> @@ -2028,6 +2028,8 @@ tsubst_simple_requirement (tree t, tree args, sat_info info)
>     tree expr = tsubst_valid_expression_requirement (t0, args, info);
>     if (expr == error_mark_node)
>       return error_mark_node;
> +  if (processing_template_decl)
> +    return finish_simple_requirement (EXPR_LOCATION (t), expr);
>     return boolean_true_node;
>   }
>   
> @@ -2068,6 +2070,8 @@ tsubst_type_requirement (tree t, tree args, sat_info info)
>     tree type = tsubst_type_requirement_1 (t0, args, info, EXPR_LOCATION (t));
>     if (type == error_mark_node)
>       return error_mark_node;
> +  if (processing_template_decl)
> +    return finish_type_requirement (EXPR_LOCATION (t), type);
>     return boolean_true_node;
>   }
>   
> @@ -2182,6 +2186,9 @@ tsubst_compound_requirement (tree t, tree args, sat_info info)
>   	}
>       }
>   
> +  if (processing_template_decl)
> +    return finish_compound_requirement (EXPR_LOCATION (t),
> +					expr, type, noexcept_p);
>     return boolean_true_node;
>   }
>   
> @@ -2190,6 +2197,15 @@ tsubst_compound_requirement (tree t, tree args, sat_info info)
>   static tree
>   tsubst_nested_requirement (tree t, tree args, sat_info info)
>   {
> +  if (processing_template_decl)
> +    {
> +      tree req = TREE_OPERAND (t, 0);
> +      req = tsubst_constraint (req, args, info.complain, info.in_decl);
> +      if (req == error_mark_node)
> +	return error_mark_node;
> +      return finish_nested_requirement (EXPR_LOCATION (t), req);
> +    }
> +
>     sat_info quiet (tf_none, info.in_decl);
>     tree result = constraint_satisfaction_value (t, args, quiet);
>     if (result == boolean_true_node)
> @@ -2330,18 +2346,25 @@ tsubst_requires_expr (tree t, tree args, sat_info info)
>   
>     args = add_extra_args (REQUIRES_EXPR_EXTRA_ARGS (t), args,
>   			 info.complain, info.in_decl);
> -  if (processing_template_decl)
> +  if (processing_template_decl
> +      && !processing_constraint_expression_p ())
>       {
>         /* We're partially instantiating a generic lambda.  Substituting into
>   	 this requires-expression now may cause its requirements to get
>   	 checked out of order, so instead just remember the template
> -	 arguments and wait until we can substitute them all at once.  */
> +	 arguments and wait until we can substitute them all at once.
> +
> +	 Except if this requires-expr is part of associated constraints
> +	 that we're substituting into directly (for e.g. declaration
> +	 matching or dguide constraint rewriting), in which case we need
> +	 to partially substitute.  */
>         t = copy_node (t);
>         REQUIRES_EXPR_EXTRA_ARGS (t) = build_extra_args (t, args, info.complain);
>         return t;
>       }
>   
> -  if (tree parms = REQUIRES_EXPR_PARMS (t))
> +  tree parms = REQUIRES_EXPR_PARMS (t);
> +  if (parms)
>       {
>         parms = tsubst_constraint_variables (parms, args, info);
>         if (parms == error_mark_node)
> @@ -2349,10 +2372,13 @@ tsubst_requires_expr (tree t, tree args, sat_info info)
>       }
>   
>     tree result = boolean_true_node;
> +  if (processing_template_decl)
> +    result = NULL_TREE;
>     for (tree reqs = REQUIRES_EXPR_REQS (t); reqs; reqs = TREE_CHAIN (reqs))
>       {
>         tree req = TREE_VALUE (reqs);
> -      if (tsubst_requirement (req, args, info) == error_mark_node)
> +      req = tsubst_requirement (req, args, info);
> +      if (req == error_mark_node)
>   	{
>   	  result = boolean_false_node;
>   	  if (info.diagnose_unsatisfaction_p ())
> @@ -2360,7 +2386,11 @@ tsubst_requires_expr (tree t, tree args, sat_info info)
>   	  else
>   	    break;
>   	}
> +      else if (processing_template_decl)
> +	result = tree_cons (NULL_TREE, req, result);
>       }
> +  if (processing_template_decl && result != boolean_false_node)
> +    result = finish_requires_expr (EXPR_LOCATION (t), parms, nreverse (result));
>     return result;
>   }
>   
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 9d30a271713..17ecd4b9c5a 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -17065,7 +17065,8 @@ tsubst_baselink (tree baselink, tree object_type,
>   {
>     bool qualified_p = BASELINK_QUALIFIED_P (baselink);
>     tree qualifying_scope = BINFO_TYPE (BASELINK_ACCESS_BINFO (baselink));
> -  qualifying_scope = tsubst (qualifying_scope, args, complain, in_decl);
> +  qualifying_scope = tsubst_aggr_type (qualifying_scope, args, complain, in_decl,
> +				       /*entering_scope=*/true);
>   
>     tree optype = BASELINK_OPTYPE (baselink);
>     optype = tsubst (optype, args, complain, in_decl);
> diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias18.C b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias18.C
> new file mode 100644
> index 00000000000..0bb11bb944c
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias18.C
> @@ -0,0 +1,13 @@
> +// PR c++/112769
> +// { dg-do compile { target c++20 } }
> +
> +template<int I, typename T>
> +struct type
> +{
> +    type(T) requires requires { T{0}; };
> +};
> +
> +template <typename T>
> +using alias = type<0, T>;
> +
> +alias foo{123};
> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-friend16.C b/gcc/testsuite/g++.dg/cpp2a/concepts-friend16.C
> new file mode 100644
> index 00000000000..18974eeb172
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-friend16.C
> @@ -0,0 +1,25 @@
> +// PR c++/110006
> +// { dg-do compile { target c++20 } }
> +
> +template<typename T>
> +class s;
> +
> +template<typename T>
> +void constraint(s<T> const&, int&);
> +
> +template<typename U, typename T2>
> +U function(s<T2> const x)
> +	requires requires (U& u) { constraint(x, u); };
> +
> +template<typename T>
> +class s
> +{
> +	template<typename U, typename T2>
> +	friend U function(s<T2> const x)
> +		requires requires (U& u) { constraint(x, u); };
> +};
> +
> +int f(s<int> q)
> +{
> +	return function<int>(q); // { dg-bogus "ambiguous" }
> +}