c++: requires-exprs and partial constraint subst [PR110006]

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.

These PRs however illustrate that we still sometimes do need to
partially substitute into a requires-expression, in particular when it
appears in associated constraints that we're directly substituting for
sake of declaration matching or dguide constraint rewriting.  In these
cases we're being called from tsubst_constraint during which
processing_constraint_expression_p is true, so this patch checks this
predicate to control whether we defer substitution or partially
substitute.

In turn, we now need to propagate semantic tsubst flags through
tsubst_requires_expr rather than just using tf_none, notably for sake of
dguide constraint rewriting which sets tf_dguide.

	PR c++/110006
	PR c++/112769

gcc/cp/ChangeLog:

	* constraint.cc (subst_info::quiet): Accomodate non-diagnostic
	tsubst flags.
	(tsubst_valid_expression_requirement): Likewise.
	(tsubst_simple_requirement): Return a substituted _REQ node when
	processing_template_decl.
	(tsubst_type_requirement_1): Accomodate non-diagnostic tsubst
	flags.
	(tsubst_type_requirement): Return a substituted _REQ node when
	processing_template_decl.
	(tsubst_compound_requirement): Likewise.  Accomodate non-diagnostic
	tsubst flags.
	(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_expr) <case REQUIRES_EXPR>: Accomodate
	non-diagnostic tsubst flags.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/class-deduction-alias18.C: New test.
	* g++.dg/cpp2a/concepts-friend16.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

gcc/cp/constraint.cc

@@ -85,7 +85,7 @@ struct subst_info
   /* True if we should not diagnose errors.  */
   bool quiet() const
   {
-    return complain == tf_none;
+    return !(complain & tf_warning_or_error);
   }
 
   /* True if we should diagnose errors.  */
@@ -1991,8 +1991,9 @@ hash_placeholder_constraint (tree c)
 static tree
 tsubst_valid_expression_requirement (tree t, tree args, sat_info info)
 {
-  tree r = tsubst_expr (t, args, tf_none, info.in_decl);
-  if (convert_to_void (r, ICV_STATEMENT, tf_none) != error_mark_node)
+  tsubst_flags_t quiet = info.complain & ~tf_warning_or_error;
+  tree r = tsubst_expr (t, args, quiet, info.in_decl);
+  if (convert_to_void (r, ICV_STATEMENT, quiet) != error_mark_node)
     return r;
 
   if (info.diagnose_unsatisfaction_p ())
@@ -2028,6 +2029,8 @@ 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;
 }
 
@@ -2037,7 +2040,8 @@ tsubst_simple_requirement (tree t, tree args, sat_info info)
 static tree
 tsubst_type_requirement_1 (tree t, tree args, sat_info info, location_t loc)
 {
-  tree r = tsubst (t, args, tf_none, info.in_decl);
+  tsubst_flags_t quiet = info.complain & ~tf_warning_or_error;
+  tree r = tsubst (t, args, quiet, info.in_decl);
   if (r != error_mark_node)
     return r;
 
@@ -2068,6 +2072,8 @@ 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;
 }
 
@@ -2124,9 +2130,11 @@ tsubst_compound_requirement (tree t, tree args, sat_info info)
 
   location_t loc = cp_expr_loc_or_input_loc (expr);
 
+  subst_info quiet (info.complain & ~tf_warning_or_error, info.in_decl);
+
   /* Check the noexcept condition.  */
   bool noexcept_p = COMPOUND_REQ_NOEXCEPT_P (t);
-  if (noexcept_p && !expr_noexcept_p (expr, tf_none))
+  if (noexcept_p && !expr_noexcept_p (expr, quiet.complain))
     {
       if (info.diagnose_unsatisfaction_p ())
 	inform (loc, "%qE is not %<noexcept%>", expr);
@@ -2139,8 +2147,6 @@ tsubst_compound_requirement (tree t, tree args, sat_info info)
   if (type == error_mark_node)
     return error_mark_node;
 
-  subst_info quiet (tf_none, info.in_decl);
-
   /* Check expression against the result type.  */
   if (type)
     {
@@ -2182,6 +2188,9 @@ 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;
 }
 
@@ -2190,7 +2199,16 @@ tsubst_compound_requirement (tree t, tree args, sat_info info)
 static tree
 tsubst_nested_requirement (tree t, tree args, sat_info info)
 {
-  sat_info quiet (tf_none, info.in_decl);
+  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 (info.complain & ~tf_warning_or_error, info.in_decl);
   tree result = constraint_satisfaction_value (t, args, quiet);
   if (result == boolean_true_node)
     return boolean_true_node;
@@ -2330,18 +2348,25 @@ 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 +2374,13 @@ 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 +2388,11 @@ 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;
 }
 

gcc/cp/pt.cc

@@ -21706,7 +21706,8 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 
     case REQUIRES_EXPR:
       {
-	tree r = tsubst_requires_expr (t, args, tf_none, in_decl);
+	complain &= ~tf_warning_or_error;
+	tree r = tsubst_requires_expr (t, args, complain, in_decl);
 	RETURN (r);
       }
 

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};

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" }
+}