c++: requires-expressions and partial instantiation [PR96410]

This patch makes tsubst_requires_expr avoid substituting into a
requires-expression when partially instantiating a generic lambda.
This is necessary in general to ensure that we always check requirements
in lexical order (as in the first testcase below).  A mechanism similar
to PACK_EXPANSION_EXTRA_ARGS is added to remember template arguments and
defer substitution of requires-expressions.

Incidentally, this change also fixes the two mentioned PRs -- the
problem there is that tsubst_requires_expr was performing semantic
checks on template trees, and some of the checks are not prepared to
handle such trees.  With this patch, tsubst_requires_expr no longer
does any semantic checking at all when processing_template_decl.

gcc/cp/ChangeLog:

	PR c++/96409
	PR c++/96410
	* constraint.cc (tsubst_requires_expr): Use REQUIRES_EXPR_PARMS
	and REQUIRES_EXPR_REQS.  Use REQUIRES_EXPR_EXTRA_ARGS,
	add_extra_args and build_extra_args to defer substitution until
	we have all the template arguments.
	(finish_requires_expr): Adjust the call to build_min so that
	REQUIRES_EXPR_EXTRA_ARGS gets set to NULL_TREE.
	* cp-tree.def (REQUIRES_EXPR): Give it a third operand.
	* cp-tree.h (REQUIRES_EXPR_PARMS, REQUIRES_EXPR_REQS,
	REQUIRES_EXPR_EXTRA_ARGS): Define.
	(add_extra_args, build_extra_args): Declare.

gcc/testsuite/ChangeLog:

	PR c++/96409
	PR c++/96410
	* g++.dg/cpp2a/concepts-lambda13.C: New test.
	* g++.dg/cpp2a/concepts-lambda14.C: New test.

1 files changed, 15 insertions, 6 deletions

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index ad9d470..0aab307 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -2175,7 +2175,19 @@ tsubst_requires_expr (tree t, tree args,
   /* A requires-expression is an unevaluated context.  */
   cp_unevaluated u;
 
-  tree parms = TREE_OPERAND (t, 0);
+  args = add_extra_args (REQUIRES_EXPR_EXTRA_ARGS (t), args);
+  if (processing_template_decl)
+    {
+      /* 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.  */
+      t = copy_node (t);
+      REQUIRES_EXPR_EXTRA_ARGS (t) = build_extra_args (t, args, complain);
+      return t;
+    }
+
+  tree parms = REQUIRES_EXPR_PARMS (t);
   if (parms)
     {
       parms = tsubst_constraint_variables (parms, args, info);
@@ -2183,14 +2195,11 @@ tsubst_requires_expr (tree t, tree args,
 	return boolean_false_node;
     }
 
-  tree reqs = TREE_OPERAND (t, 1);
+  tree reqs = REQUIRES_EXPR_REQS (t);
   reqs = tsubst_requirement_body (reqs, args, info);
   if (reqs == error_mark_node)
     return boolean_false_node;
 
-  if (processing_template_decl)
-    return finish_requires_expr (cp_expr_location (t), parms, reqs);
-
   return boolean_true_node;
 }
 
@@ -2933,7 +2942,7 @@ finish_requires_expr (location_t loc, tree parms, tree reqs)
     }
 
   /* Build the node. */
-  tree r = build_min (REQUIRES_EXPR, boolean_type_node, parms, reqs);
+  tree r = build_min (REQUIRES_EXPR, boolean_type_node, parms, reqs, NULL_TREE);
   TREE_SIDE_EFFECTS (r) = false;
   TREE_CONSTANT (r) = true;
   SET_EXPR_LOCATION (r, loc);