c++: requires-expression outside of a template is misevaluated [PR94252]

This PR shows that a REQUIRES_EXPR outside of a template can sometimes be
misevaluated.  This happens because the evaluation routine tsubst_requires_expr
(and diagnose_requires_expr) assumes the REQUIRES_EXPR's subtrees are templated
trees and that therefore it's safe to call tsubst_expr on them.  But this
assumption isn't valid when we've parsed a REQUIRES_EXPR outside of a template
context.  In order to make this assumption valid here, this patch sets
processing_template_decl to non-zero before parsing the body of a REQUIRES_EXPR
so that its subtrees are indeed always templated trees.

gcc/cp/ChangeLog:

	PR c++/94252
	* constraint.cc (tsubst_compound_requirement): Always suppress errors
	from type_deducible_p and expression_convertible_p, as they're not
	substitution errors.
	(diagnose_atomic_constraint) <case INTEGER_CST>: Remove this case so
	that we diagnose INTEGER_CST expressions of non-bool type via the
	default case.
	* cp-gimplify.c (cp_genericize_r) <case REQUIRES_EXPR>: New case.
	* parser.c (cp_parser_requires_expression): Always parse the requirement
	body as if we're processing a template, by temporarily incrementing
	processing_template_decl.  Afterwards, if we're not actually in a
	template context, perform semantic processing to diagnose any invalid
	types and expressions.
	* pt.c (tsubst_copy_and_build) <case REQUIRES_EXPR>: Remove dead code.
	* semantics.c (finish_static_assert): Explain an assertion failure
	when the condition is a REQUIRES_EXPR like we do when it is a concept
	check.

gcc/testsuite/ChangeLog:

	PR c++/94252
	* g++.dg/concepts/diagnostic7.C: New test.
	* g++.dg/concepts/pr94252.C: New test.
	* g++.dg/cpp2a/concepts-requires18.C: Adjust to expect an additional
	diagnostic.

1 files changed, 13 insertions, 1 deletions

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 3ca8eb9..bf7387d 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -27762,7 +27762,9 @@ cp_parser_requires_expression (cp_parser *parser)
       parms = NULL_TREE;
 
     /* Parse the requirement body. */
+    ++processing_template_decl;
     reqs = cp_parser_requirement_body (parser);
+    --processing_template_decl;
     if (reqs == error_mark_node)
       return error_mark_node;
   }
@@ -27771,7 +27773,17 @@ cp_parser_requires_expression (cp_parser *parser)
      the parm chain.  */
   grokparms (parms, &parms);
   loc = make_location (loc, loc, parser->lexer);
-  return finish_requires_expr (loc, parms, reqs);
+  tree expr = finish_requires_expr (loc, parms, reqs);
+  if (!processing_template_decl)
+    {
+      /* Perform semantic processing now to diagnose any invalid types and
+	 expressions.  */
+      int saved_errorcount = errorcount;
+      tsubst_requires_expr (expr, NULL_TREE, tf_warning_or_error, NULL_TREE);
+      if (errorcount > saved_errorcount)
+	return error_mark_node;
+    }
+  return expr;
 }
 
 /* Parse a parameterized requirement.