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

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.