* cp-tree.h (decl_in_std_namespace_p): Declare.
	* semantics.c (is_std_constant_evaluated_p): New.
	(finish_if_stmt_cond): Warn about "std::is_constant_evaluated ()" in
	an if-constexpr.
	* typeck.c (decl_in_std_namespace_p): No longer static.

	* g++.dg/cpp2a/is-constant-evaluated9.C: New test.

From-SVN: r274981