PR c++/77467
	* constexpr.c (enum constexpr_switch_state): New.
	(struct constexpr_ctx): Add css_state field.
	(label_matches): Add CTX and STMT arguments, remove I and
	DEFAULT_LABEL.  For CASE_LABEL_EXPR assert ctx->css_state != NULL,
	handle default labels according to css_state.
	(cxx_eval_statement_list): Remove statement skipping, label_matches
	and default_label handling code.
	(cxx_eval_loop_expr): Exit after first iteration even if
	switches (jump_target).
	(cxx_eval_switch_expr): Set up css_state field in ctx, if default
	label has been seen in the body, but no cases matched, evaluate
	the body second time.
	(cxx_eval_constant_expression): Handle stmt skipping and label_matches
	here.  Handle PREDICT_EXPR.  For MODIFY_EXPR or INIT_EXPR, assert
	statement is not skipped.  For COND_EXPR during skipping, don't
	evaluate condition, just the then block and if still skipping at the
	end also the else block.
	(cxx_eval_outermost_constant_expr): Adjust constexpr_ctx initializer.
	(is_sub_constant_expr): Likewise.

	* g++.dg/cpp1y/constexpr-77467.C: New test.

From-SVN: r240591