c++: constexpr std::construct_at on empty field [PR101663]

Here during constexpr evaluation of

  std::construct_at(&a._M_value)

we find ourselves in cxx_eval_store_expression where the target object
is 'a._M_value' and the initializer is {}.  Since _M_value is an empty
[[no_unique_address]] member we don't create a sub-CONSTRUCTOR for it,
so we end up in the early exit code path for empty stores with mismatched
types and trip over the assert therein

  gcc_assert (is_empty_class (TREE_TYPE (init)) && !lval);

because lval is true.  The reason it's true is because the INIT_EXPR in
question is the LHS of a COMPOUND_EXPR, and evaluation of the LHS is
always performed with lval=true (to indicate there's no lvalue-to-rvalue
conversion).

This patch makes the code path in question handle the lval=true case
appropriately rather than asserting.  In passing, it also consolidates
the duplicate implementations of std::construct_at/destroy_at in some
of the C++20 constexpr tests into a common header file.

	PR c++/101663

gcc/cp/ChangeLog:

	* constexpr.c (cxx_eval_store_expression): Handle the lval=true
	case in the early exit code path for empty stores with mismatched
	types.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/construct_at.h: New convenience header file that
	defines minimal implementations of std::construct_at/destroy_at,
	split out from ...
	* g++.dg/cpp2a/constexpr-new5.C: ... here.
	* g++.dg/cpp2a/constexpr-new6.C: Use the header.
	* g++.dg/cpp2a/constexpr-new14.C: Likewise.
	* g++.dg/cpp2a/constexpr-new20.C: New test.

1 files changed, 2 insertions, 2 deletions

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 1af365d..25d84a3 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -5588,8 +5588,8 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
 	   argument, which has the derived type rather than the base type.  In
 	   this situation, just evaluate the initializer and return, since
 	   there's no actual data to store.  */
-	  gcc_assert (is_empty_class (TREE_TYPE (init)) && !lval);
-	  return init;
+	  gcc_assert (is_empty_class (TREE_TYPE (init)));
+	  return lval ? target : init;
 	}
       CONSTRUCTOR_ELTS (*valp) = CONSTRUCTOR_ELTS (init);
       TREE_CONSTANT (*valp) = TREE_CONSTANT (init);