c++: constexpr aggregate destruction [PR109357]

We were assuming that the result of evaluation of TARGET_EXPR_INITIAL would
always be the new value of the temporary, but that's not necessarily true
when the initializer is complex (i.e. target_expr_needs_replace).  In that
case evaluating the initializer initializes the temporary as a side-effect.

	PR c++/109357

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_constant_expression) [TARGET_EXPR]:
	Check for complex initializer.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constexpr-dtor15.C: New test.

2 files changed, 30 insertions, 4 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 3de60cf..d109776 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -7230,16 +7230,23 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 					  non_constant_p, overflow_p);
 	if (*non_constant_p)
 	  break;
-	/* Adjust the type of the result to the type of the temporary.  */
-	r = adjust_temp_type (type, r);
+	/* If the initializer is complex, evaluate it to initialize slot.  */
+	bool is_complex = target_expr_needs_replace (t);
+	if (!is_complex)
+	  {
+	    r = unshare_constructor (r);
+	    /* Adjust the type of the result to the type of the temporary.  */
+	    r = adjust_temp_type (type, r);
+	    ctx->global->put_value (slot, r);
+	  }
 	if (TARGET_EXPR_CLEANUP (t) && !CLEANUP_EH_ONLY (t))
 	  ctx->global->cleanups->safe_push (TARGET_EXPR_CLEANUP (t));
-	r = unshare_constructor (r);
-	ctx->global->put_value (slot, r);
 	if (ctx->save_exprs)
 	  ctx->save_exprs->safe_push (slot);
 	if (lval)
 	  return slot;
+	if (is_complex)
+	  r = ctx->global->get_value (slot);
       }
       break;
 
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-dtor15.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-dtor15.C
new file mode 100644
index 0000000..d34c27e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-dtor15.C
@@ -0,0 +1,19 @@
+// PR c++/109357
+// { dg-do compile { target c++20 } }
+// { dg-prune-output "used but never defined" }
+
+struct basic_string {
+  char _M_local_buf;
+  basic_string();
+  constexpr basic_string(const char *) {}
+  constexpr ~basic_string();
+  constexpr basic_string& operator=(basic_string);
+};
+struct S1 {
+  basic_string x;
+  basic_string y;
+} s1;
+struct s2 {
+  ~s2();
+};
+s2::~s2() { s1 = {"", ""}; }