[pushed] c++: constexpr aggregate destruction [PR109357]

[Jason Merrill via Gcc-patches]

Tested x86_64-pc-linux-gnu, applying to trunk.

-- 8< --

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.
---
 gcc/cp/constexpr.cc                           | 15 +++++++++++----
 gcc/testsuite/g++.dg/cpp2a/constexpr-dtor15.C | 19 +++++++++++++++++++
 2 files changed, 30 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-dtor15.C

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 3de60cfd0f8..d1097764b10 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 00000000000..d34c27eee45
--- /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 = {"", ""}; }

base-commit: 9964df74a9e99e850bf9b0b6ff5c47133f846db8
-- 
2.31.1