c++: ICE with ()-init and TARGET_EXPR eliding [PR116424]

Here we crash on a cp_gimplify_expr/TARGET_EXPR assert:

      gcc_checking_assert (!TARGET_EXPR_ELIDING_P (*expr_p)
                           || !TREE_ADDRESSABLE (TREE_TYPE (*expr_p)));

We cannot elide the TARGET_EXPR because we're taking its address.

It is set as eliding in massage_init_elt.  I've tried to not set
TARGET_EXPR_ELIDING_P when the context is not direct-initialization.
That didn't work: even when it's not direct-initialization now, it
can become one later, for instance, after split_nonconstant_init.
One problem is that replace_placeholders_for_class_temp_r will replace
placeholders in non-eliding TARGET_EXPRs with the slot, but if we then
elide the TARGET_EXPR, we end up with a "stray" VAR_DECL and crash.
(Only some TARGET_EXPRs are handled by replace_decl.)

I thought I'd have to go back to
<https://gcc.gnu.org/pipermail/gcc-patches/2024-May/651163.html> but
then I realized that this problem occurrs only with ()-init but not
{}-init.  With {}-init, there is no problem, because we are clearing
TARGET_EXPR_ELIDING_P in process_init_constructor_record:

       /* We can't actually elide the temporary when initializing a
          potentially-overlapping field from a function that returns by
          value.  */
       if (ce->index
           && TREE_CODE (next) == TARGET_EXPR
           && unsafe_copy_elision_p (ce->index, next))
         TARGET_EXPR_ELIDING_P (next) = false;

But that does not happen for ()-init because we have no ce->index.
()-init doesn't allow brace elision so we don't really reshape them.

But I can just move the clearing a few lines down and then it handles
both ()-init and {}-init.

	PR c++/116424

gcc/cp/ChangeLog:

	* typeck2.cc (process_init_constructor_record): Move the clearing of
	TARGET_EXPR_ELIDING_P down.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/paren-init38.C: New test.

2 files changed, 27 insertions, 7 deletions

diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc
index 30a6fbe..a0c8f83 100644
--- a/gcc/cp/typeck2.cc
+++ b/gcc/cp/typeck2.cc
@@ -1774,13 +1774,6 @@ process_init_constructor_record (tree type, tree init, int nested, int flags,
 	    {
 	      gcc_assert (ce->value);
 	      next = massage_init_elt (fldtype, next, nested, flags, complain);
-	      /* We can't actually elide the temporary when initializing a
-		 potentially-overlapping field from a function that returns by
-		 value.  */
-	      if (ce->index
-		  && TREE_CODE (next) == TARGET_EXPR
-		  && unsafe_copy_elision_p (ce->index, next))
-		TARGET_EXPR_ELIDING_P (next) = false;
 	      ++idx;
 	    }
 	}
@@ -1873,6 +1866,13 @@ process_init_constructor_record (tree type, tree init, int nested, int flags,
 	    }
 	}
 
+      /* We can't actually elide the temporary when initializing a
+	 potentially-overlapping field from a function that returns by
+	 value.  */
+      if (TREE_CODE (next) == TARGET_EXPR
+	  && unsafe_copy_elision_p (field, next))
+	TARGET_EXPR_ELIDING_P (next) = false;
+
       if (is_empty_field (field)
 	  && !TREE_SIDE_EFFECTS (next))
 	/* Don't add trivial initialization of an empty base/field to the
diff --git a/gcc/testsuite/g++.dg/cpp2a/paren-init38.C b/gcc/testsuite/g++.dg/cpp2a/paren-init38.C
new file mode 100644
index 0000000..58743e0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/paren-init38.C
@@ -0,0 +1,20 @@
+// PR c++/116424
+// { dg-do compile { target c++20 } }
+
+struct dd {
+  char *ptr;
+  dd();
+  dd(dd &&__str);
+};
+struct v {
+  dd n{};
+  int f = -1;
+  v operator|(const v &other) const;
+};
+struct cc : v {};
+static const cc a;
+static const cc b;
+static const cc c1(a | b);
+static const cc c2{a | b};
+static const cc c3 = cc(a | b);
+static const cc c4 = cc{a | b};