[PATCH] c++: Relax checking assert about elision to support -fno-elide-constructors [PR114619]

[Simon Martin]

We currently ICE in checking mode with cxx_dialect < 17 on the following
valid code

=== cut here ===
struct X {
  X(const X&) {}
};
extern X x;
void foo () {
  new X[1]{x};
}
=== cut here ===

The problem is that cp_gimplify_expr gcc_checking_asserts that a
TARGET_EXPR is not TARGET_EXPR_ELIDING_P (or cannot be elided), while in
this case with cxx_dialect < 17, it is TARGET_EXPR_ELIDING_P but we have
not even tried to elide.

This patch relaxes that gcc_checking_assert to not fail when using
cxx_dialect < 17 and -fno-elide-constructors (I considered being more
clever at setting TARGET_EXPR_ELIDING_P appropriately but it looks more
risky and not worth the extra complexity for a checking assert).

Successfully tested on x86_64-pc-linux-gnu.

        PR c++/114619

gcc/cp/ChangeLog:

        * cp-gimplify.cc (cp_gimplify_expr): Relax gcc_checking_assert
        to support the usage of -fno-elide-constructors with c++ < 17.

gcc/testsuite/ChangeLog:

        * g++.dg/init/no-elide3.C: New test.

---
 gcc/cp/cp-gimplify.cc                 |  7 ++++++-
 gcc/testsuite/g++.dg/init/no-elide3.C | 11 +++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/init/no-elide3.C

diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index 003e68f1ea7..354ea73c63b 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -908,7 +908,12 @@ cp_gimplify_expr (tree *expr_p, gimple_seq *pre_p, 
gimple_seq *post_p)
         gimplify_init_ctor_preeval can materialize subobjects of a CONSTRUCTOR
         on the rhs of an assignment, as in constexpr-aggr1.C.  */
       gcc_checking_assert (!TARGET_EXPR_ELIDING_P (*expr_p)
-                          || !TREE_ADDRESSABLE (TREE_TYPE (*expr_p)));
+                          || !TREE_ADDRESSABLE (TREE_TYPE (*expr_p))
+                          /* If we explicitly asked not to elide and it's not
+                             required by the standard, we can't expect elision
+                             to have happened.  */
+                          || (cxx_dialect < cxx17
+                              && !flag_elide_constructors));
       ret = GS_UNHANDLED;
       break;
 
diff --git a/gcc/testsuite/g++.dg/init/no-elide3.C 
b/gcc/testsuite/g++.dg/init/no-elide3.C
new file mode 100644
index 00000000000..9377d9f0161
--- /dev/null
+++ b/gcc/testsuite/g++.dg/init/no-elide3.C
@@ -0,0 +1,11 @@
+// PR c++/114619
+// { dg-do "compile" { target c++11 } }
+// { dg-options "-fno-elide-constructors" }
+
+struct X {
+  X(const X&) {}
+};
+extern X x;
+void foo () {
+  new X[1]{x};
+}
-- 
2.44.0