c++: c++20 constexpr default ctor and array init

The implicit constexpr patch revealed that marking the constructor in the
PR70690 testcase as constexpr made the bug reappear, because build_vec_init
assumed that a constexpr default constructor initialized the whole object,
so it was equivalent to value-initialization.  But this is no longer true in
C++20.

	PR c++/70690

gcc/cp/ChangeLog:

	* init.c (build_vec_init): Check default_init_uninitialized_part in
	C++20.

gcc/testsuite/ChangeLog:

	* g++.dg/init/array41a.C: New test.

2 files changed, 32 insertions, 2 deletions

diff --git a/gcc/cp/init.c b/gcc/cp/init.c
index 771a19b..3ba2e3b 100644
--- a/gcc/cp/init.c
+++ b/gcc/cp/init.c
@@ -4470,11 +4470,14 @@ build_vec_init (tree base, tree maxindex, tree init,
 
      We do need to keep going if we're copying an array.  */
 
-  if (try_const && !init)
+  if (try_const && !init
+      && (cxx_dialect < cxx20
+	  || !default_init_uninitialized_part (inner_elt_type)))
     /* With a constexpr default constructor, which we checked for when
        setting try_const above, default-initialization is equivalent to
        value-initialization, and build_value_init gives us something more
-       friendly to maybe_constant_init.  */
+       friendly to maybe_constant_init.  Except in C++20 and up a constexpr
+       constructor need not initialize all the members.  */
     explicit_value_init_p = true;
   if (from_array
       || ((type_build_ctor_call (type) || init || explicit_value_init_p)
diff --git a/gcc/testsuite/g++.dg/init/array41a.C b/gcc/testsuite/g++.dg/init/array41a.C
new file mode 100644
index 0000000..aa9fdc6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/init/array41a.C
@@ -0,0 +1,27 @@
+// PR c++/70690
+// { dg-do run { target c++11 } }
+
+struct A {
+  constexpr A() {}
+};
+
+struct APadded : public A {
+  char pad[63];
+};
+
+int f();
+int i = f();
+APadded cache[50];
+APadded *p = cache;
+
+int f()
+{
+  cache[0].pad[0] = 42;
+  return 1;
+}
+
+int main()
+{
+  if (cache[0].pad[0] != 42)
+    __builtin_abort();
+}