c++: constexpr, empty base after non-empty [PR106369]

Here the CONSTRUCTOR we were providing for D{} had an entry for the B base
subobject at offset 0 following the entry for the C base, causing
output_constructor_regular_field to ICE due to going backwards.  It might be
nice for that function to be more tolerant of empty fields, but it also
seems reasonable for the front end to prune the useless entry.

	PR c++/106369

gcc/cp/ChangeLog:

	* constexpr.cc (reduced_constant_expression_p): Return false
	if a CONSTRUCTOR initializes an empty field.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/constexpr-lambda27.C: New test.

2 files changed, 33 insertions, 1 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 5f7fc6f..5e0d339 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -3081,7 +3081,13 @@ reduced_constant_expression_p (tree t)
 	     element.  */
 	  if (!reduced_constant_expression_p (e.value))
 	    return false;
-	  /* Empty class field may or may not have an initializer.  */
+	  /* We want to remove initializers for empty fields in a struct to
+	     avoid confusing output_constructor.  */
+	  if (is_empty_field (e.index)
+	      && TREE_CODE (TREE_TYPE (t)) == RECORD_TYPE)
+	    return false;
+	  /* Check for non-empty fields between initialized fields when
+	     CONSTRUCTOR_NO_CLEARING.  */
 	  for (; field && e.index != field;
 	       field = next_subobject_field (DECL_CHAIN (field)))
 	    if (!is_really_empty_class (TREE_TYPE (field),
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-lambda27.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-lambda27.C
new file mode 100644
index 0000000..24e2e9b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-lambda27.C
@@ -0,0 +1,26 @@
+// PR c++/106369
+// { dg-do compile { target c++17 } }
+
+struct A {
+  int a[256];
+  constexpr int &operator[] (int n) noexcept { return a[n]; }
+  constexpr const int &operator[] (int n) const noexcept { return a[n]; }
+};
+struct B {};
+template <typename T>
+struct C {
+  constexpr T &foo (const char x) noexcept { c = T::d[x]; return static_cast<T &>(*this); }
+  int c;
+};
+struct D : public C<D>, public B
+{
+  D () noexcept = default;
+  static constexpr char e[9] { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I' };
+  static constexpr A d = [] () constexpr {
+    A f {};
+    for (int i = 0; i < 9; ++i)
+      f[e[i]] = 1;
+    return f;
+  } ();
+};
+constexpr auto g = D{}.foo ('E');