C++ PATCH for c++/48911 (constexpr and implicit aggregate initializers)

[Jason Merrill <jason at redhat dot com>]

In 48911, the constexpr expander wasn't properly dealing with 
aggregate/string constant array initializers with omitted elements.  We 
should build up a value-initialization as needed.

Tested x86_64-pc-linux-gnu, applying to trunk and 4.6.
commit b557b9384f1a6509735c25574f1c1d09703e6252
Author: Jason Merrill <jason@redhat.com>
Date:   Fri May 6 10:21:38 2011 -0400

    	PR c++/48911
    	* semantics.c (cxx_eval_array_reference): Handle implicit
    	initializers.

diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 8bf5a52..d0c559b 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -6324,6 +6324,7 @@ cxx_eval_array_reference (const constexpr_call *call, tree t,
 					   non_constant_p);
   tree index, oldidx;
   HOST_WIDE_INT i;
+  tree elem_type;
   unsigned len, elem_nchars = 1;
   if (*non_constant_p)
     return t;
@@ -6336,16 +6337,27 @@ cxx_eval_array_reference (const constexpr_call *call, tree t,
     return t;
   else if (addr)
     return build4 (ARRAY_REF, TREE_TYPE (t), ary, index, NULL, NULL);
+  elem_type = TREE_TYPE (TREE_TYPE (ary));
   if (TREE_CODE (ary) == CONSTRUCTOR)
     len = CONSTRUCTOR_NELTS (ary);
   else
     {
-      elem_nchars = (TYPE_PRECISION (TREE_TYPE (TREE_TYPE (ary)))
+      elem_nchars = (TYPE_PRECISION (elem_type)
 		     / TYPE_PRECISION (char_type_node));
       len = (unsigned) TREE_STRING_LENGTH (ary) / elem_nchars;
     }
   if (compare_tree_int (index, len) >= 0)
     {
+      if (tree_int_cst_lt (index, array_type_nelts_top (TREE_TYPE (ary))))
+	{
+	  /* If it's within the array bounds but doesn't have an explicit
+	     initializer, it's value-initialized.  */
+	  tree val = build_value_init (elem_type, tf_warning_or_error);
+	  return cxx_eval_constant_expression (call, val,
+					       allow_non_constant, addr,
+					       non_constant_p);
+	}
+
       if (!allow_non_constant)
 	error ("array subscript out of bound");
       *non_constant_p = true;
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-missing.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-missing.C
new file mode 100644
index 0000000..547f552
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-missing.C
@@ -0,0 +1,39 @@
+// PR c++/48911
+// { dg-do compile }
+// { dg-options "-std=c++0x" }
+
+#define SA(X) static_assert((X),#X)
+
+struct A
+{
+  constexpr A () : a (6) {}
+  int a;
+};
+
+int
+main ()
+{
+  constexpr int a[2] = { 42 };
+  constexpr int i = a[1];
+  SA(i==0);
+  constexpr int b[1] = { };
+  constexpr int j = b[0];
+  SA(j==0);
+  constexpr char c[2] = "a";
+  constexpr char k = c[1];
+  SA(k==0);
+  constexpr char d[2] = "";
+  constexpr char l = d[1];
+  SA(l==0);
+  constexpr wchar_t e[2] = L"a";
+  constexpr wchar_t m = e[1];
+  SA(m==0);
+  constexpr wchar_t f[2] = L"";
+  constexpr wchar_t n = f[1];
+  SA(n==0);
+  constexpr A g[2] = { A () };
+  constexpr A o = g[0];
+  SA(o.a == 6);
+  constexpr A p = g[1];
+  SA(p.a == 6);
+}