Fix a number of bugs in __val_expr's subset operator[].

The current definitions were entirely broken. They didn't call any
existing constructor and the forgot to friend the expression types they
were trying to construct.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@357453 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/numerics/numarray/template.valarray/valarray.sub/slice_non_const.pass.cpp

@@ -17,28 +17,38 @@
 
 int main(int, char**)
 {
-    int a1[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15};
-    int a2[] = {-1, -2, -3, -4, -5};
-    std::valarray<int> v1(a1, sizeof(a1)/sizeof(a1[0]));
-    std::valarray<int> v2(a2, sizeof(a2)/sizeof(a2[0]));
-    v1[std::slice(1, 5, 3)] = v2;
-    assert(v1.size() == 16);
-    assert(v1[ 0] ==  0);
-    assert(v1[ 1] == -1);
-    assert(v1[ 2] ==  2);
-    assert(v1[ 3] ==  3);
-    assert(v1[ 4] == -2);
-    assert(v1[ 5] ==  5);
-    assert(v1[ 6] ==  6);
-    assert(v1[ 7] == -3);
-    assert(v1[ 8] ==  8);
-    assert(v1[ 9] ==  9);
-    assert(v1[10] == -4);
-    assert(v1[11] == 11);
-    assert(v1[12] == 12);
-    assert(v1[13] == -5);
-    assert(v1[14] == 14);
-    assert(v1[15] == 15);
-
+    {
+        int a1[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15};
+        int a2[] = {-1, -2, -3, -4, -5};
+        std::valarray<int> v1(a1, sizeof(a1)/sizeof(a1[0]));
+        std::valarray<int> v2(a2, sizeof(a2)/sizeof(a2[0]));
+        v1[std::slice(1, 5, 3)] = v2;
+        assert(v1.size() == 16);
+        assert(v1[ 0] ==  0);
+        assert(v1[ 1] == -1);
+        assert(v1[ 2] ==  2);
+        assert(v1[ 3] ==  3);
+        assert(v1[ 4] == -2);
+        assert(v1[ 5] ==  5);
+        assert(v1[ 6] ==  6);
+        assert(v1[ 7] == -3);
+        assert(v1[ 8] ==  8);
+        assert(v1[ 9] ==  9);
+        assert(v1[10] == -4);
+        assert(v1[11] == 11);
+        assert(v1[12] == 12);
+        assert(v1[13] == -5);
+        assert(v1[14] == 14);
+        assert(v1[15] == 15);
+    }
+    {
+        int a1[] = {0, 1, 2, 3, 4, 5};
+        std::valarray<int> v1(a1, sizeof(a1)/sizeof(a1[0]));
+        std::valarray<int> v2((v1 + 0)[std::slice(0, 3, 2)]);
+        assert(v2.size() == 3);
+        assert(v2[0] == 0);
+        assert(v2[1] == 2);
+        assert(v2[2] == 4);
+    }
   return 0;
 }