Fix wcpncpy() return value; add test.

Author: enh-google

libc/src/wchar/wcpncpy.cpp

@@ -28,8 +28,9 @@ LLVM_LIBC_FUNCTION(wchar_t *, wcpncpy,
   for (i = 0; i < n && s2[i] != '\0'; ++i)
     s1[i] = s2[i];
   // When n>strlen(src), n-strlen(src) \0 are appended.
-  for (; i < n; ++i)
-    s1[i] = L'\0';
+  for (size_t j = i; j < n; ++j)
+    s1[j] = L'\0';
+  // ...but our result points to the first \0 (if any).
   return s1 + i;
 }
 

libc/test/src/wchar/wcpncpy_test.cpp

@@ -75,6 +75,15 @@ TEST(LlvmLibcWCPNCpyTest, CopyTwoWithNull) {
   ASSERT_EQ(dest + 2, res);
 }
 
+TEST(LlvmLibcWCPNCpyTest, CopyAndFill) {
+  wchar_t dest[] = {L'a', L'b', L'c'};
+  wchar_t *res = LIBC_NAMESPACE::wcpncpy(dest, L"x", 3);
+  ASSERT_TRUE(dest[0] == L'x');
+  ASSERT_TRUE(dest[1] == L'\0');
+  ASSERT_TRUE(dest[2] == L'\0');
+  ASSERT_EQ(dest + 1, res);
+}
+
 #if defined(LIBC_ADD_NULL_CHECKS) && !defined(LIBC_HAS_SANITIZER)
 TEST(LlvmLibcWCPNCpyTest, NullptrCrash) {
   // Passing in a nullptr should crash the program.