Reland "Fix wcpncpy() return value; add test." (#146753)

Reverts #146752, which was a revert of my accidental
push, so we can actually review and presubmit this time.

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

@@ -45,7 +45,7 @@ TEST(LlvmLibcWCPNCpyTest, CopyNull) {
   wchar_t *res = LIBC_NAMESPACE::wcpncpy(dest, src, 1);
   ASSERT_TRUE(dest[0] == L'\0');
   ASSERT_TRUE(dest[1] == L'b');
-  ASSERT_EQ(dest + 1, res);
+  ASSERT_EQ(dest, res);
 }
 
 TEST(LlvmLibcWCPNCpyTest, CopyPastSrc) {
@@ -54,7 +54,7 @@ TEST(LlvmLibcWCPNCpyTest, CopyPastSrc) {
   wchar_t *res = LIBC_NAMESPACE::wcpncpy(dest, src, 2);
   ASSERT_TRUE(dest[0] == L'\0');
   ASSERT_TRUE(dest[1] == L'\0');
-  ASSERT_EQ(dest + 2, res);
+  ASSERT_EQ(dest, res);
 }
 
 TEST(LlvmLibcWCPNCpyTest, CopyTwoNoNull) {
@@ -72,7 +72,16 @@ TEST(LlvmLibcWCPNCpyTest, CopyTwoWithNull) {
   wchar_t *res = LIBC_NAMESPACE::wcpncpy(dest, src, 2);
   ASSERT_TRUE(dest[0] == L'x');
   ASSERT_TRUE(dest[1] == L'\0');
-  ASSERT_EQ(dest + 2, res);
+  ASSERT_EQ(dest + 1, 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)