[Sema] Document+test the -Wsign-compare change for enums in C code [NFC]

rL316268 / D39122 has fixed PR35009, and now when in C,
these three(?) diagnostics properly use the enum's underlying
datatype.

While it was fixed, the test coverage was clearly insufficient,
because the -Wsign-compare change didn't show up in any of the
tests, until it was reported in the post-commit mail for rL316268.

So add the test for the -Wsign-compare diagnostic for enum
for C code, and while there, document this in the release notes.

The fix itself was obviously correct, so unless we want to silence
this new diagnosed case, i deem this commit to be NFC.

llvm-svn: 316500

clang/docs/ReleaseNotes.rst

@@ -82,6 +82,10 @@ Improvements to Clang's diagnostics
   tautological comparisons between integer variable of the type ``T`` and the
   largest/smallest possible integer constant of that same type.
 
+- For C code, ``-Wsign-compare``, ``-Wtautological-constant-compare`` and
+  ``-Wtautological-constant-out-of-range-compare`` were adjusted to use the
+  underlying datatype of ``enum``.
+
 - ``-Wnull-pointer-arithmetic`` now warns about performing pointer arithmetic
   on a null pointer. Such pointer arithmetic has an undefined behavior if the
   offset is nonzero. It also now warns about arithmetic on a null pointer

clang/test/Sema/sign-compare-enum.c

@@ -0,0 +1,24 @@
+// RUN: %clang_cc1 -triple=x86_64-pc-linux-gnu -fsyntax-only -DUNSIGNED -verify -Wsign-compare %s
+// RUN: %clang_cc1 -triple=x86_64-pc-win32 -fsyntax-only -DSIGNED -verify -Wsign-compare %s
+// RUN: %clang_cc1 -triple=x86_64-pc-linux-gnu -fsyntax-only -DUNSIGNED -DSILENCE -verify %s
+// RUN: %clang_cc1 -triple=x86_64-pc-win32 -fsyntax-only -DSIGNED -DSILENCE -verify %s
+
+int main() {
+  enum A { A_a = 0, A_b = 1 };
+  static const int message[] = {0, 1};
+  enum A a;
+
+  if (a < 2)
+    return 0;
+
+#if defined(SIGNED) && !defined(SILENCE)
+  if (a < sizeof(message)/sizeof(message[0])) // expected-warning {{comparison of integers of different signs: 'enum A' and 'unsigned long long'}}
+    return 0;
+#else
+  // expected-no-diagnostics
+  if (a < 2U)
+    return 0;
+  if (a < sizeof(message)/sizeof(message[0]))
+    return 0;
+#endif
+}