Enable -Wtautological-unsigned-zero-compare under -Wextra

[eandrews]

Summary

GCC enables -Wtype-limits under -Wextra. Warnings under GCC's -Wtype-limits is covered in Clang by -Wtautological-type-limit-compare and -Wtautological-unsigned-zero-compare.

Since -Wtautological-type-limit-compare can fire spuriously when a type's size is platform-dependent (as detailed in D41512), I am adding only -Wtautological-unsigned-zero-compare to -Wextra.

I should point out that GCC does not throw warnings on pointless unsigned expression comparisons inside templates while -Wtautological-unsigned-zero-compare does. I figure this is a limitation in GCC's implementation and don't think this is an issue but please feel free to let me know if I am mistaken.

[eandrews]

@lebedev.ri is there a specific reason -Wtautological-unsigned-zero-compare was removed? All the issues I am aware of talks about comparisons with min/max.

[lebedev.ri]

In D51545#1220967, @eandrews wrote:

@lebedev.ri is there a specific reason -Wtautological-unsigned-zero-compare was removed? All the issues I am aware of talks about comparisons with min/max.

Dunno.
Comparison with zero, i would imagine, has the same problem as comparisons with min/max, since comparisons with min == comparison with zero, for unsigned types.

[thakis]

We don't match gcc's -Wextra behvior. We generally try to not put a ton of stuff in Wextra that isn't in -Wall. (We also generally don't put a lot of stuff in -Wall that isn't enabled by default.) So I don't think we want this.

[eandrews]

Alright. Thanks for the review. I will abandon this patch

include/clang/Basic/DiagnosticGroups.td

 def Extra : DiagGroup<"extra", [
     MissingFieldInitializers,
     IgnoredQualifiers,
     InitializerOverrides,
     SemiBeforeMethodBody,
     MissingMethodReturnType,
     SignCompare,
     UnusedParameter,
-    NullPointerArithmetic
+    NullPointerArithmetic,
+    TautologicalUnsignedZeroCompare
   ]>;
 
 def Most : DiagGroup<"most", [
     CharSubscript,
     Comment,
     DeleteNonVirtualDtor,
     ForLoopAnalysis,
     Format,

test/Sema/tautological-unsigned-zero-compare.c

 // RUN: %clang_cc1 -fsyntax-only \
 // RUN:            -Wtautological-unsigned-zero-compare \
 // RUN:            -verify %s
 // RUN: %clang_cc1 -fsyntax-only \
 // RUN:            -verify=silence %s
 // RUN: %clang_cc1 -fsyntax-only \
 // RUN:            -Wtautological-unsigned-zero-compare \
 // RUN:            -verify -x c++ %s
 // RUN: %clang_cc1 -fsyntax-only \
 // RUN:            -verify=silence -x c++ %s
+// RUN: %clang_cc1 -fsyntax-only \
+// RUN:            -Wextra -Wno-sign-compare\
+// RUN:            -verify %s
+// RUN: %clang_cc1 -fsyntax-only \
+// RUN:            -Wextra -Wno-sign-compare\
+// RUN:            -verify -x c++ %s
 
 unsigned uvalue(void);
 signed int svalue(void);
 
 #define macro(val) val
 
 #ifdef __cplusplus
 template<typename T>