[Sema] -Wtautological-constant-compare is too good. Cripple it.

Summary:
The diagnostic was mostly introduced in D38101 by me, as a reaction to wasting a lot of time, see [[ https://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20171009/206427.html | mail ]].
However, the diagnostic is pretty dumb. While it works with no false-positives,
there are some questionable cases that are diagnosed when one would argue that they should not be.

The common complaint is that it diagnoses the comparisons between an `int` and
`long` when compiling for a 32-bit target as tautological, but not when
compiling for 64-bit targets. The underlying problem is obvious: data model.
In most cases, 64-bit target is `LP64` (`int` is 32-bit, `long` and pointer are
64-bit), and the 32-bit target is `ILP32` (`int`, `long`, and pointer are 32-bit).

I.e. the common pattern is: (pseudocode)
```
#include <limits>
#include <cstdint>
int main() {
  using T1 = long;
  using T2 = int;

  T1 r;
  if (r < std::numeric_limits<T2>::min()) {}
  if (r > std::numeric_limits<T2>::max()) {}
}
```
As an example, D39149 was trying to fix this diagnostic in libc++, and it was not well-received.

This *could* be "fixed", by changing the diagnostics logic to something like
`if the types of the values being compared are different, but are of the same size, then do diagnose`,
and i even attempted to do so in D39462, but as @rjmccall rightfully commented,
that implementation is incomplete to say the least.

So to stop causing trouble, and avoid contaminating upcoming release, lets do this workaround:
* move these three diags (`warn_unsigned_always_true_comparison`, `warn_unsigned_enum_always_true_comparison`, `warn_tautological_constant_compare`) into it's own `-Wtautological-constant-in-range-compare`
* Disable them by default
* Make them part of `-Wextra`
* Additionally, give `warn_tautological_constant_compare` it's own flag `-Wtautological-type-limit-compare`.
  I'm not happy about that name, but i can't come up with anything better.

This way all three of them can be enabled/disabled either altogether, or one-by-one.

Reviewers: aaron.ballman, rsmith, smeenai, rjmccall, rnk, mclow.lists, dim

Reviewed By: aaron.ballman, rsmith, dim

Subscribers: thakis, compnerd, mehdi_amini, dim, hans, cfe-commits, rjmccall

Tags: #clang

Differential Revision: https://reviews.llvm.org/D41512

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@321691 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 10 insertions, 5 deletions

diff --git a/include/clang/Basic/DiagnosticGroups.td b/include/clang/Basic/DiagnosticGroups.td
index c23183c81a..79b2766ae1 100644
--- a/include/clang/Basic/DiagnosticGroups.td
+++ b/include/clang/Basic/DiagnosticGroups.td
@@ -435,12 +435,16 @@ def StringCompare : DiagGroup<"string-compare">;
 def StringPlusInt : DiagGroup<"string-plus-int">;
 def StringPlusChar : DiagGroup<"string-plus-char">;
 def StrncatSize : DiagGroup<"strncat-size">;
+def TautologicalTypeLimitCompare : DiagGroup<"tautological-type-limit-compare">;
 def TautologicalUnsignedZeroCompare : DiagGroup<"tautological-unsigned-zero-compare">;
 def TautologicalUnsignedEnumZeroCompare : DiagGroup<"tautological-unsigned-enum-zero-compare">;
+def TautologicalInRangeCompare : DiagGroup<"tautological-constant-in-range-compare",
+                                           [TautologicalTypeLimitCompare,
+                                            TautologicalUnsignedZeroCompare,
+                                            TautologicalUnsignedEnumZeroCompare]>;
 def TautologicalOutOfRangeCompare : DiagGroup<"tautological-constant-out-of-range-compare">;
 def TautologicalConstantCompare : DiagGroup<"tautological-constant-compare",
-                                            [TautologicalUnsignedZeroCompare,
-                                             TautologicalUnsignedEnumZeroCompare,
+                                            [TautologicalInRangeCompare,
                                              TautologicalOutOfRangeCompare]>;
 def TautologicalPointerCompare : DiagGroup<"tautological-pointer-compare">;
 def TautologicalOverlapCompare : DiagGroup<"tautological-overlap-compare">;
@@ -715,6 +719,7 @@ def IntToPointerCast : DiagGroup<"int-to-pointer-cast",
 def Move : DiagGroup<"move", [PessimizingMove, RedundantMove, SelfMove]>;
 
 def Extra : DiagGroup<"extra", [
+    TautologicalInRangeCompare,
     MissingFieldInitializers,
     IgnoredQualifiers,
     InitializerOverrides,
diff --git a/include/clang/Basic/DiagnosticSemaKinds.td b/include/clang/Basic/DiagnosticSemaKinds.td
index e4649d3a44..ec624a0cc1 100644
--- a/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/include/clang/Basic/DiagnosticSemaKinds.td
@@ -5946,15 +5946,15 @@ def note_typecheck_assign_const : Note<
 def warn_unsigned_always_true_comparison : Warning<
   "result of comparison of %select{%3|unsigned expression}0 %2 "
   "%select{unsigned expression|%3}0 is always %4">,
-  InGroup<TautologicalUnsignedZeroCompare>;
+  InGroup<TautologicalUnsignedZeroCompare>, DefaultIgnore;
 def warn_unsigned_enum_always_true_comparison : Warning<
   "result of comparison of %select{%3|unsigned enum expression}0 %2 "
   "%select{unsigned enum expression|%3}0 is always %4">,
-  InGroup<TautologicalUnsignedEnumZeroCompare>;
+  InGroup<TautologicalUnsignedEnumZeroCompare>, DefaultIgnore;
 def warn_tautological_constant_compare : Warning<
   "result of comparison %select{%3|%1}0 %2 "
   "%select{%1|%3}0 is always %4">,
-  InGroup<TautologicalConstantCompare>;
+  InGroup<TautologicalTypeLimitCompare>, DefaultIgnore;
 
 def warn_mixed_sign_comparison : Warning<
   "comparison of integers of different signs: %0 and %1">,