[Sema] Re-land: Diagnose tautological comparison with type's min/max values

The first attempt, rL315614 was reverted because one libcxx
test broke, and i did not know at the time how to deal with it.

Summary:
Currently, clang only diagnoses completely out-of-range comparisons (e.g. `char` and constant `300`),
and comparisons of unsigned and `0`. But gcc also does diagnose the comparisons with the
`std::numeric_limits<>::max()` / `std::numeric_limits<>::min()` so to speak

Finally Fixes https://bugs.llvm.org/show_bug.cgi?id=34147
Continuation of https://reviews.llvm.org/D37565

Reviewers: rjmccall, rsmith, aaron.ballman

Reviewed By: rsmith

Subscribers: rtrieu, jroelofs, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D38101

llvm-svn: 315875

1 files changed, 4 insertions, 0 deletions

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 49c26b449d0..293144a8327 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -78,6 +78,10 @@ Improvements to Clang's diagnostics
   when the signed integer is coerced to an unsigned type for the comparison.
   ``-Wsign-compare`` was adjusted not to warn in this case.
 
+- ``-Wtautological-constant-compare`` is a new warning that warns on
+  tautological comparisons between integer variable of the type ``T`` and the
+  largest/smallest possible integer constant of that same type.
+
 - ``-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