[PATCH] Add warnings for undefined behaviors with pointers

[Richard Smith]

Some suggestions on diagnostic wording.

================
Comment at: include/clang/Basic/DiagnosticSemaKinds.td:2392-2393
@@ +2391,4 @@
+def warn_this_bool_conversion : Warning<
+  "this pointer can only be false in undefined contexts, which may be "
+  "removed during optimization">,
+  InGroup<UndefinedBoolConversion>, DefaultIgnore;
----------------
"undefined contexts" is rather unclear here. Maybe something like:

    "'this' pointer cannot be null in well-defined C++ code; comparison may be removed"

(I don't like "during optimization" either, since it suggests we guarantee to leave this alone at `-O0`)

================
Comment at: include/clang/Basic/DiagnosticSemaKinds.td:2396-2398
@@ +2395,5 @@
+def warn_address_of_reference_bool_conversion : Warning<
+  "pointer to reference can only be false in undefined contexts, which may be "
+  "removed during optimization">,
+  InGroup<UndefinedBoolConversion>, DefaultIgnore;
+
----------------
Likewise:

  "reference cannot be bound to dereferenced null pointer in well-defined C++ code; comparison may be removed"

http://reviews.llvm.org/D3999