[PATCH] D37042: Teach clang to tolerate the 'p = nullptr + n' idiom used by glibc

[Andy Kaylor via Phabricator via cfe-commits]

andrew.w.kaylor added inline comments.


================
Comment at: lib/AST/Expr.cpp:1857
+  if (!PExp->IgnoreParenCasts()
+          ->isNullPointerConstant(Ctx, Expr::NPC_ValueDependentIsNull))
+    return false;
----------------
rsmith wrote:
> If we get here with a value-dependent expression, we should treat it as non-null (do not warn on `(char*)PtrTemplateParameter + N`).
OK.  I wasn't sure about that.

So how do I test that?  Is this right?
```
template<typename T, T *P>
T* f(intptr_t offset) {
  return P + offset;
}

char *test(intptr_t offset) {
  return f<char, nullptr>(offset);
}
```


================
Comment at: lib/Sema/SemaExpr.cpp:8877
     if (RHS.get()->getType()->isIntegerType()) {
+      // Subtracting from a null pointer should produce a warning.
+      // The last argument to the diagnose call says this doesn't match the
----------------
rsmith wrote:
> Subtracting zero from a null pointer should not warn in C++.
> 
> (Conversely, subtracting a non-null pointer from a pointer should warn in C++, and subtracting any pointer from a null pointer should warn in C.)
Is it OK if I just add a FIXME in the section below that handles pointer-pointer?


https://reviews.llvm.org/D37042