[clang] [C] Don't diagnose null pointer macros in -Wimplicit-void-ptr-cast (PR #140724)

[Aaron Ballman via cfe-commits]

================
@@ -9966,8 +9966,13 @@ AssignConvertType Sema::CheckSingleAssignmentConstraints(QualType LHSType,
       // If there is a conversion of some kind, check to see what kind of
       // pointer conversion happened so we can diagnose a C++ compatibility
       // diagnostic if the conversion is invalid. This only matters if the RHS
-      // is some kind of void pointer.
-      if (Kind != CK_NoOp && !getLangOpts().CPlusPlus) {
+      // is some kind of void pointer. We have a carve-out when the RHS is from
+      // a macro expansion because the use of a macro may indicate different
+      // code between C and C++. Consider: char *s = NULL; where NULL is
+      // defined as (void *)0 in C (which would be invalid in C++), but 0 in
+      // C++, which is valid in C++.
+      if (Kind != CK_NoOp && !getLangOpts().CPlusPlus &&
+          !RHS.get()->getBeginLoc().isMacroID()) {
----------------
AaronBallman wrote:

The predicate already checks that:
```
RHS.get()->isNullPointerConstant(Context, Expr::NPC_ValueDependentIsNull)))
```
so this allows for users who define their own null macros (which does happen): https://sourcegraph.com/search?q=context:global+%23define%5B+%5Ct%5D%2B%5BA-Za-z0-9_%5D%2B+%5B%5C%28%5D%3F%5C%28void%5B+%5Ct%5D*%5C*%5C%290%5B%5C%29%5D%3F&patternType=regexp&case=yes&sm=0

https://github.com/llvm/llvm-project/pull/140724