[clang-tidy] modernize-use-nullptr matches "NULL" in templates (#109169)

Make modernize-use-nullptr matcher also match "NULL", but not "0", when
it appears on a substituted type of a template specialization.

Previously, any matches on a substituted type were excluded, but this
meant that a situation like the following is not diagnosed:

```c++
template <typename T>
struct X {
  T val;
  X() { val = NULL; }  // should diagnose
};
```

When the user says `NULL`, we expect that the destination type is always
meant to be a pointer type, so this should be converted to `nullptr`. By
contrast, we do not propose changing a literal `0` in that case, which
appears as initializers of both pointer and integer specializations in
reasonable real code. (If `NULL` is used erroneously in such a
situation, it should be changed to `0` or `{}`.)

1 files changed, 11 insertions, 1 deletions

diff --git a/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
index 6a003a3..108717e 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
@@ -35,10 +35,20 @@ AST_MATCHER(Type, sugaredNullptrType) {
 /// to null within.
 /// Finding sequences of explicit casts is necessary so that an entire sequence
 /// can be replaced instead of just the inner-most implicit cast.
+///
+/// TODO/NOTE: The second "anyOf" below discards matches on a substituted type,
+/// since we don't know if that would _always_ be a pointer type for all other
+/// specializations, unless the expression was "__null", in which case we assume
+/// that all specializations are expected to be for pointer types. Ideally this
+/// would check for the "NULL" macro instead, but that'd be harder to express.
+/// In practice, "NULL" is often defined as "__null", and this is a useful
+/// condition.
 StatementMatcher makeCastSequenceMatcher(llvm::ArrayRef<StringRef> NameList) {
   auto ImplicitCastToNull = implicitCastExpr(
       anyOf(hasCastKind(CK_NullToPointer), hasCastKind(CK_NullToMemberPointer)),
-      unless(hasImplicitDestinationType(qualType(substTemplateTypeParmType()))),
+      anyOf(hasSourceExpression(gnuNullExpr()),
+            unless(hasImplicitDestinationType(
+                qualType(substTemplateTypeParmType())))),
       unless(hasSourceExpression(hasType(sugaredNullptrType()))),
       unless(hasImplicitDestinationType(
           qualType(matchers::matchesAnyListedTypeName(NameList)))));