[clang-tidy] 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 `{}`.)

Author: tkoeppe

clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp

@@ -38,7 +38,9 @@ AST_MATCHER(Type, sugaredNullptrType) {
 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)))));

clang-tools-extra/test/clang-tidy/checkers/modernize/use-nullptr.cpp

@@ -84,6 +84,17 @@ void test_macro_expansion4() {
 #undef MY_NULL
 }
 
+template <typename T> struct pear {
+  pear() { x = __null; }
+  // CHECK-MESSAGES: :[[@LINE-1]]:16: warning: use nullptr [modernize-use-nullptr]
+  // CHECK-FIXES: x = nullptr;
+  T x;
+};
+void test_templated() {
+  pear<int*> p;
+  dummy(p.x);
+}
+
 #define IS_EQ(x, y) if (x != y) return;
 void test_macro_args() {
   int i = 0;