[clang-tidy] Fix a false positive in modernize-use-nullptr.

Summary:
The FP happens when a casting nullptr expression is used within a NULL-default-arguemnt cxx constructor.

Before the fix, the check will give a warning on nullptr when running
with the test case, which should not happen:

```
G(g(static_cast<char*>(nullptr)));
    ^~~~~~~~~~~
    nullptr
```

Reviewers: alexfh

Reviewed By: alexfh

Subscribers: cfe-commits, xazax.hun

Differential Revision: https://reviews.llvm.org/D34524

llvm-svn: 306091

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

@@ -200,12 +200,14 @@ public:
       return true;
     }
 
-    if (!FirstSubExpr)
-      FirstSubExpr = C->getSubExpr()->IgnoreParens();
-
-    // Ignore the expr if it is already a nullptr literal expr.
-    if (isa<CXXNullPtrLiteralExpr>(FirstSubExpr))
+    auto* CastSubExpr = C->getSubExpr()->IgnoreParens();
+    // Ignore cast expressions which cast nullptr literal.
+    if (isa<CXXNullPtrLiteralExpr>(CastSubExpr)) {
       return true;
+    }
+
+    if (!FirstSubExpr)
+      FirstSubExpr = CastSubExpr;
 
     if (C->getCastKind() != CK_NullToPointer &&
         C->getCastKind() != CK_NullToMemberPointer) {

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

@@ -261,3 +261,17 @@ class TemplateClass {
 void IgnoreSubstTemplateType() {
   TemplateClass<int*> a(1);
 }
+
+// Test on casting nullptr.
+struct G {
+  explicit G(bool, const char * = NULL) {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:35: warning: use nullptr
+  // CHECK-FIXES: explicit G(bool, const char * = nullptr) {}
+};
+bool g(const char*);
+void test_cast_nullptr() {
+  G(g(nullptr));
+  G(g((nullptr)));
+  G(g(static_cast<char*>(nullptr)));
+  G(g(static_cast<const char*>(nullptr)));
+}