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

[hokein]

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

[alexfh]

LG. Thank you for the fix!

clang-tidy/modernize/UseNullptrCheck.cpp

if (auto *E = dyn_cast<CXXDefaultArgExpr>(S)) {
       C = dyn_cast<CastExpr>(E->getExpr());
       FirstSubExpr = nullptr;
     }
     if (!C) {
       FirstSubExpr = nullptr;
       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) {
       return true;
     }
 
     SourceLocation StartLoc = FirstSubExpr->getLocStart();
     SourceLocation EndLoc = FirstSubExpr->getLocEnd();

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

public:
   void f(int* p = 0) {}
 // CHECK-MESSAGES: :[[@LINE-1]]:19: warning: use nullptr
 // CHECK-FIXES: void f(int* p = nullptr) {}
 };
 
 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)));
+}