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

[hokein]

Summary

The false positive happens on two neighbour CXXDefaultArgExpr AST nodes.
like below:

CXXFunctionalCastExpr 0x85c9670 <col:7, col:23> 'struct ZZ' functional cast to struct ZZ <ConstructorConversion>
 `-CXXConstructExpr 0x85c9518 <col:7, col:23> 'struct ZZ' 'void (uint64, const uint64 *)'
   |-CallExpr 0x85a0a90 <col:10, col:22> 'uint64':'unsigned long long'
   | |-ImplicitCastExpr 0x85a0a78 <col:10> 'uint64 (*)(uint64)' <FunctionToPointerDecay>
   | | `-DeclRefExpr 0x85a09f0 <col:10> 'uint64 (uint64)' lvalue Function 0x85a06a0 'Hash' 'uint64 (uint64)'
   | `-CXXDefaultArgExpr 0x85a0ac8 <<invalid sloc>> 'uint64':'unsigned long long'
   `-CXXDefaultArgExpr 0x85c94f8 <<invalid sloc>> 'const uint64 *'

For each particular CXXDefaultArgExpr node, we need to reset
FirstSubExpr, otherwise FirstSubExpr will refer to an incorrect expr.

[alexfh]

LG. Thanks for tracking this down!

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

bool TraverseStmt(Stmt *S) {
     return RecursiveASTVisitor<CastSequenceVisitor>::TraverseStmt(S);
   }
 
   // Only VisitStmt is overridden as we shouldn't find other base AST types
   // within a cast expression.
   bool VisitStmt(Stmt *S) {
     auto *C = dyn_cast<CastExpr>(S);
     // Catch the castExpr inside cxxDefaultArgExpr.
-    if (auto *E = dyn_cast<CXXDefaultArgExpr>(S))
+    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();
 

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

struct D {
   explicit D(void *t, int *c = NULL) {}
   // CHECK-MESSAGES: :[[@LINE-1]]:32: warning: use nullptr
   // CHECK-FIXES: explicit D(void *t, int *c = nullptr) {}
 };
 
 void test_default_argument() {
   D(nullptr);
 }
+
+// Test on two neighbour CXXDefaultArgExprs nodes.
+typedef unsigned long long uint64;
+struct ZZ {
+  explicit ZZ(uint64, const uint64* = NULL) {}
+// CHECK-MESSAGES: :[[@LINE-1]]:39: warning: use nullptr
+// CHECK-FIXES: explicit ZZ(uint64, const uint64* = nullptr) {}
+  operator bool()  { return true; }
+};
+
+uint64 Hash(uint64 seed = 0) { return 0; }
+
+void f() {
+  bool a;
+  a = ZZ(Hash());
+}