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

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.

Reviewers: alexfh

Reviewed By: alexfh

Subscribers: JDevlieghere, cfe-commits

Differential Revision: https://reviews.llvm.org/D30412

llvm-svn: 296479

1 files changed, 3 insertions, 1 deletions

diff --git a/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
index 50c2f89..967b6ed 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.cpp
@@ -190,8 +190,10 @@ public:
   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;