[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, 16 insertions, 0 deletions

diff --git a/clang-tools-extra/test/clang-tidy/modernize-use-nullptr.cpp b/clang-tools-extra/test/clang-tidy/modernize-use-nullptr.cpp
index e1ab843404e..42cdb2d5fe8 100644
--- a/clang-tools-extra/test/clang-tidy/modernize-use-nullptr.cpp
+++ b/clang-tools-extra/test/clang-tidy/modernize-use-nullptr.cpp
@@ -228,3 +228,19 @@ struct D {
 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());
+}