[clang-tidy] Do not dereference a null BaseType

[chh]

Summary

Check BaseType before dereference.
Simplified test case is derived from Android Open Source code.

[aaron.ballman]

LGTM!

clang-tidy/misc/ForwardingReferenceOverloadCheck.cpp

AST_MATCHER(QualType, isEnableIf) {
   // Case: pointer or reference to enable_if.
   while (BaseType->isPointerType() || BaseType->isReferenceType()) {
     BaseType = BaseType->getPointeeType().getTypePtr();
   }
   // Case: type parameter dependent (enable_if<is_integral<T>>).
   if (const auto *Dependent = BaseType->getAs<DependentNameType>()) {
     BaseType = Dependent->getQualifier()->getAsType();
   }
+  if (!BaseType)
+    return false;
   if (CheckTemplate(BaseType->getAs<TemplateSpecializationType>())) {
     return true; // Case: enable_if_t< >.
   } else if (const auto *Elaborated = BaseType->getAs<ElaboratedType>()) {
     if (const auto *Qualifier = Elaborated->getQualifier()->getAsType()) {
       if (CheckTemplate(Qualifier->getAs<TemplateSpecializationType>())) {
         return true; // Case: enable_if< >::type.
       }
     }

test/clang-tidy/misc-forwarding-reference-overload.cpp

public:
   template <typename T> Test6(T &&n);
   // CHECK-MESSAGES: :[[@LINE-1]]:25: warning: constructor accepting a forwarding reference can hide the move constructor
 
   Test6(Test6 &&rhs);
   // CHECK-MESSAGES: :[[@LINE-1]]:3: note: move constructor declared here
 private:
   Test6(const Test6 &rhs);
 };
+
+// Do not dereference a null BaseType.
+template <class _Callable> class result_of;
+template <class _Fp, class ..._Args> class result_of<_Fp(_Args...)> { };
+template <class _Tp> using result_of_t = typename result_of<_Tp>::type;
+
+template <class... _Types> struct __overload;
+template <class _Tp, class... _Types>
+struct __overload<_Tp, _Types...> : __overload<_Types...> {
+  using __overload<_Types...>::operator();
+};
+
+template <class _Tp, class... _Types>
+using __best_match_t = typename result_of_t<__overload<_Types...>(_Tp&&)>::type;
+
+template <class... _Types>
+class variant {
+public:
+  template <class _Arg, class _Tp = __best_match_t<_Arg, _Types...> >
+  constexpr variant(_Arg&& __arg) {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:13: warning: constructor accepting a forwarding reference can hide the copy and move constructors
+};