Index: clang-tidy/modernize/UseNullptrCheck.cpp
===================================================================
--- clang-tidy/modernize/UseNullptrCheck.cpp
+++ clang-tidy/modernize/UseNullptrCheck.cpp
@@ -435,12 +435,14 @@
         Loc = D->getLocStart();
       else if (const auto *S = Parent.get<Stmt>())
         Loc = S->getLocStart();
-      else
-        llvm_unreachable("Expected to find Decl or Stmt containing ancestor");
 
-      if (!expandsFrom(Loc, MacroLoc)) {
-        Result = Parent;
-        return true;
+      // TypeLoc and NestedNameSpecifierLoc are members of the parent map. Skip
+      // them and keep going up.
+      if (Loc.isValid()) {
+        if (!expandsFrom(Loc, MacroLoc)) {
+          Result = Parent;
+          return true;
+        }
       }
       Start = Parent;
     }
Index: test/clang-tidy/modernize-use-nullptr.cpp
===================================================================
--- test/clang-tidy/modernize-use-nullptr.cpp
+++ test/clang-tidy/modernize-use-nullptr.cpp
@@ -184,3 +184,15 @@
   // CHECK-FIXES: a[2] = {ENTRY(nullptr), {nullptr}};
 #undef ENTRY
 }
+
+// One of the ancestor of the cast is a NestedNameSpecifierLoc.
+class NoDef;
+char function(NoDef *p);
+#define F(x) \
+    (sizeof(function(x)) == 1)
+template<class T, T t>
+class C {};
+C<bool, F(0)> c;
+// CHECK-MESSAGES: :[[@LINE-1]]:11: warning: use nullptr
+// CHECK-FIXES: C<bool, F(nullptr)> c;
+#undef F