Index: clang-tools-extra/trunk/test/clang-tidy/check_clang_tidy.py
===================================================================
--- clang-tools-extra/trunk/test/clang-tidy/check_clang_tidy.py
+++ clang-tools-extra/trunk/test/clang-tidy/check_clang_tidy.py
@@ -17,6 +17,7 @@
 
 Usage:
   check_clang_tidy.py [-resource-dir <resource-dir>] \
+    [-assume-filename <file-with-source-extension>] \
     <source-file> <check-name> <temp-file> \
     -- [optional clang-tidy arguments]
 
@@ -38,6 +39,7 @@
 def main():
   parser = argparse.ArgumentParser()
   parser.add_argument('-resource-dir')
+  parser.add_argument('-assume-filename')
   parser.add_argument('input_file_name')
   parser.add_argument('check_name')
   parser.add_argument('temp_file_name')
@@ -45,14 +47,17 @@
   args, extra_args = parser.parse_known_args()
 
   resource_dir = args.resource_dir
+  assume_file_name = args.assume_filename
   input_file_name = args.input_file_name
   check_name = args.check_name
   temp_file_name = args.temp_file_name
 
+  file_name_with_extension = assume_file_name or input_file_name
+
   extension = '.cpp'
-  if (input_file_name.endswith('.c')):
+  if (file_name_with_extension.endswith('.c')):
     extension = '.c'
-  if (input_file_name.endswith('.hpp')):
+  if (file_name_with_extension.endswith('.hpp')):
     extension = '.hpp'
   temp_file_name = temp_file_name + extension
 
Index: clang-tools-extra/trunk/test/clang-tidy/nonstandard-file-extension.test
===================================================================
--- clang-tools-extra/trunk/test/clang-tidy/nonstandard-file-extension.test
+++ clang-tools-extra/trunk/test/clang-tidy/nonstandard-file-extension.test
@@ -0,0 +1,6 @@
+// RUN: %check_clang_tidy -assume-filename=const-cast.cpp %s cppcoreguidelines-pro-type-const-cast %t
+
+const int *i;
+int *j;
+void f() { j = const_cast<int *>(i); }
+// CHECK-MESSAGES: :[[@LINE-1]]:16: warning: do not use const_cast [cppcoreguidelines-pro-type-const-cast]