Remove assumption of case sensitivity (#11708)

Originally added in #10093

Fixes #11690, fixes #10661, fixes #10822

Co-authored-by: hauntsaninja <>

mypy/fscache.py

@@ -30,7 +30,6 @@
 
 import os
 import stat
-import sys
 from typing import Dict, List, Set
 from mypy.util import hash_digest
 from mypy_extensions import mypyc_attr
@@ -202,9 +201,6 @@ def isfile_case(self, path: str, prefix: str) -> bool:
 
         The caller must ensure that prefix is a valid file system prefix of path.
         """
-        if sys.platform == "linux":
-            # Assume that the file system on Linux is case sensitive
-            return self.isfile(path)
         if not self.isfile(path):
             # Fast path
             return False