Suppress unsubscriptable-object warnings in type-checking blocks (#5720)

* Add a regression test for issue #3979

* Don't emit `unsubscriptable-object` for statements in type-checking blocks

Co-authored-by: Jacob Walls <[email protected]>

Author: Pierre-Sassoulas

ChangeLog

@@ -44,6 +44,11 @@ Release date: TBA
 
   Closes #4020
 
+* Fix false positive for ``unsubscriptable-object`` in Python 3.8 and below for
+  statements guarded by ``if TYPE_CHECKING``.
+
+  Closes #3979
+
 
 What's New in Pylint 2.13.7?
 ============================

doc/whatsnew/2.13.rst

@@ -630,6 +630,11 @@ Other Changes
 
   Closes #6419
 
+* Fix false positive for ``unsubscriptable-object`` in Python 3.8 and below for
+  statements guarded by ``if TYPE_CHECKING``.
+
+  Closes #3979
+
 * Fix a crash when linting a file that passes an integer ``mode=`` to
   ``open``
 

pylint/checkers/typecheck.py

@@ -1915,7 +1915,11 @@ def visit_subscript(self, node: nodes.Subscript) -> None:
                 return  # It would be better to handle function
                 # decorators, but let's start slow.
 
-        if supported_protocol and not supported_protocol(inferred, node):
+        if (
+            supported_protocol
+            and not supported_protocol(inferred, node)
+            and not utils.in_type_checking_block(node)
+        ):
             self.add_message(msg, args=node.value.as_string(), node=node.value)
 
     @check_messages("dict-items-missing-iter")

tests/functional/r/regression_02/regression_3979.py

@@ -0,0 +1,13 @@
+"""
+Regression test for https://github.com/PyCQA/pylint/issues/3979
+"""
+
+import os
+from typing import TYPE_CHECKING, Any, Union
+
+if TYPE_CHECKING:
+    BasePathLike = os.PathLike[Any]  # <-- pylint used to emit E1136 here
+else:
+    BasePathLike = os.PathLike
+
+foo: Union[str, BasePathLike] = "bar"