Suppress unsubscriptable-object warnings in type-checking blocks

ChangeLog

@@ -59,6 +59,11 @@ Release date: TBA
 
   Closes #4525
 
+* Fix false positive for ``unsubscriptable-object`` in Python 3.8 and below for
+  statements guarded by ``if TYPE_CHECKING``.
+
+  Closes #3979
+
 * Fix false negative for ``no-member`` when attempting to assign an instance
   attribute to itself without any prior assignment.
 

doc/whatsnew/2.14.rst

@@ -77,6 +77,11 @@ Other Changes
 
   Closes #5205
 
+* Fix false positive for ``unsubscriptable-object`` in Python 3.8 and below for
+  statements guarded by ``if TYPE_CHECKING``.
+
+  Closes #3979
+
 * Fix false negative for ``no-member`` when attempting to assign an instance
   attribute to itself without any prior assignment.
 

pylint/checkers/typecheck.py

@@ -1951,7 +1951,11 @@ 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"