Allow TypedDict assignment of Required item to NotRequired ReadOnly item (#18164)

Fixes #18162

Author: brianschubert

Diff for: mypy/subtypes.py

@@ -910,14 +910,12 @@ def visit_typeddict_type(self, left: TypedDictType) -> bool:
                     return False
                 # Non-required key is not compatible with a required key since
                 # indexing may fail unexpectedly if a required key is missing.
-                # Required key is not compatible with a non-required key since
-                # the prior doesn't support 'del' but the latter should support
-                # it.
-                #
-                # NOTE: 'del' support is currently not implemented (#3550). We
-                #       don't want to have to change subtyping after 'del' support
-                #       lands so here we are anticipating that change.
-                if (name in left.required_keys) != (name in right.required_keys):
+                # Required key is not compatible with a non-read-only non-required
+                # key since the prior doesn't support 'del' but the latter should
+                # support it.
+                # Required key is compatible with a read-only non-required key.
+                required_differ = (name in left.required_keys) != (name in right.required_keys)
+                if not right_readonly and required_differ:
                     return False
                 # Readonly fields check:
                 #

Diff for: test-data/unit/check-typeddict.test

@@ -3894,6 +3894,20 @@ accepts_B(b)
 [builtins fixtures/dict.pyi]
 [typing fixtures/typing-typeddict.pyi]
 
+[case testTypedDictRequiredConsistentWithNotRequiredReadOnly]
+from typing import NotRequired, ReadOnly, Required, TypedDict
+
+class A(TypedDict):
+    x: NotRequired[ReadOnly[str]]
+
+class B(TypedDict):
+    x: Required[str]
+
+def f(b: B):
+    a: A = b  # ok
+[builtins fixtures/dict.pyi]
+[typing fixtures/typing-typeddict.pyi]
+
 [case testTypedDictReadOnlyCall]
 from typing import ReadOnly, TypedDict