Fix TypeVar defaults with None (PEP 696) (#16859)

Ref: #14851

Author: cdce8p

Diff for: mypy/typeanal.py

@@ -39,6 +39,7 @@
 from mypy.options import Options
 from mypy.plugin import AnalyzeTypeContext, Plugin, TypeAnalyzerPluginInterface
 from mypy.semanal_shared import SemanticAnalyzerCoreInterface, paramspec_args, paramspec_kwargs
+from mypy.state import state
 from mypy.tvar_scope import TypeVarLikeScope
 from mypy.types import (
     ANNOTATED_TYPE_NAMES,
@@ -1893,7 +1894,8 @@ def fix_instance(
     t.args = tuple(args)
     fix_type_var_tuple_argument(t)
     if not t.type.has_type_var_tuple_type:
-        fixed = expand_type(t, env)
+        with state.strict_optional_set(options.strict_optional):
+            fixed = expand_type(t, env)
         assert isinstance(fixed, Instance)
         t.args = fixed.args
 

Diff for: test-data/unit/check-typevar-defaults.test

@@ -124,6 +124,7 @@ from typing import Generic, TypeVar, Union, overload
 T1 = TypeVar("T1")
 T2 = TypeVar("T2", default=int)
 T3 = TypeVar("T3", default=str)
+T4 = TypeVar("T4", default=Union[int, None])
 
 class ClassA1(Generic[T2, T3]): ...
 
@@ -200,6 +201,20 @@ def func_a3(
     n = ClassA3[float, float, float]()  # E: Type application has too many types (expected between 1 and 2)
     reveal_type(n)  # N: Revealed type is "Any"
 
+class ClassA4(Generic[T4]): ...
+
+def func_a4(
+    a: ClassA4,
+    b: ClassA4[float],
+) -> None:
+    reveal_type(a)  # N: Revealed type is "__main__.ClassA4[Union[builtins.int, None]]"
+    reveal_type(b)  # N: Revealed type is "__main__.ClassA4[builtins.float]"
+
+    k = ClassA4()
+    reveal_type(k)  # N: Revealed type is "__main__.ClassA4[Union[builtins.int, None]]"
+    l = ClassA4[float]()
+    reveal_type(l)  # N: Revealed type is "__main__.ClassA4[builtins.float]"
+
 [case testTypeVarDefaultsClass2]
 # flags: --disallow-any-generics
 from typing import Generic, ParamSpec