From 8b617551711a4e8061a3cacc023ebacb19beab80 Mon Sep 17 00:00:00 2001
From: Alex Waygood <Alex.Waygood@Gmail.com>
Date: Wed, 5 Jan 2022 20:50:30 +0000
Subject: [PATCH 1/6] Initial commit

---
 mypy/semanal.py | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/mypy/semanal.py b/mypy/semanal.py
index 29dfc96c2422..cf9d21ce8a60 100644
--- a/mypy/semanal.py
+++ b/mypy/semanal.py
@@ -2201,11 +2201,14 @@ def can_be_type_alias(self, rv: Expression, allow_none: bool = False) -> bool:
             return True
         if allow_none and isinstance(rv, NameExpr) and rv.fullname == 'builtins.None':
             return True
-        if (isinstance(rv, OpExpr)
-                and rv.op == '|'
-                and self.can_be_type_alias(rv.left, allow_none=True)
-                and self.can_be_type_alias(rv.right, allow_none=True)):
-            return True
+        if isinstance(rv, OpExpr) and rv.op == '|':
+            if self.is_stub_file:
+                return True
+            if (
+                self.can_be_type_alias(rv.left, allow_none=True)
+                and self.can_be_type_alias(rv.right, allow_none=True)
+            ):
+                return True
         return False
 
     def is_type_ref(self, rv: Expression, bare: bool = False) -> bool:

From dfd1a272ff10ec3fd86ff39785c4d291aa8a3952 Mon Sep 17 00:00:00 2001
From: Alex Waygood <Alex.Waygood@Gmail.com>
Date: Thu, 20 Jan 2022 00:49:34 +0000
Subject: [PATCH 2/6] Add a regression test

---
 test-data/unit/pythoneval.test | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/test-data/unit/pythoneval.test b/test-data/unit/pythoneval.test
index 9bf511e1aba3..834afb2c0142 100644
--- a/test-data/unit/pythoneval.test
+++ b/test-data/unit/pythoneval.test
@@ -1573,3 +1573,28 @@ x: OrderedDict[str, str] = OrderedDict({})
 reveal_type(x)  # Revealed type is "collections.OrderedDict[builtins.str, builtins.int]"
 [out]
 _testTypingExtensionsOrderedDictAlias.py:3: note: Revealed type is "collections.OrderedDict[builtins.str, builtins.str]"
+
+[case testImplicit604TypeAliasWithCyclicImport]
+from was_builtins import foo
+reveal_type(foo)
+[file was_abc.pyi]
+from was_builtins import *
+[file was_builtins.pyi]
+import was_typing
+from was_typeshed import ReadableBuffer
+foo: ReadableBuffer
+[file was_collections.pyi]
+from was_builtins import *
+[file was_mmap.pyi]
+from was_builtins import *
+class mmap: ...
+[file was_typeshed.pyi]
+import was_mmap
+WriteableBuffer = was_mmap.mmap
+ReadableBuffer = str | WriteableBuffer
+[file was_typing.pyi]
+from was_builtins import *
+import was_abc
+import was_collections
+[out]
+_testImplicit604TypeAliasWithCyclicImport.py:2: note: Revealed type is "Union[builtins.str, was_mmap.mmap]"
\ No newline at end of file

From 6ad779026f31ae24149408dc4795adcaeb4c94dd Mon Sep 17 00:00:00 2001
From: Alex Waygood <Alex.Waygood@Gmail.com>
Date: Thu, 20 Jan 2022 13:27:57 +0000
Subject: [PATCH 3/6] Move test

---
 test-data/unit/check-union-or-syntax.test | 24 ++++++++++++++++++++
 test-data/unit/pythoneval.test            | 27 +----------------------
 2 files changed, 25 insertions(+), 26 deletions(-)

diff --git a/test-data/unit/check-union-or-syntax.test b/test-data/unit/check-union-or-syntax.test
index 3e5a19b8fe61..ca38c2ab0e26 100644
--- a/test-data/unit/check-union-or-syntax.test
+++ b/test-data/unit/check-union-or-syntax.test
@@ -196,3 +196,27 @@ def f(x: Union[int, str, None]) -> None:
     else:
         reveal_type(x)  # N: Revealed type is "None"
 [builtins fixtures/isinstance.pyi]
+
+[case testImplicit604TypeAliasWithCyclicImport]
+# flags: --python-version 3.10
+from was_builtins import foo
+reveal_type(foo)  # N: Revealed type is "Union[builtins.str, was_mmap.mmap]"
+[file was_abc.pyi]
+from was_builtins import *
+[file was_builtins.pyi]
+import was_typing
+from was_typeshed import ReadableBuffer
+foo: ReadableBuffer
+[file was_collections.pyi]
+from was_builtins import *
+[file was_mmap.pyi]
+from was_builtins import *
+class mmap: ...
+[file was_typeshed.pyi]
+import was_mmap
+WriteableBuffer = was_mmap.mmap
+ReadableBuffer = str | WriteableBuffer
+[file was_typing.pyi]
+from was_builtins import *
+import was_abc
+import was_collections
diff --git a/test-data/unit/pythoneval.test b/test-data/unit/pythoneval.test
index eff8ebf6fba2..827c5b7c7c31 100644
--- a/test-data/unit/pythoneval.test
+++ b/test-data/unit/pythoneval.test
@@ -1584,29 +1584,4 @@ class Foo(Enum):
 [out]
 _testEnumValueWithPlaceholderNodeType.py:5: error: Incompatible types in assignment (expression has type "object", variable has type "Foo")
 _testEnumValueWithPlaceholderNodeType.py:6: error: Incompatible types in assignment (expression has type "object", variable has type "Foo")
-_testEnumValueWithPlaceholderNodeType.py:6: error: Name "Missing" is not defined
-
-[case testImplicit604TypeAliasWithCyclicImport]
-from was_builtins import foo
-reveal_type(foo)
-[file was_abc.pyi]
-from was_builtins import *
-[file was_builtins.pyi]
-import was_typing
-from was_typeshed import ReadableBuffer
-foo: ReadableBuffer
-[file was_collections.pyi]
-from was_builtins import *
-[file was_mmap.pyi]
-from was_builtins import *
-class mmap: ...
-[file was_typeshed.pyi]
-import was_mmap
-WriteableBuffer = was_mmap.mmap
-ReadableBuffer = str | WriteableBuffer
-[file was_typing.pyi]
-from was_builtins import *
-import was_abc
-import was_collections
-[out]
-_testImplicit604TypeAliasWithCyclicImport.py:2: note: Revealed type is "Union[builtins.str, was_mmap.mmap]"
+_testEnumValueWithPlaceholderNodeType.py:6: error: Name "Missing" is not defined
\ No newline at end of file

From 2890f8f40577d6d00076412ffe76b21b2a9279e0 Mon Sep 17 00:00:00 2001
From: Alex Waygood <Alex.Waygood@Gmail.com>
Date: Thu, 20 Jan 2022 13:43:56 +0000
Subject: [PATCH 4/6] Add x-fail test for `.py` files

---
 test-data/unit/check-union-or-syntax.test | 27 ++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/test-data/unit/check-union-or-syntax.test b/test-data/unit/check-union-or-syntax.test
index ca38c2ab0e26..5d4b95481052 100644
--- a/test-data/unit/check-union-or-syntax.test
+++ b/test-data/unit/check-union-or-syntax.test
@@ -197,7 +197,7 @@ def f(x: Union[int, str, None]) -> None:
         reveal_type(x)  # N: Revealed type is "None"
 [builtins fixtures/isinstance.pyi]
 
-[case testImplicit604TypeAliasWithCyclicImport]
+[case testImplicit604TypeAliasWithCyclicImportInStub]
 # flags: --python-version 3.10
 from was_builtins import foo
 reveal_type(foo)  # N: Revealed type is "Union[builtins.str, was_mmap.mmap]"
@@ -220,3 +220,28 @@ ReadableBuffer = str | WriteableBuffer
 from was_builtins import *
 import was_abc
 import was_collections
+
+# TODO: Get this test to pass
+[case testImplicit604TypeAliasWithCyclicImportNotInStub-xfail]
+# flags: --python-version 3.10
+from was_builtins import foo
+reveal_type(foo)  # N: Revealed type is "Union[builtins.str, was_mmap.mmap]"
+[file was_abc.py]
+from was_builtins import *
+[file was_builtins.py]
+import was_typing
+from was_typeshed import ReadableBuffer
+foo: ReadableBuffer
+[file was_collections.py]
+from was_builtins import *
+[file was_mmap.py]
+from was_builtins import *
+class mmap: ...
+[file was_typeshed.py]
+import was_mmap
+WriteableBuffer = was_mmap.mmap
+ReadableBuffer = str | WriteableBuffer
+[file was_typing.py]
+from was_builtins import *
+import was_abc
+import was_collections

From 8f2cc842b72066d40e369688bdb75cb5dc64404f Mon Sep 17 00:00:00 2001
From: Alex Waygood <Alex.Waygood@Gmail.com>
Date: Thu, 20 Jan 2022 14:28:18 +0000
Subject: [PATCH 5/6] Add back whitespace

---
 test-data/unit/pythoneval.test | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/test-data/unit/pythoneval.test b/test-data/unit/pythoneval.test
index 827c5b7c7c31..ab1593dac754 100644
--- a/test-data/unit/pythoneval.test
+++ b/test-data/unit/pythoneval.test
@@ -1584,4 +1584,4 @@ class Foo(Enum):
 [out]
 _testEnumValueWithPlaceholderNodeType.py:5: error: Incompatible types in assignment (expression has type "object", variable has type "Foo")
 _testEnumValueWithPlaceholderNodeType.py:6: error: Incompatible types in assignment (expression has type "object", variable has type "Foo")
-_testEnumValueWithPlaceholderNodeType.py:6: error: Name "Missing" is not defined
\ No newline at end of file
+_testEnumValueWithPlaceholderNodeType.py:6: error: Name "Missing" is not defined

From 964a7de191445944c2caa5d5079cc6ca15a140af Mon Sep 17 00:00:00 2001
From: Alex Waygood <Alex.Waygood@Gmail.com>
Date: Sun, 30 Jan 2022 22:53:21 +0000
Subject: [PATCH 6/6] Reduce size of regression tests

---
 test-data/unit/check-union-or-syntax.test | 34 ++++-------------------
 1 file changed, 6 insertions(+), 28 deletions(-)

diff --git a/test-data/unit/check-union-or-syntax.test b/test-data/unit/check-union-or-syntax.test
index 5d4b95481052..58526cfd0623 100644
--- a/test-data/unit/check-union-or-syntax.test
+++ b/test-data/unit/check-union-or-syntax.test
@@ -201,47 +201,25 @@ def f(x: Union[int, str, None]) -> None:
 # flags: --python-version 3.10
 from was_builtins import foo
 reveal_type(foo)  # N: Revealed type is "Union[builtins.str, was_mmap.mmap]"
-[file was_abc.pyi]
-from was_builtins import *
 [file was_builtins.pyi]
-import was_typing
-from was_typeshed import ReadableBuffer
-foo: ReadableBuffer
-[file was_collections.pyi]
-from was_builtins import *
-[file was_mmap.pyi]
-from was_builtins import *
-class mmap: ...
-[file was_typeshed.pyi]
 import was_mmap
 WriteableBuffer = was_mmap.mmap
 ReadableBuffer = str | WriteableBuffer
-[file was_typing.pyi]
+foo: ReadableBuffer
+[file was_mmap.pyi]
 from was_builtins import *
-import was_abc
-import was_collections
+class mmap: ...
 
 # TODO: Get this test to pass
 [case testImplicit604TypeAliasWithCyclicImportNotInStub-xfail]
 # flags: --python-version 3.10
 from was_builtins import foo
 reveal_type(foo)  # N: Revealed type is "Union[builtins.str, was_mmap.mmap]"
-[file was_abc.py]
-from was_builtins import *
 [file was_builtins.py]
-import was_typing
-from was_typeshed import ReadableBuffer
-foo: ReadableBuffer
-[file was_collections.py]
-from was_builtins import *
-[file was_mmap.py]
-from was_builtins import *
-class mmap: ...
-[file was_typeshed.py]
 import was_mmap
 WriteableBuffer = was_mmap.mmap
 ReadableBuffer = str | WriteableBuffer
-[file was_typing.py]
+foo: ReadableBuffer
+[file was_mmap.py]
 from was_builtins import *
-import was_abc
-import was_collections
+class mmap: ...