Fix "Implicit type aliases not recognised with PEP 604 + import cycles" (but only for stubs)

mypy/semanal.py

@@ -2177,11 +2177,14 @@ 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:

test-data/unit/check-union-or-syntax.test

@@ -196,3 +196,30 @@ def f(x: Union[int, str, None]) -> None:
     else:
         reveal_type(x)  # N: Revealed type is "None"
 [builtins fixtures/isinstance.pyi]
+
+[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]"
+[file was_builtins.pyi]
+import was_mmap
+WriteableBuffer = was_mmap.mmap
+ReadableBuffer = str | WriteableBuffer
+foo: ReadableBuffer
+[file was_mmap.pyi]
+from was_builtins import *
+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_builtins.py]
+import was_mmap
+WriteableBuffer = was_mmap.mmap
+ReadableBuffer = str | WriteableBuffer
+foo: ReadableBuffer
+[file was_mmap.py]
+from was_builtins import *
+class mmap: ...