Update imported Vars in the third pass (#5635)

Fixes #5275 
Fixes #4498 
Fixes #4442 

This is a simple _band-aid_ fix for `Invalid type` in import cycles where type aliases, named tuples, or typed dicts appear. Note that this is a partial fix that only fixes the `Invalid type` error when a type alias etc. appears in type context. This doesn't fix errors (e.g. `Cannot determine type of X`) that may appear if the type alias etc. appear in runtime context.

The motivation for partial fix is two-fold:
* The error often appears in stub files (especially for large libraries/frameworks) where we have more import cycles, but no runtime context at all.
* Ideally we should refactor semantic analysis to have deferred nodes, and process them in smaller passes when there is more info (like we do in type checking phase). But this is _much_ harder since this requires a large refactoring.  Also an alternative fix of updating node of every `NameExpr` and `MemberExpr` in third pass is costly from performance point of view, and still nontrivial.

mypy/semanal_pass3.py

@@ -67,12 +67,55 @@ def visit_file(self, file_node: MypyFile, fnam: str, options: Options,
         self.sem.globals = file_node.names
         with experiments.strict_optional_set(options.strict_optional):
             self.scope.enter_file(file_node.fullname())
+            self.update_imported_vars()
             self.accept(file_node)
             self.analyze_symbol_table(file_node.names)
             self.scope.leave()
         del self.cur_mod_node
         self.patches = []
 
+    def update_imported_vars(self) -> None:
+        """Update nodes for imported names, if they got updated from Var to TypeInfo or TypeAlias.
+
+        This is a simple _band-aid_ fix for "Invalid type" error in import cycles where type
+        aliases, named tuples, or typed dicts appear. The root cause is that during first pass
+        definitions like:
+
+            A = List[int]
+
+        are seen by mypy as variables, because it doesn't know yet that `List` refers to a type.
+        In the second pass, such `Var` is replaced with a `TypeAlias`. But in import cycle,
+        import of `A` will still refer to the old `Var` node. Therefore we need to update it.
+
+        Note that this is a partial fix that only fixes the "Invalid type" error when a type alias
+        etc. appears in type context. This doesn't fix errors (e.g. "Cannot determine type of A")
+        that may appear if the type alias etc. appear in runtime context.
+
+        The motivation for partial fix is two-fold:
+          * The "Invalid type" error often appears in stub files (especially for large
+            libraries/frameworks) where we have more import cycles, but no runtime
+            context at all.
+          * Ideally we should refactor semantic analysis to have deferred nodes, and process
+            them in smaller passes when there is more info (like we do in type checking phase).
+            But this is _much_ harder since this requires a large refactoring. Also an alternative
+            fix of updating node of every `NameExpr` and `MemberExpr` in third pass is costly
+            from performance point of view, and still nontrivial.
+        """
+        for sym in self.cur_mod_node.names.values():
+            if sym and isinstance(sym.node, Var):
+                fullname = sym.node.fullname()
+                if '.' not in fullname:
+                    continue
+                mod_name, _, name = fullname.rpartition('.')
+                if mod_name not in self.sem.modules:
+                    continue
+                if mod_name != self.sem.cur_mod_id:  # imported
+                    new_sym = self.sem.modules[mod_name].names.get(name)
+                    if new_sym and isinstance(new_sym.node, (TypeInfo, TypeAlias)):
+                        # This Var was replaced with a class (like named tuple)
+                        # or alias, update this.
+                        sym.node = new_sym.node
+
     def refresh_partial(self, node: Union[MypyFile, FuncDef, OverloadedFuncDef],
                         patches: List[Tuple[int, Callable[[], None]]]) -> None:
         """Refresh a stale target in fine-grained incremental mode."""

test-data/unit/check-incremental.test

@@ -5123,6 +5123,50 @@ def outer() -> None:
 [out]
 [out2]
 
+[case testRecursiveAliasImported]
+import a
+[file a.py]
+import lib
+x: int
+[file a.py.2]
+import lib
+x: lib.A
+reveal_type(x)
+[file lib.pyi]
+from typing import List
+from other import B
+A = List[B]  # type: ignore
+[file other.pyi]
+from typing import List
+from lib import A
+B = List[A]
+[builtins fixtures/list.pyi]
+[out]
+[out2]
+tmp/a.py:3: error: Revealed type is 'builtins.list[builtins.list[builtins.list[Any]]]'
+
+[case testRecursiveNamedTupleTypedDict]
+import a
+[file a.py]
+import lib
+x: int
+[file a.py.2]
+import lib
+x: lib.A
+reveal_type(x.x['x'])
+[file lib.pyi]
+from typing import NamedTuple
+from other import B
+A = NamedTuple('A', [('x', B)])  # type: ignore
+[file other.pyi]
+from mypy_extensions import TypedDict
+from lib import A
+B = TypedDict('B', {'x': A})
+[builtins fixtures/dict.pyi]
+[out]
+[out2]
+tmp/a.py:3: error: Revealed type is 'Tuple[TypedDict('other.B', {'x': Any}), fallback=lib.A]'
+
 [case testFollowImportSkipNotInvalidatedOnPresent]
 # flags: --follow-imports=skip
 # cmd: mypy -m main

test-data/unit/check-namedtuple.test

@@ -691,6 +691,28 @@ my_eval(A([B(1), B(2)])) # OK
 [builtins fixtures/isinstancelist.pyi]
 [out]
 
+[case testNamedTupleImportCycle]
+import b
+[file a.py]
+class C:
+    pass
+
+from b import tp
+x: tp
+reveal_type(x.x)  # E: Revealed type is 'builtins.int'
+
+# Unfortunately runtime part doesn't work yet, see docstring in SemanticAnalyzerPass3.update_imported_vars()
+reveal_type(tp)  # E: Revealed type is 'Any' \
+                 # E: Cannot determine type of 'tp'
+tp('x')  # E: Cannot determine type of 'tp'
+
+[file b.py]
+from a import C
+from typing import NamedTuple
+
+tp = NamedTuple('tp', [('x', int)])
+[out]
+
 [case testSubclassOfRecursiveNamedTuple]
 from typing import List, NamedTuple
 

test-data/unit/check-type-aliases.test

@@ -427,6 +427,60 @@ reveal_type(D().meth(1))  # E: Revealed type is 'Union[__main__.D*, builtins.int
 [builtins fixtures/classmethod.pyi]
 [out]
 
+[case testAliasInImportCycle]
+# cmd: mypy -m t t2
+[file t.py]
+MYPY = False
+if MYPY:
+    from t2 import A
+x: A
+[file t2.py]
+import t
+from typing import Callable
+A = Callable[[], None]
+[builtins fixtures/bool.pyi]
+[out]
+
+[case testAliasInImportCycle2]
+import a
+[file a.pyi]
+from b import Parameter
+
+class _ParamType:
+    p: Parameter
+
+_ConvertibleType = _ParamType
+
+def convert_type(ty: _ConvertibleType):
+    ...
+
+[file b.pyi]
+from a import _ConvertibleType
+
+class Parameter:
+    type: _ConvertibleType
+[out]
+
+[case testAliasInImportCycle3]
+# cmd: mypy -m t t2
+[file t.py]
+MYPY = False
+if MYPY:
+    from t2 import A
+x: A
+reveal_type(x)  # E: Revealed type is 't2.D'
+
+# Unfortunately runtime part doesn't work yet, see docstring in SemanticAnalyzerPass3.update_imported_vars()
+reveal_type(A)  # E: Revealed type is 'Any' \
+                # E: Cannot determine type of 'A'
+A()  # E: Cannot determine type of 'A'
+[file t2.py]
+import t
+class D: pass
+A = D
+[builtins fixtures/bool.pyi]
+[out]
+
 [case testFlexibleAlias1]
 from typing import TypeVar, List, Tuple
 from mypy_extensions import FlexibleAlias

test-data/unit/check-typeddict.test

@@ -1390,3 +1390,26 @@ def f(x: a.N) -> None:
 [out]
 tmp/b.py:4: error: Revealed type is 'TypedDict('a.N', {'a': builtins.str})'
 tmp/b.py:5: error: Revealed type is 'builtins.str'
+
+[case testTypedDictImportCycle]
+import b
+[file a.py]
+class C:
+    pass
+
+from b import tp
+x: tp
+reveal_type(x['x'])  # E: Revealed type is 'builtins.int'
+
+# Unfortunately runtime part doesn't work yet, see docstring in SemanticAnalyzerPass3.update_imported_vars()
+reveal_type(tp)  # E: Revealed type is 'Any' \
+                 # E: Cannot determine type of 'tp'
+tp('x')  # E: Cannot determine type of 'tp'
+
+[file b.py]
+from a import C
+from mypy_extensions import TypedDict
+
+tp = TypedDict('tp', {'x': int})
+[builtins fixtures/dict.pyi]
+[out]