Fix meta level on member access (#2809)

Fixes #2804

The problem is that analyze_member_access calls freshen_function_type_vars 
if the member is a function (i.e. method). This is necessary to not mix the type 
variables during type inference of generic methods inside generic functions. 
However, if the method does not participate in type inference, type variables 
are left in meta_level = 1 state. This causes the error, since the types could not 
be serialized in the middle of type inference.

In this PR I propose to restore meta_level = 0 on member access, this is safe, 
since anyway, check_call always calls freshen_function_type_vars on callee (for 
generic functions, generic methods, etc).

mypy/checkexpr.py

@@ -993,6 +993,13 @@ def analyze_ordinary_member_access(self, e: MemberExpr,
                 e.name, original_type, e, is_lvalue, False, False,
                 self.named_type, self.not_ready_callback, self.msg,
                 original_type=original_type, chk=self.chk)
+            if isinstance(member_type, CallableType):
+                for v in member_type.variables:
+                    v.id.meta_level = 0
+            if isinstance(member_type, Overloaded):
+                for it in member_type.items():
+                    for v in it.variables:
+                        v.id.meta_level = 0
             if is_lvalue:
                 return member_type
             else:

test-data/unit/check-incremental.test

@@ -1767,6 +1767,25 @@ main:2: error: Revealed type is 'TypedDict(x=builtins.int, _fallback=typing.Mapp
 main:3: error: Revealed type is 'TypedDict(x=builtins.int, _fallback=ntcrash.C.A@4)'
 main:4: error: Revealed type is 'def () -> ntcrash.C.A@4'
 
+[case testGenericMethodRestoreMetaLevel]
+from typing import Dict
+
+d = {}  # type: Dict[str, int]
+g = d.get  # This should not crash: see https://github.com/python/mypy/issues/2804
+[builtins fixtures/dict.pyi]
+
+[case testGenericMethodRestoreMetaLevel2]
+from typing import TypeVar
+
+T = TypeVar('T')
+
+class D:
+    def m(self, x: T) -> T:
+        return x
+
+g = D().m  # This should not crash: see https://github.com/python/mypy/issues/2804
+[builtins fixtures/dict.pyi]
+
 [case testIncrementalPerFileFlags]
 # flags: --config-file tmp/mypy.ini
 import a

test-data/unit/fixtures/dict.pyi

@@ -1,6 +1,6 @@
 # Builtins stub used in dictionary-related test cases.
 
-from typing import TypeVar, Generic, Iterable, Iterator, Mapping, Tuple, overload
+from typing import TypeVar, Generic, Iterable, Iterator, Mapping, Tuple, overload, Optional, Union
 
 T = TypeVar('T')
 KT = TypeVar('KT')
@@ -19,6 +19,10 @@ class dict(Iterable[KT], Mapping[KT, VT], Generic[KT, VT]):
     def __setitem__(self, k: KT, v: VT) -> None: pass
     def __iter__(self) -> Iterator[KT]: pass
     def update(self, a: Mapping[KT, VT]) -> None: pass
+    @overload
+    def get(self, k: KT) -> Optional[VT]: pass
+    @overload
+    def get(self, k: KT, default: Union[KT, T]) -> Union[VT, T]: pass
 
 class int: # for convenience
     def __add__(self, x: int) -> int: pass