Make constraint inference in bind_self() more principled (#7938)

* Moves TypeVarExtractor to typeops.py
* Infers and applies all and only type variables that appear in an explicit self type (instead of just always the first one as currently)

Fixes #7925

Author: ilevkivskyi

mypy/checker.py

@@ -5,8 +5,7 @@
 from contextlib import contextmanager
 
 from typing import (
-    Dict, Set, List, cast, Tuple, TypeVar, Union, Optional, NamedTuple, Iterator, Iterable,
-    Sequence
+    Dict, Set, List, cast, Tuple, TypeVar, Union, Optional, NamedTuple, Iterator, Sequence
 )
 from typing_extensions import Final
 
@@ -48,7 +47,7 @@
 from mypy.typeops import (
     map_type_from_supertype, bind_self, erase_to_bound, make_simplified_union,
     erase_def_to_union_or_bound, erase_to_union_or_bound,
-    true_only, false_only, function_type,
+    true_only, false_only, function_type, TypeVarExtractor
 )
 from mypy import message_registry
 from mypy.subtypes import (
@@ -4527,20 +4526,6 @@ def detach_callable(typ: CallableType) -> CallableType:
     return out
 
 
-class TypeVarExtractor(TypeQuery[List[TypeVarType]]):
-    def __init__(self) -> None:
-        super().__init__(self._merge)
-
-    def _merge(self, iter: Iterable[List[TypeVarType]]) -> List[TypeVarType]:
-        out = []
-        for item in iter:
-            out.extend(item)
-        return out
-
-    def visit_type_var(self, t: TypeVarType) -> List[TypeVarType]:
-        return [t]
-
-
 def overload_can_never_match(signature: CallableType, other: CallableType) -> bool:
     """Check if the 'other' method can never be matched due to 'signature'.
 

mypy/typeops.py

@@ -5,14 +5,14 @@
       since these may assume that MROs are ready.
 """
 
-from typing import cast, Optional, List, Sequence, Set
+from typing import cast, Optional, List, Sequence, Set, Iterable
 import sys
 
 from mypy.types import (
     TupleType, Instance, FunctionLike, Type, CallableType, TypeVarDef, Overloaded,
     TypeVarType, UninhabitedType, FormalArgument, UnionType, NoneType,
     AnyType, TypeOfAny, TypeType, ProperType, LiteralType, get_proper_type, get_proper_types,
-    copy_type, TypeAliasType
+    copy_type, TypeAliasType, TypeQuery
 )
 from mypy.nodes import (
     FuncBase, FuncItem, OverloadedFuncDef, TypeInfo, TypeVar, ARG_STAR, ARG_STAR2, ARG_POS,
@@ -215,23 +215,29 @@ class B(A): pass
             original_type = erase_to_bound(self_param_type)
         original_type = get_proper_type(original_type)
 
-        ids = [x.id for x in func.variables]
-        typearg = get_proper_type(infer_type_arguments(ids, self_param_type,
-                                                       original_type, is_supertype=True)[0])
-        if (is_classmethod and isinstance(typearg, UninhabitedType)
+        all_ids = [x.id for x in func.variables]
+        typeargs = infer_type_arguments(all_ids, self_param_type, original_type,
+                                        is_supertype=True)
+        if (is_classmethod
+                # TODO: why do we need the extra guards here?
+                and any(isinstance(get_proper_type(t), UninhabitedType) for t in typeargs)
                 and isinstance(original_type, (Instance, TypeVarType, TupleType))):
             # In case we call a classmethod through an instance x, fallback to type(x)
-            typearg = get_proper_type(infer_type_arguments(ids, self_param_type,
-                                                           TypeType(original_type),
-                                                           is_supertype=True)[0])
+            typeargs = infer_type_arguments(all_ids, self_param_type, TypeType(original_type),
+                                            is_supertype=True)
+
+        ids = [tid for tid in all_ids
+               if any(tid == t.id for t in get_type_vars(self_param_type))]
+
+        # Technically, some constrains might be unsolvable, make them <nothing>.
+        to_apply = [t if t is not None else UninhabitedType() for t in typeargs]
 
         def expand(target: Type) -> Type:
-            assert typearg is not None
-            return expand_type(target, {func.variables[0].id: typearg})
+            return expand_type(target, {id: to_apply[all_ids.index(id)] for id in ids})
 
         arg_types = [expand(x) for x in func.arg_types[1:]]
         ret_type = expand(func.ret_type)
-        variables = func.variables[1:]
+        variables = [v for v in func.variables if v.id not in ids]
     else:
         arg_types = func.arg_types[1:]
         ret_type = func.ret_type
@@ -587,3 +593,21 @@ def coerce_to_literal(typ: Type) -> ProperType:
             if len(enum_values) == 1:
                 return LiteralType(value=enum_values[0], fallback=typ)
     return typ
+
+
+def get_type_vars(tp: Type) -> List[TypeVarType]:
+    return tp.accept(TypeVarExtractor())
+
+
+class TypeVarExtractor(TypeQuery[List[TypeVarType]]):
+    def __init__(self) -> None:
+        super().__init__(self._merge)
+
+    def _merge(self, iter: Iterable[List[TypeVarType]]) -> List[TypeVarType]:
+        out = []
+        for item in iter:
+            out.extend(item)
+        return out
+
+    def visit_type_var(self, t: TypeVarType) -> List[TypeVarType]:
+        return [t]

test-data/unit/check-selftype.test

@@ -824,6 +824,46 @@ ab: Union[A, B, C]
 reveal_type(ab.x)  # N: Revealed type is 'builtins.int'
 [builtins fixtures/property.pyi]
 
+[case testSelfTypeNoTypeVars]
+from typing import Generic, List, Optional, TypeVar, Any
+
+Q = TypeVar("Q")
+T = TypeVar("T", bound=Super[Any])
+
+class Super(Generic[Q]):
+    @classmethod
+    def meth(cls, arg: List[T]) -> List[T]:
+        pass
+
+class Sub(Super[int]): ...
+
+def test(x: List[Sub]) -> None:
+    reveal_type(Sub.meth(x))  # N: Revealed type is 'builtins.list[__main__.Sub*]'
+[builtins fixtures/isinstancelist.pyi]
+
+[case testSelfTypeNoTypeVarsRestrict]
+from typing import Generic, TypeVar
+
+T = TypeVar('T')
+S = TypeVar('S')
+
+class C(Generic[T]):
+    def limited(self: C[str], arg: S) -> S: ...
+
+reveal_type(C[str]().limited(0))  # N: Revealed type is 'builtins.int*'
+
+[case testSelfTypeMultipleTypeVars]
+from typing import Generic, TypeVar, Tuple
+
+T = TypeVar('T')
+S = TypeVar('S')
+U = TypeVar('U')
+
+class C(Generic[T]):
+    def magic(self: C[Tuple[S, U]]) -> Tuple[T, S, U]: ...
+
+reveal_type(C[Tuple[int, str]]().magic())  # N: Revealed type is 'Tuple[Tuple[builtins.int, builtins.str], builtins.int, builtins.str]'
+
 [case testSelfTypeOnUnion]
 from typing import TypeVar, Union