Improve usage of outer context for inference (#5699)

Fixes #4872 
Fixes #3876
Fixes #2678 
Fixes #5199 
Fixes #5493 
(It also fixes a bunch of similar issues previously closed as duplicates, except one, see below).

This PR fixes a problems when mypy commits to soon to using outer context for type inference. This is done by:
* Postponing inference to inner (argument) context in situations where type inferred from outer (return) context doesn't satisfy bounds or constraints.
* Adding a special case for situation where optional return is inferred against optional context. In such situation, unwrapping the optional is a better idea in 99% of cases. (Note: this doesn't affect type safety, only gives empirically more reasonable inferred types.)

In general, instead of adding a special case, it would be better to use inner and outer context at the same time, but this a big change (see comment in code), and using the simple special case fixes majority of issues. Among reported issues, only #5311 will stay unfixed.

mypy/applytype.py

@@ -9,13 +9,17 @@
 
 
 def apply_generic_arguments(callable: CallableType, orig_types: Sequence[Optional[Type]],
-                            msg: MessageBuilder, context: Context) -> CallableType:
+                            msg: MessageBuilder, context: Context,
+                            skip_unsatisfied: bool = False) -> CallableType:
     """Apply generic type arguments to a callable type.
 
     For example, applying [int] to 'def [T] (T) -> T' results in
     'def (int) -> int'.
 
     Note that each type can be None; in this case, it will not be applied.
+
+    If `skip_unsatisfied` is True, then just skip the types that don't satisfy type variable
+    bound or constraints, instead of giving an error.
     """
     tvars = callable.variables
     assert len(tvars) == len(orig_types)
@@ -25,7 +29,9 @@ def apply_generic_arguments(callable: CallableType, orig_types: Sequence[Optiona
     for i, type in enumerate(types):
         assert not isinstance(type, PartialType), "Internal error: must never apply partial type"
         values = callable.variables[i].values
-        if values and type:
+        if type is None:
+            continue
+        if values:
             if isinstance(type, AnyType):
                 continue
             if isinstance(type, TypeVarType) and type.values:
@@ -34,15 +40,31 @@ def apply_generic_arguments(callable: CallableType, orig_types: Sequence[Optiona
                 if all(any(is_same_type(v, v1) for v in values)
                        for v1 in type.values):
                     continue
+            matching = []
             for value in values:
                 if mypy.subtypes.is_subtype(type, value):
-                    types[i] = value
-                    break
+                    matching.append(value)
+            if matching:
+                best = matching[0]
+                # If there are more than one matching value, we select the narrowest
+                for match in matching[1:]:
+                    if mypy.subtypes.is_subtype(match, best):
+                        best = match
+                types[i] = best
             else:
-                msg.incompatible_typevar_value(callable, type, callable.variables[i].name, context)
-        upper_bound = callable.variables[i].upper_bound
-        if type and not mypy.subtypes.is_subtype(type, upper_bound):
-            msg.incompatible_typevar_value(callable, type, callable.variables[i].name, context)
+                if skip_unsatisfied:
+                    types[i] = None
+                else:
+                    msg.incompatible_typevar_value(callable, type, callable.variables[i].name,
+                                                   context)
+        else:
+            upper_bound = callable.variables[i].upper_bound
+            if not mypy.subtypes.is_subtype(type, upper_bound):
+                if skip_unsatisfied:
+                    types[i] = None
+                else:
+                    msg.incompatible_typevar_value(callable, type, callable.variables[i].name,
+                                                   context)
 
     # Create a map from type variable id to target type.
     id_to_type = {}  # type: Dict[TypeVarId, Type]

mypy/checker.py

@@ -31,7 +31,8 @@
     Type, AnyType, CallableType, FunctionLike, Overloaded, TupleType, TypedDictType,
     Instance, NoneTyp, strip_type, TypeType, TypeOfAny,
     UnionType, TypeVarId, TypeVarType, PartialType, DeletedType, UninhabitedType, TypeVarDef,
-    true_only, false_only, function_type, is_named_instance, union_items, TypeQuery
+    true_only, false_only, function_type, is_named_instance, union_items, TypeQuery,
+    is_optional, remove_optional
 )
 from mypy.sametypes import is_same_type, is_same_types
 from mypy.messages import MessageBuilder, make_inferred_type_note
@@ -3792,17 +3793,6 @@ def is_literal_none(n: Expression) -> bool:
     return isinstance(n, NameExpr) and n.fullname == 'builtins.None'
 
 
-def is_optional(t: Type) -> bool:
-    return isinstance(t, UnionType) and any(isinstance(e, NoneTyp) for e in t.items)
-
-
-def remove_optional(typ: Type) -> Type:
-    if isinstance(typ, UnionType):
-        return UnionType.make_union([t for t in typ.items if not isinstance(t, NoneTyp)])
-    else:
-        return typ
-
-
 def is_literal_not_implemented(n: Expression) -> bool:
     return isinstance(n, NameExpr) and n.fullname == 'builtins.NotImplemented'
 

mypy/checkexpr.py

@@ -18,8 +18,9 @@
 from mypy.types import (
     Type, AnyType, CallableType, Overloaded, NoneTyp, TypeVarDef,
     TupleType, TypedDictType, Instance, TypeVarType, ErasedType, UnionType,
-    PartialType, DeletedType, UninhabitedType, TypeType, TypeOfAny, true_only,
-    false_only, is_named_instance, function_type, callable_type, FunctionLike, StarType,
+    PartialType, DeletedType, UninhabitedType, TypeType, TypeOfAny,
+    true_only, false_only, is_named_instance, function_type, callable_type, FunctionLike,
+    StarType, is_optional, remove_optional, is_invariant_instance
 )
 from mypy.nodes import (
     NameExpr, RefExpr, Var, FuncDef, OverloadedFuncDef, TypeInfo, CallExpr,
@@ -30,7 +31,7 @@
     ConditionalExpr, ComparisonExpr, TempNode, SetComprehension,
     DictionaryComprehension, ComplexExpr, EllipsisExpr, StarExpr, AwaitExpr, YieldExpr,
     YieldFromExpr, TypedDictExpr, PromoteExpr, NewTypeExpr, NamedTupleExpr, TypeVarExpr,
-    TypeAliasExpr, BackquoteExpr, EnumCallExpr, TypeAlias, ClassDef, Block, SymbolNode,
+    TypeAliasExpr, BackquoteExpr, EnumCallExpr, TypeAlias, SymbolNode,
     ARG_POS, ARG_OPT, ARG_NAMED, ARG_STAR, ARG_STAR2, MODULE_REF, LITERAL_TYPE, REVEAL_TYPE
 )
 from mypy.literals import literal
@@ -819,20 +820,36 @@ def infer_function_type_arguments_using_context(
         # valid results.
         erased_ctx = replace_meta_vars(ctx, ErasedType())
         ret_type = callable.ret_type
-        if isinstance(ret_type, TypeVarType):
-            if ret_type.values or (not isinstance(ctx, Instance) or
-                                   not ctx.args):
-                # The return type is a type variable. If it has values, we can't easily restrict
-                # type inference to conform to the valid values. If it's unrestricted, we could
-                # infer a too general type for the type variable if we use context, and this could
-                # result in confusing and spurious type errors elsewhere.
-                #
-                # Give up and just use function arguments for type inference. As an exception,
-                # if the context is a generic instance type, actually use it as context, as
-                # this *seems* to usually be the reasonable thing to do.
-                #
-                # See also github issues #462 and #360.
-                ret_type = NoneTyp()
+        if is_optional(ret_type) and is_optional(ctx):
+            # If both the context and the return type are optional, unwrap the optional,
+            # since in 99% cases this is what a user expects. In other words, we replace
+            #     Optional[T] <: Optional[int]
+            # with
+            #     T <: int
+            # while the former would infer T <: Optional[int].
+            ret_type = remove_optional(ret_type)
+            erased_ctx = remove_optional(erased_ctx)
+            #
+            # TODO: Instead of this hack and the one below, we need to use outer and
+            # inner contexts at the same time. This is however not easy because of two
+            # reasons:
+            #   * We need to support constraints like [1 <: 2, 2 <: X], i.e. with variables
+            #     on both sides. (This is not too hard.)
+            #   * We need to update all the inference "infrastructure", so that all
+            #     variables in an expression are inferred at the same time.
+            #     (And this is hard, also we need to be careful with lambdas that require
+            #     two passes.)
+        if isinstance(ret_type, TypeVarType) and not is_invariant_instance(ctx):
+            # Another special case: the return type is a type variable. If it's unrestricted,
+            # we could infer a too general type for the type variable if we use context,
+            # and this could result in confusing and spurious type errors elsewhere.
+            #
+            # Give up and just use function arguments for type inference. As an exception,
+            # if the context is an invariant instance type, actually use it as context, as
+            # this *seems* to usually be the reasonable thing to do.
+            #
+            # See also github issues #462 and #360.
+            return callable.copy_modified()
         args = infer_type_arguments(callable.type_var_ids(), ret_type, erased_ctx)
         # Only substitute non-Uninhabited and non-erased types.
         new_args = []  # type: List[Optional[Type]]
@@ -841,7 +858,10 @@ def infer_function_type_arguments_using_context(
                 new_args.append(None)
             else:
                 new_args.append(arg)
-        return self.apply_generic_arguments(callable, new_args, error_context)
+        # Don't show errors after we have only used the outer context for inference.
+        # We will use argument context to infer more variables.
+        return self.apply_generic_arguments(callable, new_args, error_context,
+                                            skip_unsatisfied=True)
 
     def infer_function_type_arguments(self, callee_type: CallableType,
                                       args: List[Expression],
@@ -1609,9 +1629,10 @@ def check_arg(caller_type: Type, original_caller_type: Type, caller_kind: int,
             return False
 
     def apply_generic_arguments(self, callable: CallableType, types: Sequence[Optional[Type]],
-                                context: Context) -> CallableType:
+                                context: Context, skip_unsatisfied: bool = False) -> CallableType:
         """Simple wrapper around mypy.applytype.apply_generic_arguments."""
-        return applytype.apply_generic_arguments(callable, types, self.msg, context)
+        return applytype.apply_generic_arguments(callable, types, self.msg, context,
+                                                 skip_unsatisfied=skip_unsatisfied)
 
     def visit_member_expr(self, e: MemberExpr, is_lvalue: bool = False) -> Type:
         """Visit member expression (of form e.id)."""

mypy/types.py

@@ -1918,6 +1918,23 @@ def union_items(typ: Type) -> List[Type]:
         return [typ]
 
 
+def is_invariant_instance(tp: Type) -> bool:
+    if not isinstance(tp, Instance) or not tp.args:
+        return False
+    return any(v.variance == INVARIANT for v in tp.type.defn.type_vars)
+
+
+def is_optional(t: Type) -> bool:
+    return isinstance(t, UnionType) and any(isinstance(e, NoneTyp) for e in t.items)
+
+
+def remove_optional(typ: Type) -> Type:
+    if isinstance(typ, UnionType):
+        return UnionType.make_union([t for t in typ.items if not isinstance(t, NoneTyp)])
+    else:
+        return typ
+
+
 names = globals().copy()  # type: Final
 names.pop('NOT_READY', None)
 deserialize_map = {

test-data/unit/check-generics.test

@@ -849,7 +849,7 @@ def fun2(v: Vec[T], scale: T) -> Vec[T]:
     return v
 
 reveal_type(fun1([(1, 1)])) # E: Revealed type is 'builtins.int*'
-fun1(1) # E: Argument 1 to "fun1" has incompatible type "int"; expected "List[Tuple[int, int]]"
+fun1(1) # E: Argument 1 to "fun1" has incompatible type "int"; expected "List[Tuple[bool, bool]]"
 fun1([(1, 'x')]) # E: Cannot infer type argument 1 of "fun1"
 
 reveal_type(fun2([(1, 1)], 1)) # E: Revealed type is 'builtins.list[Tuple[builtins.int*, builtins.int*]]'