Allow subscripted aliases at function scope (#4000)

Fixes #3145.

This PR allows subscripted type aliases at function scope, as discussed in 
#3145. This will simplify the rules and have other pluses (apart from fixing 
the actual issue).

In addition, I prohibit reuse of bound type variables to define generic 
aliases. Situations like this were always ambiguous.

mypy/checkexpr.py

@@ -652,7 +652,7 @@ def analyze_type_type_callee(self, item: Type, context: Context) -> Type:
             res = type_object_type(item.type, self.named_type)
             if isinstance(res, CallableType):
                 res = res.copy_modified(from_type_type=True)
-            return res
+            return expand_type_by_instance(res, item)
         if isinstance(item, UnionType):
             return UnionType([self.analyze_type_type_callee(item, context)
                               for item in item.relevant_items()], item.line)

mypy/semanal.py

@@ -1725,11 +1725,10 @@ def alias_fallback(self, tp: Type) -> Instance:
         return Instance(fb_info, [])
 
     def analyze_alias(self, rvalue: Expression,
-                      allow_unnormalized: bool) -> Tuple[Optional[Type], List[str]]:
+                      warn_bound_tvar: bool = False) -> Tuple[Optional[Type], List[str]]:
         """Check if 'rvalue' represents a valid type allowed for aliasing
         (e.g. not a type variable). If yes, return the corresponding type and a list of
         qualified type variable names for generic aliases.
-        If 'allow_unnormalized' is True, allow types like builtins.list[T].
         """
         dynamic = bool(self.function_stack and self.function_stack[-1].is_dynamic())
         global_scope = not self.type and not self.function_stack
@@ -1744,7 +1743,8 @@ def analyze_alias(self, rvalue: Expression,
                                  self.is_typeshed_stub_file,
                                  allow_unnormalized=True,
                                  in_dynamic_func=dynamic,
-                                 global_scope=global_scope)
+                                 global_scope=global_scope,
+                                 warn_bound_tvar=warn_bound_tvar)
         if res:
             alias_tvars = [name for (name, _) in
                            res.accept(TypeVariableQuery(self.lookup_qualified, self.tvar_scope))]
@@ -1758,50 +1758,62 @@ def check_and_set_up_type_alias(self, s: AssignmentStmt) -> None:
         For subscripted (including generic) aliases the resulting types are stored
         in rvalue.analyzed.
         """
-        # Type aliases are created only at module scope and class scope (for subscripted types),
-        # at function scope assignments always create local variables with type object types.
         lvalue = s.lvalues[0]
-        if not isinstance(lvalue, NameExpr):
+        if len(s.lvalues) > 1 or not isinstance(lvalue, NameExpr):
+            # First rule: Only simple assignments like Alias = ... create aliases.
             return
-        if (len(s.lvalues) == 1 and not self.is_func_scope() and
-                not (self.type and isinstance(s.rvalue, NameExpr) and lvalue.is_def)
-                and not s.type):
-            rvalue = s.rvalue
-            res, alias_tvars = self.analyze_alias(rvalue, allow_unnormalized=True)
-            if not res:
-                return
-            node = self.lookup(lvalue.name, lvalue)
-            if not lvalue.is_def:
-                # Only a definition can create a type alias, not regular assignment.
-                if node and node.kind == TYPE_ALIAS or isinstance(node.node, TypeInfo):
-                    self.fail('Cannot assign multiple types to name "{}"'
-                              ' without an explicit "Type[...]" annotation'
-                              .format(lvalue.name), lvalue)
-                return
-            check_for_explicit_any(res, self.options, self.is_typeshed_stub_file, self.msg,
-                                   context=s)
-            # when this type alias gets "inlined", the Any is not explicit anymore,
-            # so we need to replace it with non-explicit Anys
-            res = make_any_non_explicit(res)
-            if isinstance(res, Instance) and not res.args and isinstance(rvalue, RefExpr):
-                # For simple (on-generic) aliases we use aliasing TypeInfo's
-                # to allow using them in runtime context where it makes sense.
-                node.node = res.type
-                if isinstance(rvalue, RefExpr):
-                    sym = self.lookup_type_node(rvalue)
-                    if sym:
-                        node.normalized = sym.normalized
-                return
-            node.kind = TYPE_ALIAS
-            node.type_override = res
-            node.alias_tvars = alias_tvars
-            if isinstance(rvalue, (IndexExpr, CallExpr)):
-                # We only need this for subscripted aliases, since simple aliases
-                # are already processed using aliasing TypeInfo's above.
-                rvalue.analyzed = TypeAliasExpr(res, node.alias_tvars,
-                                                fallback=self.alias_fallback(res))
-                rvalue.analyzed.line = rvalue.line
-                rvalue.analyzed.column = rvalue.column
+        if s.type:
+            # Second rule: Explicit type (cls: Type[A] = A) always creates variable, not alias.
+            return
+        non_global_scope = self.type or self.is_func_scope()
+        if isinstance(s.rvalue, NameExpr) and non_global_scope and lvalue.is_def:
+            # Third rule: Non-subscripted right hand side creates a variable
+            # at class and function scopes. For example:
+            #
+            #   class Model:
+            #       ...
+            #   class C:
+            #       model = Model # this is automatically a variable with type 'Type[Model]'
+            #
+            # without this rule, this typical use case will require a lot of explicit
+            # annotations (see the second rule).
+            return
+        rvalue = s.rvalue
+        res, alias_tvars = self.analyze_alias(rvalue, warn_bound_tvar=True)
+        if not res:
+            return
+        node = self.lookup(lvalue.name, lvalue)
+        if not lvalue.is_def:
+            # Type aliases can't be re-defined.
+            if node and (node.kind == TYPE_ALIAS or isinstance(node.node, TypeInfo)):
+                self.fail('Cannot assign multiple types to name "{}"'
+                          ' without an explicit "Type[...]" annotation'
+                          .format(lvalue.name), lvalue)
+            return
+        check_for_explicit_any(res, self.options, self.is_typeshed_stub_file, self.msg,
+                               context=s)
+        # when this type alias gets "inlined", the Any is not explicit anymore,
+        # so we need to replace it with non-explicit Anys
+        res = make_any_non_explicit(res)
+        if isinstance(res, Instance) and not res.args and isinstance(rvalue, RefExpr):
+            # For simple (on-generic) aliases we use aliasing TypeInfo's
+            # to allow using them in runtime context where it makes sense.
+            node.node = res.type
+            if isinstance(rvalue, RefExpr):
+                sym = self.lookup_type_node(rvalue)
+                if sym:
+                    node.normalized = sym.normalized
+            return
+        node.kind = TYPE_ALIAS
+        node.type_override = res
+        node.alias_tvars = alias_tvars
+        if isinstance(rvalue, (IndexExpr, CallExpr)):
+            # We only need this for subscripted aliases, since simple aliases
+            # are already processed using aliasing TypeInfo's above.
+            rvalue.analyzed = TypeAliasExpr(res, node.alias_tvars,
+                                            fallback=self.alias_fallback(res))
+            rvalue.analyzed.line = rvalue.line
+            rvalue.analyzed.column = rvalue.column
 
     def analyze_lvalue(self, lval: Lvalue, nested: bool = False,
                        add_global: bool = False,
@@ -3366,7 +3378,7 @@ def visit_index_expr(self, expr: IndexExpr) -> None:
         elif isinstance(expr.base, RefExpr) and expr.base.kind == TYPE_ALIAS:
             # Special form -- subscripting a generic type alias.
             # Perform the type substitution and create a new alias.
-            res, alias_tvars = self.analyze_alias(expr, allow_unnormalized=self.is_stub_file)
+            res, alias_tvars = self.analyze_alias(expr)
             expr.analyzed = TypeAliasExpr(res, alias_tvars, fallback=self.alias_fallback(res),
                                           in_runtime=True)
             expr.analyzed.line = expr.line

mypy/typeanal.py

@@ -63,7 +63,8 @@ def analyze_type_alias(node: Expression,
                        is_typeshed_stub: bool,
                        allow_unnormalized: bool = False,
                        in_dynamic_func: bool = False,
-                       global_scope: bool = True) -> Optional[Type]:
+                       global_scope: bool = True,
+                       warn_bound_tvar: bool = False) -> Optional[Type]:
     """Return type if node is valid as a type alias rvalue.
 
     Return None otherwise. 'node' must have been semantically analyzed.
@@ -117,7 +118,7 @@ def analyze_type_alias(node: Expression,
         return None
     analyzer = TypeAnalyser(lookup_func, lookup_fqn_func, tvar_scope, fail_func, note_func,
                             plugin, options, is_typeshed_stub, aliasing=True,
-                            allow_unnormalized=allow_unnormalized)
+                            allow_unnormalized=allow_unnormalized, warn_bound_tvar=warn_bound_tvar)
     analyzer.in_dynamic_func = in_dynamic_func
     analyzer.global_scope = global_scope
     return type.accept(analyzer)
@@ -154,7 +155,8 @@ def __init__(self,
                  aliasing: bool = False,
                  allow_tuple_literal: bool = False,
                  allow_unnormalized: bool = False,
-                 third_pass: bool = False) -> None:
+                 third_pass: bool = False,
+                 warn_bound_tvar: bool = False) -> None:
         self.lookup = lookup_func
         self.lookup_fqn_func = lookup_fqn_func
         self.fail_func = fail_func
@@ -168,6 +170,7 @@ def __init__(self,
         self.plugin = plugin
         self.options = options
         self.is_typeshed_stub = is_typeshed_stub
+        self.warn_bound_tvar = warn_bound_tvar
         self.third_pass = third_pass
 
     def visit_unbound_type(self, t: UnboundType) -> Type:
@@ -194,7 +197,11 @@ def visit_unbound_type(self, t: UnboundType) -> Type:
                 tvar_def = self.tvar_scope.get_binding(sym)
             else:
                 tvar_def = None
-            if sym.kind == TVAR and tvar_def is not None:
+            if self.warn_bound_tvar and sym.kind == TVAR and tvar_def is not None:
+                self.fail('Can\'t use bound type variable "{}"'
+                          ' to define generic alias'.format(t.name), t)
+                return AnyType(TypeOfAny.from_error)
+            elif sym.kind == TVAR and tvar_def is not None:
                 if len(t.args) > 0:
                     self.fail('Type variable "{}" used with arguments'.format(
                         t.name), t)

test-data/unit/check-classes.test

@@ -2119,17 +2119,17 @@ reveal_type(C().aa) # E: Revealed type is '__main__.A'
 [out]
 
 [case testClassValuedAttributesGeneric]
-from typing import Generic, TypeVar
+from typing import Generic, TypeVar, Type
 T = TypeVar('T')
 
 class A(Generic[T]):
     def __init__(self, x: T) -> None:
         self.x = x
 class B(Generic[T]):
-    a = A[T]
+    a: Type[A[T]] = A
 
-reveal_type(B[int]().a) # E: Revealed type is 'def (x: builtins.int*) -> __main__.A[builtins.int*]'
-B[int]().a('hi') # E: Argument 1 has incompatible type "str"; expected "int"
+reveal_type(B[int]().a) # E: Revealed type is 'Type[__main__.A[builtins.int*]]'
+B[int]().a('hi') # E: Argument 1 to "A" has incompatible type "str"; expected "int"
 
 class C(Generic[T]):
     a = A

test-data/unit/check-isinstance.test

@@ -1447,16 +1447,16 @@ def f(x: Union[Type[int], Type[str], Type[List]]) -> None:
         x()[1]  # E: Value of type "Union[int, str]" is not indexable
     else:
         reveal_type(x)  # E: Revealed type is 'Type[builtins.list[Any]]'
-        reveal_type(x())  # E: Revealed type is 'builtins.list[<nothing>]'
+        reveal_type(x())  # E: Revealed type is 'builtins.list[Any]'
         x()[1]
     reveal_type(x)  # E: Revealed type is 'Union[Type[builtins.int], Type[builtins.str], Type[builtins.list[Any]]]'
-    reveal_type(x())  # E: Revealed type is 'Union[builtins.int, builtins.str, builtins.list[<nothing>]]'
+    reveal_type(x())  # E: Revealed type is 'Union[builtins.int, builtins.str, builtins.list[Any]]'
     if issubclass(x, (str, (list,))):
         reveal_type(x)  # E: Revealed type is 'Union[Type[builtins.str], Type[builtins.list[Any]]]'
-        reveal_type(x())  # E: Revealed type is 'Union[builtins.str, builtins.list[<nothing>]]'
+        reveal_type(x())  # E: Revealed type is 'Union[builtins.str, builtins.list[Any]]'
         x()[1]
     reveal_type(x)  # E: Revealed type is 'Union[Type[builtins.int], Type[builtins.str], Type[builtins.list[Any]]]'
-    reveal_type(x())  # E: Revealed type is 'Union[builtins.int, builtins.str, builtins.list[<nothing>]]'
+    reveal_type(x())  # E: Revealed type is 'Union[builtins.int, builtins.str, builtins.list[Any]]'
 [builtins fixtures/isinstancelist.pyi]
 
 [case testIssubclasDestructuringUnions2]
@@ -1469,40 +1469,40 @@ def f(x: Type[Union[int, str, List]]) -> None:
         x()[1]  # E: Value of type "Union[int, str]" is not indexable
     else:
         reveal_type(x)  # E: Revealed type is 'Type[builtins.list[Any]]'
-        reveal_type(x())  # E: Revealed type is 'builtins.list[<nothing>]'
+        reveal_type(x())  # E: Revealed type is 'builtins.list[Any]'
         x()[1]
     reveal_type(x)  # E: Revealed type is 'Union[Type[builtins.int], Type[builtins.str], Type[builtins.list[Any]]]'
-    reveal_type(x())  # E: Revealed type is 'Union[builtins.int, builtins.str, builtins.list[<nothing>]]'
+    reveal_type(x())  # E: Revealed type is 'Union[builtins.int, builtins.str, builtins.list[Any]]'
     if issubclass(x, (str, (list,))):
         reveal_type(x)  # E: Revealed type is 'Union[Type[builtins.str], Type[builtins.list[Any]]]'
-        reveal_type(x())  # E: Revealed type is 'Union[builtins.str, builtins.list[<nothing>]]'
+        reveal_type(x())  # E: Revealed type is 'Union[builtins.str, builtins.list[Any]]'
         x()[1]
     reveal_type(x)  # E: Revealed type is 'Union[Type[builtins.int], Type[builtins.str], Type[builtins.list[Any]]]'
-    reveal_type(x())  # E: Revealed type is 'Union[builtins.int, builtins.str, builtins.list[<nothing>]]'
+    reveal_type(x())  # E: Revealed type is 'Union[builtins.int, builtins.str, builtins.list[Any]]'
 [builtins fixtures/isinstancelist.pyi]
 
 [case testIssubclasDestructuringUnions3]
 from typing import Union, List, Tuple, Dict, Type
 
 def f(x: Type[Union[int, str, List]]) -> None:
     reveal_type(x)  # E: Revealed type is 'Union[Type[builtins.int], Type[builtins.str], Type[builtins.list[Any]]]'
-    reveal_type(x())  # E: Revealed type is 'Union[builtins.int, builtins.str, builtins.list[<nothing>]]'
+    reveal_type(x())  # E: Revealed type is 'Union[builtins.int, builtins.str, builtins.list[Any]]'
     if issubclass(x, (str, (int,))):
         reveal_type(x)  # E: Revealed type is 'Union[Type[builtins.int], Type[builtins.str]]'
         reveal_type(x())  # E: Revealed type is 'Union[builtins.int, builtins.str]'
         x()[1]  # E: Value of type "Union[int, str]" is not indexable
     else:
         reveal_type(x)  # E: Revealed type is 'Type[builtins.list[Any]]'
-        reveal_type(x())  # E: Revealed type is 'builtins.list[<nothing>]'
+        reveal_type(x())  # E: Revealed type is 'builtins.list[Any]'
         x()[1]
     reveal_type(x)  # E: Revealed type is 'Union[Type[builtins.int], Type[builtins.str], Type[builtins.list[Any]]]'
-    reveal_type(x())  # E: Revealed type is 'Union[builtins.int, builtins.str, builtins.list[<nothing>]]'
+    reveal_type(x())  # E: Revealed type is 'Union[builtins.int, builtins.str, builtins.list[Any]]'
     if issubclass(x, (str, (list,))):
         reveal_type(x)  # E: Revealed type is 'Union[Type[builtins.str], Type[builtins.list[Any]]]'
-        reveal_type(x())  # E: Revealed type is 'Union[builtins.str, builtins.list[<nothing>]]'
+        reveal_type(x())  # E: Revealed type is 'Union[builtins.str, builtins.list[Any]]'
         x()[1]
     reveal_type(x)  # E: Revealed type is 'Union[Type[builtins.int], Type[builtins.str], Type[builtins.list[Any]]]'
-    reveal_type(x())  # E: Revealed type is 'Union[builtins.int, builtins.str, builtins.list[<nothing>]]'
+    reveal_type(x())  # E: Revealed type is 'Union[builtins.int, builtins.str, builtins.list[Any]]'
 [builtins fixtures/isinstancelist.pyi]
 
 [case testIssubclass]

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

@@ -96,6 +96,48 @@ GenAlias = Sequence[T]
 def fun(x: Alias) -> GenAlias[int]: pass
 [out]
 
+[case testCorrectQualifiedAliasesAlsoInFunctions]
+from typing import TypeVar, Generic
+T = TypeVar('T')
+S = TypeVar('S')
+
+class X(Generic[T]):
+    A = X[S]
+    def f(self) -> X[T]:
+        pass
+
+    a: X[T]
+    b: A = a
+    c: A[T] = a
+    d: A[int] = a # E: Incompatible types in assignment (expression has type "X[T]", variable has type "X[int]")
+
+    def g(self) -> None:
+        a: X[T]
+        b: X.A = a
+        c: X.A[T] = a
+        d: X.A[int] = a  # E: Incompatible types in assignment (expression has type "X[T]", variable has type "X[int]")
+
+def g(arg: X[int]) -> None:
+    p: X[int] = arg.f()
+    q: X.A = arg.f()
+    r: X.A[str] = arg.f() # E: Incompatible types in assignment (expression has type "X[int]", variable has type "X[str]")
+[out]
+
+[case testProhibitBoundTypeVariableReuseForAliases]
+from typing import TypeVar, Generic, List
+T = TypeVar('T')
+class C(Generic[T]):
+    A = List[T] # E: Can't use bound type variable "T" to define generic alias
+
+x: C.A
+reveal_type(x) # E: Revealed type is 'builtins.list[Any]'
+
+def f(x: T) -> T:
+    A = List[T] # E: Can't use bound type variable "T" to define generic alias
+    return x
+[builtins fixtures/list.pyi]
+[out]
+
 [case testTypeAliasInBuiltins]
 def f(x: bytes): pass
 bytes

test-data/unit/check-typevar-values.test

@@ -556,16 +556,15 @@ def outer(x: T) -> T:
 
 [case testClassMemberTypeVarInFunctionBody]
 from typing import TypeVar, List
+S = TypeVar('S')
 class C:
     T = TypeVar('T', bound=int)
     def f(self, x: T) -> T:
-        L = List[C.T] # this creates a variable, not an alias
-        reveal_type(L)  # E: Revealed type is 'Overload(def () -> builtins.list[T`-1], def (x: typing.Iterable[T`-1]) -> builtins.list[T`-1])'
-        y: C.T = x
-        L().append(x)
+        L = List[S]
+        y: L[C.T] = [x]
         C.T  # E: Type variable "C.T" cannot be used as an expression
         A = C.T  # E: Type variable "C.T" cannot be used as an expression
-        return L()[0]
+        return y[0]
 
 [builtins fixtures/list.pyi]