Allow generics in runtime expressions (#2302)

As discussed in python/typing#303 generics in ``typing.py`` are now cached and are therefore much faster at runtime than before. Here I propose (and it looks like Guido is not against this) to allow generic types as runtime expressions.

Author: ilevkivskyi

mypy/checkexpr.py

@@ -1373,8 +1373,8 @@ def visit_reveal_type_expr(self, expr: RevealTypeExpr) -> Type:
 
     def visit_type_application(self, tapp: TypeApplication) -> Type:
         """Type check a type application (expr[type, ...])."""
-        self.chk.fail(messages.GENERIC_TYPE_NOT_VALID_AS_EXPRESSION, tapp)
-        return AnyType()
+        tp = cast(CallableType, self.accept(tapp.expr))
+        return self.apply_generic_arguments(tp, tapp.types, tapp)
 
     def visit_type_alias_expr(self, alias: TypeAliasExpr) -> Type:
         return AnyType()

mypy/messages.py

@@ -77,8 +77,6 @@
 READ_ONLY_PROPERTY_OVERRIDES_READ_WRITE = \
     'Read-only property cannot override read-write property'
 FORMAT_REQUIRES_MAPPING = 'Format requires a mapping'
-GENERIC_TYPE_NOT_VALID_AS_EXPRESSION = \
-    "Generic type is prohibited as a runtime expression (use a type alias or '# type:' comment)"
 RETURN_TYPE_CANNOT_BE_CONTRAVARIANT = "Cannot use a contravariant type variable as return type"
 FUNCTION_PARAMETER_CANNOT_BE_COVARIANT = "Cannot use a covariant type variable as a parameter"
 INCOMPATIBLE_IMPORT_OF = "Incompatible import of"
@@ -681,7 +679,7 @@ def incompatible_type_application(self, expected_arg_count: int,
                                       actual_arg_count: int,
                                       context: Context) -> None:
         if expected_arg_count == 0:
-            self.fail('Type application targets a non-generic function',
+            self.fail('Type application targets a non-generic function or class',
                       context)
         elif actual_arg_count > expected_arg_count:
             self.fail('Type application has too many types ({} expected)'

test-data/unit/check-generics.test

@@ -422,33 +422,83 @@ main: note: In member "f" of class "A":
 -- ----------------------------
 
 
-[case testInvalidTypeApplicationTarget]
+[case testTypeApplicationArgs]
 from typing import TypeVar, Generic
 T = TypeVar('T')
-class A(Generic[T]): pass
-A[A]() # E: Generic type is prohibited as a runtime expression (use a type alias or '# type:' comment)
-A[int, str]() # E: Generic type is prohibited as a runtime expression (use a type alias or '# type:' comment)
+class Node(Generic[T]):
+    def __init__(self, x: T) -> None:
+        ...
+Node[int]()  # E: Too few arguments for "Node"
+Node[int](1, 1, 1)  # E: Too many arguments for "Node"
 [out]
 
-[case testInvalidTypeApplicationTarget2]
+[case testTypeApplicationTvars]
 from typing import TypeVar, Generic
 T = TypeVar('T')
 S = TypeVar('S')
 class A(Generic[T, S]): pass
-A[int, str]() # E: Generic type is prohibited as a runtime expression (use a type alias or '# type:' comment)
-A[int]() # E: Generic type is prohibited as a runtime expression (use a type alias or '# type:' comment)
+A[int]()  # E: Type application has too few types (2 expected)
+A[int, str, int]() # E: Type application has too many types (2 expected)
 [out]
 
-[case testInvalidTypeApplicationTarget3]
-
+[case testInvalidTypeApplicationType]
 a = None # type: A
 class A: pass
-a[A]() # Fail
-A[A]() # Fail
+a[A]()  # E: Value of type "A" is not indexable
+A[A]()  # E: Type application targets a non-generic function or class
 [out]
-main:4: error: Value of type "A" is not indexable
-main:5: error: Generic type is prohibited as a runtime expression (use a type alias or '# type:' comment)
 
+[case testTypeApplicationArgTypes]
+from typing import TypeVar, Generic
+T = TypeVar('T')
+class Node(Generic[T]):
+    def __init__(self, x: T) -> None:
+        ...
+
+Node[int](1)
+Node[int]('a')  # E: Argument 1 to "Node" has incompatible type "str"; expected "int"
+
+class Dummy(Generic[T]):
+    def meth(self, x: T) -> None:
+        ...
+    def methout(self) -> T:
+        ...
+
+Dummy[int]().meth(1)
+Dummy[int]().meth('a')  # E: Argument 1 to "meth" of "Dummy" has incompatible type "str"; expected "int"
+reveal_type(Dummy[int]())  # E: Revealed type is '__main__.Dummy[builtins.int*]'
+reveal_type(Dummy[int]().methout())  # E: Revealed type is 'builtins.int*'
+[out]
+
+[case testTypeApplicationArgTypesSubclasses]
+from typing import TypeVar, Generic
+T = TypeVar('T')
+S = TypeVar('S')
+class C(Generic[T, S]):
+    def __init__(self, x: T, y: S) -> None:
+        ...
+
+class D(C[int, T], Generic[T]): ...
+
+D[str](1, 'a')
+D[str](1, 1)  # E: Argument 2 to "D" has incompatible type "int"; expected "str"
+
+class E(D[str]): ...
+E(1, 'a')
+E(1, 1)  # E: Argument 2 to "E" has incompatible type "int"; expected "str"
+[out]
+
+[case testTypeApplicationAlias]
+from typing import TypeVar, Generic
+T = TypeVar('T')
+class Node(Generic[T]):
+    def __init__(self, x: T) -> None:
+        ...
+
+Alias = Node
+Alias[int](1)
+Alias[int]("a")  # E: Argument 1 to "Node" has incompatible type "str"; expected "int"
+[out]
 
 -- Multiple assignment with lists
 -- ------------------------------