Don't raise in TypeAnalyser.visit_type_var().

The raise caused two specific errors that are now fixed.  The fix
causes no new errors.  I don't understand what's going on, but the
raise has been there since prehistoric times, and may well have been
misguided in the first place.

Fixes #1898.

Author: Guido van Rossum

mypy/typeanal.py

@@ -201,7 +201,7 @@ def visit_instance(self, t: Instance) -> Type:
         return t
 
     def visit_type_var(self, t: TypeVarType) -> Type:
-        raise RuntimeError('TypeVarType is already analyzed')
+        return t
 
     def visit_callable_type(self, t: CallableType) -> Type:
         return t.copy_modified(arg_types=self.anal_array(t.arg_types),

test-data/unit/check-generics.test

@@ -857,3 +857,21 @@ from typing import TypeVar, Container
 T = TypeVar('T')
 def f(x: Container[T]) -> T: ...
 reveal_type(f((1, 2))) # E: Revealed type is 'builtins.int*'
+
+[case testClassMethodInGenericClassWithGenericConstructorArg]
+from typing import TypeVar, Generic
+T = TypeVar('T')
+class A(Generic[T]):
+    def __init__(self, a: T) -> None: pass
+    @classmethod
+    def f(cls) -> None: pass
+[builtins fixtures/classmethod.py]
+
+[case testClassMethodInClassWithGenericConstructor]
+from typing import TypeVar, Generic
+T = TypeVar('T')
+class A:
+    def __init__(self, a: T) -> None: pass
+    @classmethod
+    def f(cls) -> None: pass
+[builtins fixtures/classmethod.py]