Fix internal error on invalid number of type args

Fixes #996. This is a more general alternative to PR #997.

Author: JukkaL

mypy/test/data/check-semanal-error.test

@@ -54,3 +54,10 @@ class A(X): # E: Invalid type "__main__.X"
     x = 1
 A().foo(1) # E: "A" has no attribute "foo"
 A().x = '' # E: Incompatible types in assignment (expression has type "str", variable has type "int")
+
+[case testInvalidNumberOfTypeArgs]
+from typing import TypeVar
+T = TypeVar('T')
+class C:  # Forgot to add type params here
+    def __init__(self, t: T) -> None: pass
+c = C(t=3)  # type: C[int]  # E: "C" expects no type arguments, but 1 given

mypy/typeanal.py

@@ -301,6 +301,10 @@ def visit_instance(self, t: Instance) -> None:
                 act = 'none'
             self.fail('"{}" expects {}, but {} given'.format(
                 info.name(), s, act), t)
+            # Construct the correct number of type arguments, as
+            # otherwise the type checker may crash as it expects
+            # things to be right.
+            t.args = [AnyType() for _ in info.type_vars]
         elif info.defn.type_vars:
             # Check type argument values.
             for arg, TypeVar in zip(t.args, info.defn.type_vars):