Fix #996

mypy/constraints.py

@@ -169,7 +169,7 @@ def visit_instance(self, template: Instance) -> List[Constraint]:
             if (self.direction == SUBTYPE_OF and
                     template.type.has_base(instance.type.fullname())):
                 mapped = map_instance_to_supertype(template, instance.type)
-                for i in range(len(instance.args)):
+                for i in range(min(len(mapped.args), len(instance.args))):
                     # The constraints for generic type parameters are
                     # invariant. Include the default constraint and its
                     # negation to achieve the effect.

mypy/test/data/semanal-errors.test

@@ -72,6 +72,24 @@ y = 0 # type: A[A]
 main:5: error: "B" expects 1 type argument, but 2 given
 main:6: error: "A" expects no type arguments, but 1 given
 
+[case testInvalidNumberOfGenericArgsWithExplicitArgument]
+from typing import TypeVar, Generic
+t = TypeVar('t')
+class A(object):
+    def __init__(self, a: t) -> None:
+        self.a = a
+x = A(a=1) # type: A[None]
+[out]
+main:6: error: "A" expects no type arguments, but 1 given
+
+[case testInvalidNumberOfGenericArgsInInstantiatedType]
+from typing import TypeVar, Generic
+t = TypeVar('t')
+class A(Generic[t]): pass
+x = A() # type: X[None, None]
+[out]
+main:4: error: "A" expects 1 type argument, but 2 given
+
 [case testInvalidNumberOfGenericArgsInUndefinedArg]
 
 class A: pass