[Sema] Error on generic arg lists with different lengths

Don't assert that generic arg lists have the same lengths. We don't
currently allow nested generics, but we try to type-check anyway, and
one of the effects is sticking the generic params for both the outer and
inner classes on the inner type. Instead, just error out.

<rdar://problem/21967211>

lib/Sema/CSSimplify.cpp

@@ -1038,7 +1038,9 @@ ConstraintSystem::matchDeepEqualityTypes(Type type1, Type type2,
   // Match up the generic arguments, exactly.
   auto args1 = bound1->getGenericArgs();
   auto args2 = bound2->getGenericArgs();
-  assert(args1.size() == args2.size() && "Mismatched generic args");
+  if (args1.size() != args2.size()) {
+    return SolutionKind::Error;
+  }
   for (unsigned i = 0, n = args1.size(); i != n; ++i) {
     switch (matchTypes(args1[i], args2[i], TypeMatchKind::SameType,
                        TMF_GenerateConstraints,

test/Generics/generic_types.swift

@@ -312,3 +312,13 @@ class Top {}
 class Bottom<T : Bottom<Top>> {} // expected-error 2{{type may not reference itself as a requirement}}
 // expected-error@-1{{Bottom' requires that 'Top' inherit from 'Bottom<Top>'}}
 // expected-note@-2{{requirement specified as 'T' : 'Bottom<Top>' [with T = Top]}}
+
+class X6<T> {
+  let d: D<T>
+  init(_ value: T) {
+    d = D(value) // expected-error{{cannot invoke initializer for type 'X6<T>.D<_, _>' with an argument list of type '(T)'}} expected-note{{expected an argument list of type '(T2)'}}
+  }
+  class D<T2> { // expected-error{{generic type 'D' nested in type 'X6' is not allowed}}
+    init(_ value: T2) {}
+  }
+}