Remove an unnecessary assertion. (swiftlang#60046)

I did check that the code doesn't actually rely on not seeing a
type parameter.

rdar://96571508

Author: rjmccall

lib/AST/ExistentialGeneralization.cpp

@@ -267,10 +267,14 @@ class Generalizer : public CanTypeVisitor<Generalizer, Type> {
 ExistentialTypeGeneralization
 ExistentialTypeGeneralization::get(Type rawType) {
   assert(rawType->isAnyExistentialType());
-  assert(!rawType->hasTypeParameter());
 
   // Canonicalize.  We need to generalize the canonical shape of the
   // type or else generalization parameters won't match up.
+  //
+  // TODO: in full generality, do we need to do *contextual*
+  // canonicalization in order to avoid introducing non-canonical
+  // parameters?  (That is, do we need a contextual generic
+  // signature if given an interface type?)
   CanType type = rawType->getCanonicalType();
 
   Generalizer generalizer(type->getASTContext());

test/Interpreter/parameterized_existentials.swift

@@ -44,5 +44,14 @@ ParameterizedProtocolsTestSuite.test("casting") {
   expectEqual(a.value, b.value)
 }
 
+// rdar://96571508
+struct ErasingHolder<T> {
+  let box: any Holder<T>
+}
+ParameterizedProtocolsTestSuite.test("casting") {
+  let a = ErasingHolder(box: IntHolder(value: 5))
+  expectEqual(a.box.value, 5)
+}
+
 runAllTests()