Sema: Partial revert of bbe8c20

Since it is source-breaking with existing code, we decided not to
take the TypeAliasType fix for the 5.9 branch.

Author: slavapestov

lib/Sema/TypeCheckGeneric.cpp

@@ -352,16 +352,6 @@ void TypeChecker::checkReferencedGenericParams(GenericContext *dc) {
         return Action::SkipChildren;
       }
 
-      // Don't walk into generic type alias substitutions. This does
-      // not constrain `T`:
-      //
-      //   typealias Foo<T> = Int
-      //   func foo<T>(_: Foo<T>) {}
-      if (auto *aliasTy = dyn_cast<TypeAliasType>(ty.getPointer())) {
-        Type(aliasTy->getSinglyDesugaredType()).walk(*this);
-        return Action::SkipChildren;
-      }
-
       return Action::Continue;
     }
 

test/type/pack_expansion.swift

@@ -106,4 +106,4 @@ func unusedParameterPack1<each T: Sequence>(_: repeat (each T).Element) {}
 typealias First<T, U> = T
 
 func unusedParameterPack2<each T>(_: repeat First<Int, each T>) {}
-// expected-error@-1 {{generic parameter 'T' is not used in function signature}}
+// allowed