[PreCheckExpr] Allow pack references when resolving types during TypeExpr folding.

Author: hborla

lib/Sema/PreCheckExpr.cpp

@@ -1435,8 +1435,13 @@ TypeExpr *PreCheckExpression::simplifyNestedTypeExpr(UnresolvedDotExpr *UDE) {
   // Fold 'T.U' into a nested type.
 
   // Resolve the TypeRepr to get the base type for the lookup.
+  TypeResolutionOptions options(TypeResolverContext::InExpression);
+  // Pre-check always allows pack references during TypeExpr folding.
+  // CSGen will diagnose cases that appear outside of pack expansion
+  // expressions.
+  options |= TypeResolutionFlags::AllowPackReferences;
   const auto BaseTy = TypeResolution::resolveContextualType(
-      InnerTypeRepr, DC, TypeResolverContext::InExpression,
+      InnerTypeRepr, DC, options,
       [](auto unboundTy) {
         // FIXME: Don't let unbound generic types escape type resolution.
         // For now, just return the unbound generic type.

test/Constraints/pack-expansion-expressions.swift

@@ -105,3 +105,14 @@ func tupleExpansion<each T, each U>(
   _ = zip(repeat each tuple1.element, with: repeat each tuple2.element)
   // expected-error@-1 {{global function 'zip(_:with:)' requires the type packs 'U' and 'T' have the same shape}}
 }
+
+protocol Generatable {
+  static func generate() -> Self
+}
+
+func generateTuple<each T : Generatable>() -> (repeat each T) {
+  (each T).generate()
+  // expected-error@-1 {{pack reference 'T' can only appear in pack expansion or generic requirement}}
+
+  return (repeat (each T).generate())
+}