AST: Fix swift::substOpaqueTypesWithUnderlyingTypes() to pass PreservePackExpansionLevel

Fixes rdar://problem/123645784.

Author: slavapestov

lib/AST/Type.cpp

@@ -5364,10 +5364,10 @@ CanType swift::substOpaqueTypesWithUnderlyingTypes(CanType ty,
   ReplaceOpaqueTypesWithUnderlyingTypes replacer(
       context.getContext(), context.getResilienceExpansion(),
       context.isWholeModuleContext());
-  SubstOptions flags = SubstFlags::SubstituteOpaqueArchetypes;
+  SubstOptions flags = (SubstFlags::SubstituteOpaqueArchetypes |
+                        SubstFlags::PreservePackExpansionLevel);
   if (allowLoweredTypes)
-    flags =
-        SubstFlags::SubstituteOpaqueArchetypes | SubstFlags::AllowLoweredTypes;
+    flags |= SubstFlags::AllowLoweredTypes;
   return ty.subst(replacer, replacer, flags)->getCanonicalType();
 }
 

test/SILGen/Inputs/variadic_generic_opaque_multifile_other.swift

@@ -0,0 +1,11 @@
+public protocol P {
+  func f()
+}
+
+public struct G<each T>: P {
+  public func f() {}
+}
+
+public func callee<each T>(_: repeat each T) -> some P {
+    G<repeat each T>()
+}

test/SILGen/variadic_generic_opaque_multifile.swift

@@ -0,0 +1,6 @@
+// RUN: %target-swift-frontend -emit-silgen -primary-file %s %S/Inputs/variadic_generic_opaque_multifile_other.swift -disable-availability-checking
+// RUN: %target-swift-frontend -emit-silgen %s %S/Inputs/variadic_generic_opaque_multifile_other.swift -disable-availability-checking
+
+public func caller() {
+  callee(1, 2, 3).f()
+}