Translate the pattern type of a pack expansion when emitting

type metadata for layout.

Fixes rdar://110971671

Author: rjmccall

lib/IRGen/MetadataRequest.cpp

@@ -3307,11 +3307,10 @@ class GetFormalTypeWithSameLayout
   }
 
   CanType visitPackExpansionType(CanPackExpansionType ty) {
-    return ty;
-  }
-
-  CanType visitPackElementType(CanPackElementType ty) {
-    llvm_unreachable("not implemented for PackElementType");
+    CanType pattern = ty.getPatternType();
+    CanType loweredPattern = visit(ty.getPatternType());
+    if (pattern == loweredPattern) return ty;
+    return CanPackExpansionType::get(loweredPattern, ty.getCountType());
   }
 
   CanType visitTupleType(CanTupleType ty) {

test/IRGen/variadic_generics.sil

@@ -177,3 +177,19 @@ bb0:
   %len = pack_length $Pack{repeat each T, repeat each T, Int, Float, String}
   return %len : $Builtin.Word
 }
+
+// Translate the pattern type of a pack expansion type when
+// emitting tuple type metadata for layout.  rdar://110971671
+
+// CHECK-LABEL: define{{.*}} @test_pack_expansion_for_layout
+// CHECK:         [[PACK_LEN:%.*]] = add [[INT]] %0, 1
+// CHECK-NEXT:    [[VANISHES:%.*]] = icmp eq [[INT]] [[PACK_LEN]], 1
+// CHECK-NEXT:    br i1 [[VANISHES]],
+// CHECK:         call swiftcc %swift.metadata_response @swift_getTupleTypeMetadata(
+sil @test_pack_expansion_for_layout : $<each T> () -> () {
+bb0:
+  %tuple = alloc_stack $(Int, repeat () async throws -> each T)
+  dealloc_stack %tuple : $*(Int, repeat () async throws -> each T)
+  %result = tuple ()
+  return %result : $()
+}