Sema: Relax enum parameter pack restriction for CodingKeys

Author: slavapestov

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -3226,11 +3226,13 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
 
     // Temporary restriction until we figure out pattern matching and
     // enum case construction with packs.
-    if (auto genericSig = ED->getGenericSignature()) {
-      for (auto paramTy : genericSig.getGenericParams()) {
-        if (paramTy->isParameterPack()) {
-          ED->diagnose(diag::enum_with_pack);
-          break;
+    if (!ED->isSynthesized()) {
+      if (auto genericSig = ED->getGenericSignature()) {
+        for (auto paramTy : genericSig.getGenericParams()) {
+          if (paramTy->isParameterPack()) {
+            ED->diagnose(diag::enum_with_pack);
+            break;
+          }
         }
       }
     }

test/decl/protocol/special/coding/codable_parameter_pack.swift

@@ -0,0 +1,7 @@
+// RUN: %target-typecheck-verify-swift -target %target-swift-5.9-abi-triple
+
+// We should accept this:
+
+public struct HasPack<each T>: Codable {
+  var x: String?
+}