From 6fd128ff676b9142a258e48642d123dc9d788568 Mon Sep 17 00:00:00 2001
From: Hamish Knight <hamish_knight@apple.com>
Date: Fri, 13 Jun 2025 19:33:00 +0100
Subject: [PATCH] [Sema] Scale back CodingKeys hack in
 `TypeChecker::lookupUnqualifiedType`

I previously added this hack to match the logic in
`TypeChecker::lookupUnqualified`, but it turns out that can introduce
request cycles for cases where `CodingKeys` is used in a generic
requirement for one of `Codable`'s potential value witnesses. Scale
back the hack such that it's only done when we get an initial empty
lookup result, ensuring we maintain source compatibility. Both these
lookup hacks should go away once we properly handle CodingKeys
synthesis.

rdar://153096639
---
 lib/Sema/TypeCheckNameLookup.cpp | 14 +++++++++-----
 test/Sema/rdar153096639.swift    | 18 ++++++++++++++++++
 2 files changed, 27 insertions(+), 5 deletions(-)
 create mode 100644 test/Sema/rdar153096639.swift

diff --git a/lib/Sema/TypeCheckNameLookup.cpp b/lib/Sema/TypeCheckNameLookup.cpp
index 9eb184aaffd41..7d4853b9d3738 100644
--- a/lib/Sema/TypeCheckNameLookup.cpp
+++ b/lib/Sema/TypeCheckNameLookup.cpp
@@ -325,9 +325,6 @@ TypeChecker::lookupUnqualifiedType(DeclContext *dc, DeclNameRef name,
                                    NameLookupOptions options) {
   auto &ctx = dc->getASTContext();
 
-  // HACK: Synthesize CodingKeys if needed.
-  synthesizeCodingKeysIfNeededForUnqualifiedLookup(ctx, dc, name);
-
   auto ulOptions = convertToUnqualifiedLookupOptions(options) |
                    UnqualifiedLookupFlags::TypeLookup;
   {
@@ -336,8 +333,15 @@ TypeChecker::lookupUnqualifiedType(DeclContext *dc, DeclNameRef name,
         name, dc, loc,
         ulOptions - UnqualifiedLookupFlags::AllowProtocolMembers);
 
-    auto lookup =
-        evaluateOrDefault(ctx.evaluator, UnqualifiedLookupRequest{desc}, {});
+    UnqualifiedLookupRequest req(desc);
+    auto lookup = evaluateOrDefault(ctx.evaluator, req, {});
+
+    // HACK: Try synthesize CodingKeys if we got an empty result.
+    if (lookup.allResults().empty() && name.isSimpleName(ctx.Id_CodingKeys)) {
+      synthesizeCodingKeysIfNeededForUnqualifiedLookup(ctx, dc, name);
+      lookup = evaluateOrDefault(ctx.evaluator, req, {});
+    }
+
     if (!lookup.allResults().empty())
       return lookup;
   }
diff --git a/test/Sema/rdar153096639.swift b/test/Sema/rdar153096639.swift
new file mode 100644
index 0000000000000..a5960ccb544c5
--- /dev/null
+++ b/test/Sema/rdar153096639.swift
@@ -0,0 +1,18 @@
+// RUN: %target-typecheck-verify-swift
+
+// Make sure we don't run into a request cycle for the below use of 'CodingKeys'
+// in an `init(from:)` signature.
+
+protocol P {
+  associatedtype X
+}
+
+struct S: Codable {
+  var foo: String?
+
+  enum CodingKeys: CodingKey {
+    case foo
+  }
+
+  init<T: P>(from: T) where T.X == CodingKeys {}
+}