Patch A Regression in Lookup for CodingKeys

Codable's magic previously relied on the subject of every qualified lookup in an
unqualified lookup stack to force the synthesis of this member. This
allowed users to reference CodingKeys transitively through a non-primary input
without qualification. As part of the requestification of name lookup,
this synthesis was moved out of the normal qualified lookup path and into the
Type Checker's semantic lookup entrypoints in order to prevent wild
cycles caused by protocol conformance resolution. In the process, we
forget to restore the synthesis check at this entrypoint.

To patch up the source break this caused, we need to walk the context
stack again and force synthesis. Unfortunately, we're stuck with a hack like
this until we bring Codable's implementation back out of the realm of magic
once more. A future implementation of synthesizeSemanticMembersIfNeeded
should aim to just craft the AST for CodingKeys, but not actually run
any of the semantic checks until we check the conformance to CodingKey.

rdar://65088901, SR-13137

Author: CodaFi

lib/Sema/TypeCheckNameLookup.cpp

@@ -220,8 +220,26 @@ convertToUnqualifiedLookupOptions(NameLookupOptions options) {
 LookupResult TypeChecker::lookupUnqualified(DeclContext *dc, DeclNameRef name,
                                             SourceLoc loc,
                                             NameLookupOptions options) {
-  auto ulOptions = convertToUnqualifiedLookupOptions(options);
   auto &ctx = dc->getASTContext();
+  // HACK: Qualified lookup cannot be allowed to synthesize CodingKeys because
+  // it would lead to a number of egregious cycles through
+  // QualifiedLookupRequest when we resolve the protocol conformance. Codable's
+  // magic has pushed its way so deeply into the compiler, we have to
+  // pessimistically force every nominal context above this one to synthesize
+  // it in the event the user needs it from e.g. a non-primary input.
+  // We can undo this if Codable's semantic content is divorced from its
+  // syntactic content - so we synthesize just enough to allow lookups to
+  // succeed, but don't force protocol conformances while we're doing it.
+  if (name.getBaseIdentifier() == ctx.Id_CodingKeys) {
+    for (auto typeCtx = dc->getInnermostTypeContext(); typeCtx != nullptr;
+         typeCtx = typeCtx->getParent()->getInnermostTypeContext()) {
+      if (auto *nominal = typeCtx->getSelfNominalTypeDecl()) {
+        nominal->synthesizeSemanticMembersIfNeeded(name.getFullName());
+      }
+    }
+  }
+
+  auto ulOptions = convertToUnqualifiedLookupOptions(options);
   auto descriptor = UnqualifiedLookupDescriptor(name, dc, loc, ulOptions);
   auto lookup = evaluateOrDefault(ctx.evaluator,
                                   UnqualifiedLookupRequest{descriptor}, {});

test/decl/protocol/special/coding/Inputs/struct_codable_simple_multi1.swift

@@ -21,3 +21,11 @@ struct SimpleStruct : Codable {
     let _ = SimpleStruct.CodingKeys.z // expected-error {{type 'SimpleStruct.CodingKeys' has no member 'z'}}
   }
 }
+
+// SR-13137 Ensure unqualified lookup installs CodingKeys regardless of the
+// order of primaries.
+struct A: Codable {
+  var property: String
+  static let propertyName = CodingKeys.property.stringValue
+}
+

test/decl/protocol/special/coding/Inputs/struct_codable_simple_multi2.swift

@@ -8,3 +8,11 @@ func foo() {
   // struct.
   let _ = SimpleStruct.CodingKeys.self // expected-error {{'CodingKeys' is inaccessible due to 'private' protection level}}
 }
+
+struct B {
+  static let propertyName = A.propertyName
+  
+  struct Nest {
+    static let propertyName = A.propertyName
+  }
+}