Eagerly create init(from:) when looking up 'init' on a Decodable type (#17712)

Otherwise, the initializer won't be inherited properly onto a
subclass, resulting in the base class being allocated instead of the
subclass when using Sub.init(from:).

https://bugs.swift.org/browse/SR-8083

Author: jrose-apple

lib/Sema/TypeCheckDecl.cpp

@@ -6383,6 +6383,9 @@ void TypeChecker::synthesizeMemberForLookup(NominalTypeDecl *target,
   //
   // Returns whether the target conforms to the protocol.
   auto evaluateTargetConformanceTo = [&](ProtocolDecl *protocol) {
+    if (!protocol)
+      return false;
+
     auto targetType = target->getDeclaredInterfaceType();
     if (auto ref = conformsToProtocol(
                         targetType, protocol, target,
@@ -6420,12 +6423,11 @@ void TypeChecker::synthesizeMemberForLookup(NominalTypeDecl *target,
     }
   } else {
     auto argumentNames = member.getArgumentNames();
-    if (argumentNames.size() != 1)
+    if (member.isCompoundName() && argumentNames.size() != 1)
       return;
 
-    auto argumentName = argumentNames.front();
     if (baseName == DeclBaseName::createConstructor() &&
-        argumentName == Context.Id_from) {
+        (member.isSimpleName() || argumentNames.front() == Context.Id_from)) {
       // init(from:) may be synthesized as part of derived conformance to the
       // Decodable protocol.
       // If the target should conform to the Decodable protocol, check the
@@ -6434,7 +6436,8 @@ void TypeChecker::synthesizeMemberForLookup(NominalTypeDecl *target,
       (void)evaluateTargetConformanceTo(decodableProto);
     } else if (!baseName.isSpecial() &&
                baseName.getIdentifier() == Context.Id_encode &&
-               argumentName == Context.Id_to) {
+               (member.isSimpleName() ||
+                argumentNames.front() == Context.Id_to)) {
       // encode(to:) may be synthesized as part of derived conformance to the
       // Encodable protocol.
       // If the target should conform to the Encodable protocol, check the

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

@@ -0,0 +1,25 @@
+// RUN: %target-swift-frontend -emit-silgen %s | %FileCheck %s
+
+class SR8083_Base: Codable {
+  var thing: String { return "Abstract" }
+}
+
+class SR8083_Sub: SR8083_Base {
+  override var thing: String { return "Yo" }
+}
+
+func sr8083(decoder: Decoder) throws {
+  _ = try SR8083_Sub(from: decoder)
+}
+
+// CHECK-LABEL: sil_vtable SR8083_Base {
+// CHECK-DAG: #SR8083_Base.init!initializer.1: (SR8083_Base.Type) -> () -> SR8083_Base : @$S23class_codable_inherited11SR8083_BaseCACycfc	// SR8083_Base.init()
+// CHECK-DAG: #SR8083_Base.init!allocator.1: (SR8083_Base.Type) -> (Decoder) throws -> SR8083_Base : @$S23class_codable_inherited11SR8083_BaseC4fromACs7Decoder_p_tKcfC
+// CHECK-DAG: #SR8083_Base.init!initializer.1: (SR8083_Base.Type) -> (Decoder) throws -> SR8083_Base : @$S23class_codable_inherited11SR8083_BaseC4fromACs7Decoder_p_tKcfc	// SR8083_Base.init(from:)
+// CHECK: {{^}$}}
+
+// CHECK-LABEL: sil_vtable SR8083_Sub {
+// CHECK-DAG: #SR8083_Base.init!initializer.1: (SR8083_Base.Type) -> () -> SR8083_Base : @$S23class_codable_inherited10SR8083_SubCACycfc [override]	// SR8083_Sub.init()
+// CHECK-DAG: #SR8083_Base.init!allocator.1: (SR8083_Base.Type) -> (Decoder) throws -> SR8083_Base : @$S23class_codable_inherited10SR8083_SubC4fromACs7Decoder_p_tKcfC [override]	// SR8083_Sub.__allocating_init(from:)
+// CHECK-DAG: #SR8083_Base.init!initializer.1: (SR8083_Base.Type) -> (Decoder) throws -> SR8083_Base : @$S23class_codable_inherited10SR8083_SubC4fromACs7Decoder_p_tKcfc [override]	// SR8083_Sub.init(from:)
+// CHECK: {{^}$}}