diff --git a/lib/AST/ConformanceLookup.cpp b/lib/AST/ConformanceLookup.cpp
index 4ee498b5ce322..60299b1729f59 100644
--- a/lib/AST/ConformanceLookup.cpp
+++ b/lib/AST/ConformanceLookup.cpp
@@ -728,6 +728,8 @@ LookupConformanceInModuleRequest::evaluate(
 ProtocolConformanceRef
 ModuleDecl::checkConformance(Type type, ProtocolDecl *proto,
                              bool allowMissing) {
+  assert(!type->hasTypeParameter());
+
   auto lookupResult = lookupConformance(type, proto, allowMissing);
   if (lookupResult.isInvalid()) {
     return ProtocolConformanceRef::forInvalid();
diff --git a/lib/IDE/ConformingMethodList.cpp b/lib/IDE/ConformingMethodList.cpp
index 144ff2888eb51..505aab0f2369c 100644
--- a/lib/IDE/ConformingMethodList.cpp
+++ b/lib/IDE/ConformingMethodList.cpp
@@ -174,8 +174,19 @@ void ConformingMethodListCallbacks::getMatchingMethods(
       if (FD->isStatic() || FD->isOperator())
         return false;
 
-      auto resultTy = T->getTypeOfMember(CurModule, FD,
-                                         FD->getResultInterfaceType());
+      assert(!T->hasTypeParameter());
+
+      // T may contain primary archetypes from some fixed generic signature G.
+      // This might be unrelated to the generic signature of FD. However if
+      // FD has a generic parameter of its own and it returns a type containing
+      // that parameter, we want to map it to the corresponding archetype
+      // from the generic environment of FD, because all we do with the
+      // resulting type is check conformance. If the conformance is conditional,
+      // we might run into trouble with really complicated cases but the fake
+      // archetype setup will mostly work.
+      auto substitutions = T->getMemberSubstitutionMap(
+          CurModule, FD, FD->getGenericEnvironment());
+      auto resultTy =  FD->getResultInterfaceType().subst(substitutions);
       if (resultTy->is<ErrorType>())
         return false;
 
diff --git a/lib/Sema/TypeCheckDeclPrimary.cpp b/lib/Sema/TypeCheckDeclPrimary.cpp
index ab41da9255e30..d71a6b8397082 100644
--- a/lib/Sema/TypeCheckDeclPrimary.cpp
+++ b/lib/Sema/TypeCheckDeclPrimary.cpp
@@ -1396,7 +1396,7 @@ static void diagnoseClassWithoutInitializers(ClassDecl *classDecl) {
   if (auto *superclassDecl = classDecl->getSuperclassDecl()) {
     auto *decodableProto = C.getProtocol(KnownProtocolKind::Decodable);
     auto superclassType = superclassDecl->getDeclaredInterfaceType();
-    auto ref = classDecl->getParentModule()->checkConformance(
+    auto ref = classDecl->getParentModule()->lookupConformance(
         superclassType, decodableProto);
     if (ref) {
       // super conforms to Decodable, so we've failed to inherit init(from:).
@@ -1425,7 +1425,7 @@ static void diagnoseClassWithoutInitializers(ClassDecl *classDecl) {
       // likely that the user forgot to override its encode(to:). In this case,
       // we can produce a slightly different diagnostic to suggest doing so.
       auto *encodableProto = C.getProtocol(KnownProtocolKind::Encodable);
-      auto ref = classDecl->getParentModule()->checkConformance(
+      auto ref = classDecl->getParentModule()->lookupConformance(
           superclassType, encodableProto);
       if (ref) {
         // We only want to produce this version of the diagnostic if the
diff --git a/test/decl/protocol/special/coding/class_codable_inheritance_diagnostics.swift b/test/decl/protocol/special/coding/class_codable_inheritance_diagnostics.swift
index a26f25bc64c68..c5c0c13673b0e 100644
--- a/test/decl/protocol/special/coding/class_codable_inheritance_diagnostics.swift
+++ b/test/decl/protocol/special/coding/class_codable_inheritance_diagnostics.swift
@@ -97,3 +97,13 @@ class EncodableSubWithoutInitialValue : CodableSuper { // expected-error {{class
 class CodableSubWithInitialValue : CodableSuper {
   var value2 = 10
 }
+
+class GenericCodableSuper<T>: Decodable {}
+
+class GenericCodableSub<T>: GenericCodableSuper<T> {
+// expected-error@-1 {{class 'GenericCodableSub' has no initializers}}
+// expected-note@-2 {{did you mean to override 'init(from:)'?}}
+// expected-warning@-2 {{'required' initializer 'init(from:)' must be provided by subclass of 'GenericCodableSuper<T>'; this is an error in Swift 6}}
+  var t: T
+  // expected-note@-1 {{stored property 't' without initial value prevents synthesized initializers}}
+}
\ No newline at end of file