--- a/js/src/jsfun.cpp
+++ b/js/src/jsfun.cpp
@@ -1290,16 +1290,40 @@ JSFunction::infallibleIsDefaultClassCons
     }
 
     MOZ_ASSERT_IF(isDefault, isConstructor());
     MOZ_ASSERT_IF(isDefault, isClassConstructor());
     return isDefault;
 }
 
 bool
+JSFunction::isDerivedClassConstructor()
+{
+    bool derived;
+    if (isInterpretedLazy()) {
+        // There is only one plausible lazy self-hosted derived
+        // constructor.
+        if (isSelfHostedBuiltin()) {
+            JSAtom* name = &getExtendedSlot(LAZY_FUNCTION_NAME_SLOT).toString()->asAtom();
+
+            // This function is called from places without access to a
+            // JSContext. Trace some plumbing to get what we want.
+            derived = name == compartment()->runtimeFromAnyThread()->
+                              commonNames->DefaultDerivedClassConstructor;
+        } else {
+            derived = lazyScript()->isDerivedClassConstructor();
+        }
+    } else {
+        derived = nonLazyScript()->isDerivedClassConstructor();
+    }
+    MOZ_ASSERT_IF(derived, isClassConstructor());
+    return derived;
+}
+
+bool
 JSFunction::getLength(JSContext* cx, uint16_t* length)
 {
     JS::RootedFunction self(cx, this);
     MOZ_ASSERT(!self->isBoundFunction());
     if (self->isInterpretedLazy() && !self->getOrCreateScript(cx))
         return false;
 
     *length = self->hasScript() ? self->nonLazyScript()->funLength()

--- a/js/src/jsfun.h
+++ b/js/src/jsfun.h
@@ -533,25 +533,17 @@ class JSFunction : public js::NativeObje
         return u.n.jitinfo;
     }
 
     void setJitInfo(const JSJitInfo* data) {
         MOZ_ASSERT(isNative());
         u.n.jitinfo = data;
     }
 
-    bool isDerivedClassConstructor() {
-        bool derived;
-        if (isInterpretedLazy())
-            derived = lazyScript()->isDerivedClassConstructor();
-        else
-            derived = nonLazyScript()->isDerivedClassConstructor();
-        MOZ_ASSERT_IF(derived, isClassConstructor());
-        return derived;
-    }
+    bool isDerivedClassConstructor();
 
     static unsigned offsetOfNativeOrScript() {
         static_assert(offsetof(U, n.native) == offsetof(U, i.s.script_),
                       "native and script pointers must be in the same spot "
                       "for offsetOfNativeOrScript() have any sense");
         static_assert(offsetof(U, n.native) == offsetof(U, nativeOrScript),
                       "U::nativeOrScript must be at the same offset as "
                       "native");