Give opened archetypes a generic environment.

And maybe allow nested types to live on them.

lib/AST/GenericSignatureBuilder.cpp

@@ -2811,6 +2811,8 @@ static void concretizeNestedTypeFromConcreteParent(
         ->getTypeWitness(assocType, builder.getLazyResolver());
     if (!witnessType || witnessType->hasError())
       return; // FIXME: should we delay here?
+  } else if (auto archetype = concreteParent->getAs<ArchetypeType>()) {
+    witnessType = archetype->getNestedType(assocType->getName());
   } else {
     witnessType = DependentMemberType::get(concreteParent, assocType);
   }
@@ -2902,7 +2904,7 @@ PotentialArchetype *PotentialArchetype::updateNestedTypeForConformance(
 
 void ArchetypeType::resolveNestedType(
                                     std::pair<Identifier, Type> &nested) const {
-  auto genericEnv = getPrimary()->getGenericEnvironment();
+  auto genericEnv = getGenericEnvironment();
   auto &builder = *genericEnv->getGenericSignatureBuilder();
 
   Type interfaceType = getInterfaceType();
@@ -3886,7 +3888,7 @@ ResolvedType GenericSignatureBuilder::maybeResolveEquivalenceClass(
   }
 
   // The equivalence class of a dependent member type is determined by its
-  // base equivalence class.
+  // base equivalence class, if there is one.
   if (auto depMemTy = type->getAs<DependentMemberType>()) {
     // Find the equivalence class of the base.
     auto resolvedBase =
@@ -3894,6 +3896,9 @@ ResolvedType GenericSignatureBuilder::maybeResolveEquivalenceClass(
                                    resolutionKind,
                                    wantExactPotentialArchetype);
     if (!resolvedBase) return resolvedBase;
+    // If the base is concrete, so is this member.
+    if (resolvedBase.getAsConcreteType())
+      return ResolvedType::forConcrete(type);
 
     // Find the nested type declaration for this.
     auto baseEquivClass = resolvedBase.getEquivalenceClass(*this);