SIL: Fix crash when enum is nested inside a fully-constrained extension

Fixes <https://bugs.swift.org/browse/SR-10023>.

Author: slavapestov

lib/SIL/TypeLowering.cpp

@@ -1840,8 +1840,15 @@ CanAnyFunctionType TypeConverter::makeConstantInterfaceType(SILDeclRef c) {
     return getFunctionInterfaceTypeWithCaptures(funcTy, func);
   }
 
-  case SILDeclRef::Kind::EnumElement:
-    return cast<AnyFunctionType>(vd->getInterfaceType()->getCanonicalType());
+  case SILDeclRef::Kind::EnumElement: {
+    auto funcTy = cast<AnyFunctionType>(
+                                   vd->getInterfaceType()->getCanonicalType());
+    auto sig = getEffectiveGenericSignature(vd->getDeclContext());
+    return CanAnyFunctionType::get(sig,
+                                   funcTy->getParams(),
+                                   funcTy.getResult(),
+                                   funcTy->getExtInfo());
+  }
 
   case SILDeclRef::Kind::Allocator: {
     auto *cd = cast<ConstructorDecl>(vd);

test/SILGen/constrained_extensions.swift

@@ -212,6 +212,15 @@ extension Array where Element == AnyObject {
     // CHECK-LABEL: sil hidden [ossa] @$sSa22constrained_extensionsyXlRszlE12DerivedClassCfE : $@convention(method) (@guaranteed Array<AnyObject>.DerivedClass) -> ()
     var e: Element? = nil
   }
+
+  enum NestedEnum {
+    case hay
+    case grain
+
+    func makeHay() -> NestedEnum {
+      return .hay
+    }
+  }
 }
 
 func referenceNestedTypes() {