From 4047c0a8707c17006f812700041c8d2215e76bb1 Mon Sep 17 00:00:00 2001
From: Robert Jackson <rjackson@linkedin.com>
Date: Tue, 23 Apr 2019 18:33:38 -0400
Subject: [PATCH 1/2] [FEATURE] Ensure `fn` can be nested.

---
 .../-internals/glimmer/lib/helpers/fn.ts      |  5 +--
 .../tests/integration/helpers/fn-test.js      | 32 +++++++++++++++++++
 2 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/packages/@ember/-internals/glimmer/lib/helpers/fn.ts b/packages/@ember/-internals/glimmer/lib/helpers/fn.ts
index 3a0474514b9..aae4a13e818 100644
--- a/packages/@ember/-internals/glimmer/lib/helpers/fn.ts
+++ b/packages/@ember/-internals/glimmer/lib/helpers/fn.ts
@@ -4,6 +4,7 @@ import { DEBUG } from '@glimmer/env';
 import { Arguments, VM } from '@glimmer/runtime';
 import { ICapturedArguments } from '@glimmer/runtime/dist/types/lib/vm/arguments';
 import { InternalHelperReference } from '../utils/references';
+import { Opaque } from '@glimmer/util';
 
 let context: any = null;
 if (DEBUG && HAS_NATIVE_PROXY) {
@@ -45,10 +46,10 @@ function fnHelper({ positional }: ICapturedArguments) {
     typeof positional.at(0).value() === 'function'
   );
 
-  return () => {
+  return (...invocationArgs: Opaque[]) => {
     let [fn, ...args] = positional.value();
 
-    return fn!['apply'](context, args);
+    return fn!['call'](context, ...args, ...invocationArgs);
   };
 }
 
diff --git a/packages/@ember/-internals/glimmer/tests/integration/helpers/fn-test.js b/packages/@ember/-internals/glimmer/tests/integration/helpers/fn-test.js
index 03b81dfee1d..3c38cea437b 100644
--- a/packages/@ember/-internals/glimmer/tests/integration/helpers/fn-test.js
+++ b/packages/@ember/-internals/glimmer/tests/integration/helpers/fn-test.js
@@ -158,6 +158,38 @@ if (EMBER_GLIMMER_FN_HELPER) {
 
         assert.equal(this.stashedFn(), 'arg1: foo, arg2: bar');
       }
+
+      '@test partially applies each layer when nested [GH#17959]'() {
+        this.render(`{{invoke (fn (fn (fn this.myFunc this.arg1) this.arg2) this.arg3)}}`, {
+          myFunc(arg1, arg2, arg3) {
+            return `arg1: ${arg1}, arg2: ${arg2}, arg3: ${arg3}`;
+          },
+
+          arg1: 'foo',
+          arg2: 'bar',
+          arg3: 'qux',
+        });
+
+        this.assertText('arg1: foo, arg2: bar, arg3: qux');
+        this.assertStableRerender();
+
+        runTask(() => set(this.context, 'arg1', 'qux'));
+        this.assertText('arg1: qux, arg2: bar, arg3: qux');
+
+        runTask(() => set(this.context, 'arg2', 'derp'));
+        this.assertText('arg1: qux, arg2: derp, arg3: qux');
+
+        runTask(() => set(this.context, 'arg3', 'huzzah'));
+        this.assertText('arg1: qux, arg2: derp, arg3: huzzah');
+
+        runTask(() => {
+          set(this.context, 'arg1', 'foo');
+          set(this.context, 'arg2', 'bar');
+          set(this.context, 'arg3', 'qux');
+        });
+
+        this.assertText('arg1: foo, arg2: bar, arg3: qux');
+      }
     }
   );
 }

From 33fdfdc02c8adf9eaace5ac223b2ed2d072984bb Mon Sep 17 00:00:00 2001
From: Ricardo Mendes <rokusu@gmail.com>
Date: Wed, 24 Apr 2019 10:09:12 +0100
Subject: [PATCH 2/2] Fix import ordering lint

---
 packages/@ember/-internals/glimmer/lib/helpers/fn.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/@ember/-internals/glimmer/lib/helpers/fn.ts b/packages/@ember/-internals/glimmer/lib/helpers/fn.ts
index aae4a13e818..9843f77604c 100644
--- a/packages/@ember/-internals/glimmer/lib/helpers/fn.ts
+++ b/packages/@ember/-internals/glimmer/lib/helpers/fn.ts
@@ -3,8 +3,8 @@ import { assert } from '@ember/debug';
 import { DEBUG } from '@glimmer/env';
 import { Arguments, VM } from '@glimmer/runtime';
 import { ICapturedArguments } from '@glimmer/runtime/dist/types/lib/vm/arguments';
-import { InternalHelperReference } from '../utils/references';
 import { Opaque } from '@glimmer/util';
+import { InternalHelperReference } from '../utils/references';
 
 let context: any = null;
 if (DEBUG && HAS_NATIVE_PROXY) {