Implementation for RFC #756

packages/@glimmer/manager/lib/internal/defaults.ts

@@ -0,0 +1,36 @@
+import { CAPABILITIES, CapturedArguments as Arguments } from '@glimmer/interfaces';
+
+type FnArgs<Args extends Arguments = Arguments> =
+  | [...Args['positional'], Args['named']]
+  | [...Args['positional'], {}];
+
+interface FunctionHelperState<Args extends Arguments = Arguments> {
+  fn: <Return>(...args: FnArgs<Args>) => Return;
+  args: Args;
+}
+
+export class FunctionHelperManager<State extends FunctionHelperState> {
+  capabilities = {
+    [CAPABILITIES]: true,
+    hasValue: true,
+    hasDestroyable: false,
+    hasScheduledEffect: false,
+  } as const;
+
+  createHelper(fn: State['fn'], args: State['args']) {
+    return { fn, args };
+  }
+
+  getValue({ fn, args }: State) {
+    let argsForFn: FnArgs<Arguments> = [
+      ...args.positional,
+      Object.keys(args.named).length > 0 ? args.named : {},
+    ];
+
+    return fn(...argsForFn);
+  }
+
+  getDebugName(fn: State['fn']) {
+    return fn.name || '(anonymous function)';
+  }
+}

packages/@glimmer/manager/lib/internal/index.ts

@@ -7,6 +7,7 @@ import {
   Owner,
 } from '@glimmer/interfaces';
 import { CustomHelperManager } from '../public/helper';
+import { FunctionHelperManager } from './defaults';
 
 type InternalManager =
   | InternalComponentManager
@@ -138,7 +139,18 @@ export function getInternalHelperManager(
     );
   }
 
-  const manager = getManager(HELPER_MANAGERS, definition)!;
+  let manager = getManager(HELPER_MANAGERS, definition)!;
+
+  // Functions are special-cased because functions are defined
+  // as the "default" helper, per: https://github.com/emberjs/rfcs/pull/756
+  if (manager === undefined && typeof definition === 'function') {
+    setInternalHelperManager(
+      new CustomHelperManager(() => new FunctionHelperManager()),
+      definition
+    );
+
+    manager = getManager(HELPER_MANAGERS, definition)!;
+  }
 
   if (manager === undefined) {
     if (isOptional === true) {