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Co-authored-by: Chris Krycho <hello@chriskrycho.com>

packages/@ember/debug/lib/warn.ts

@@ -9,11 +9,14 @@ export interface WarnOptions {
 }
 
 export type RegisterHandlerFunc = (handler: HandlerCallback<WarnOptions>) => void;
-export type WarnFunc = (
-  message: string,
-  test: boolean | WarnOptions,
-  options?: WarnOptions
-) => void;
+export interface WarnFunc {
+  (message: string, options: WarnOptions): void;
+  (
+    message: string,
+    test: boolean,
+    options: WarnOptions
+  ): void;
+}
 
 let registerHandler: RegisterHandlerFunc = () => {};
 let warn: WarnFunc = () => {};
@@ -111,7 +114,10 @@ if (DEBUG) {
     assert(missingOptionsDeprecation, Boolean(options));
     assert(missingOptionsIdDeprecation, Boolean(options && options.id));
 
-    invoke('warn', message, test as boolean, options);
+    // SAFETY: we have explicitly assigned `false` if the user invoked the
+    // arity-2 version of the overload, so we know `test` is always either
+    // `undefined` or a `boolean` for type-safe callers.
+    invoke('warn', message, test as boolean | undefined, options);
   };
 }