fix(types): restore AxiosError.cause type from unknown to Error (#7327)

* Initial plan

* fix(types): restore AxiosError.cause type from unknown to Error

Co-authored-by: jasonsaayman <4814473+jasonsaayman@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jasonsaayman <4814473+jasonsaayman@users.noreply.github.com>

index.d.cts

@@ -100,7 +100,7 @@ declare class AxiosError<T = unknown, D = any> extends Error {
   isAxiosError: boolean;
   status?: number;
   toJSON: () => object;
-  cause?: unknown;
+  cause?: Error;
   event?: BrowserProgressEvent;
   static from<T = unknown, D = any>(
     error: Error | unknown,

index.d.ts

@@ -431,7 +431,7 @@ export class AxiosError<T = unknown, D = any> extends Error {
   isAxiosError: boolean;
   status?: number;
   toJSON: () => object;
-  cause?: unknown;
+  cause?: Error;
   event?: BrowserProgressEvent;
   static from<T = unknown, D = any>(
     error: Error | unknown,

test/module/typings/esm/index.ts

@@ -720,3 +720,12 @@ axios.get('/user', {
     return ['127.0.0.1', 4];
   }
 });
+
+// AxiosError.cause should be typed as Error to allow accessing .message
+axios.get('/user').catch((error: AxiosError) => {
+  if (error.cause) {
+    // This should not produce a type error - cause is typed as Error
+    const causeMessage: string | undefined = error.cause.message;
+    console.log(causeMessage);
+  }
+});