IntelliSense for generic Promise function shows truncated return type

TS Template added by @mjbvz

TypeScript Version: 4.0.0-dev.20200702

Search Terms

• hover / quick info
• truncation

---

• VSCode Version: 1.46.1 (user setup)
• OS Version: Windows_NT x64 10.0.18363

Steps to Reproduce:

App.ts:

import { Repro} from "./Repro";

function test()
{
    Repro.repro<number>("test");
}

Repro.ts:

export namespace Repro
{
    export interface Arg { argName: string, argValue: any }

 // export async function repro<T>(AbcdefghijklmnopqrstuvwxyzAbcdefghijklmnopqrstuvwxyzAbcdefghijklmnopqrstuvwxyzAbcdefghijklmnopqrstuvwxyzAbcdefghijkl: string, ...methodArgs: Arg[]): Promise<T> // Working
    export async function repro<T>(AbcdefghijklmnopqrstuvwxyzAbcdefghijklmnopqrstuvwxyzAbcdefghijklmnopqrstuvwxyzAbcdefghijklmnopqrstuvwxyzAbcdefghijklX: string, ...methodArgs: Arg[]): Promise<T> // Broken
    {
        return (new Promise<T>(null));
    }
}

Problem:

In App.ts, hover over the .repro function. The IntelliSense shows repro as having a return type of Promise<...>. This is incorrect: it should be Promise<number>.

Now, in Repro.ts edit the repro function by remove the trailing X in the first parameter name. Return to App.ts and note that the IntelliSense for repro is now correct (the function return type is shown as Promise<number>).

While at first glance this may appear to be an edge case due to the long identifier name, it is not. I ran into this problem with "normal length" parameters, but the example above was the simplest repro I could find. Here's the signature of the function where I originally saw the problem:

export async function postForkAsync<T>(destinationInstance: string, methodName: string, methodVersion: number, errorTemplate: string = null, resultTimeoutInMs: number = -1, ...methodArgs: PostMethodArg[]): Promise<T>

Does this issue occur when all extensions are disabled?: Yes

Start by reproducing the hover behavior using App.ts and Repro.ts, including the long generic function parameter and the version with the trailing X. Trace the IntelliSense return-type display to determine why it shows Promise<...> instead of Promise; done means the reproduced hover shows the complete generic return type without relying on parameter-name length.