should allow type predicates when `isType(badInput as any)`

Bug Report

🔎 Search Terms

🕗 Version & Regression Information

• This is a crash
• This changed between versions ______ and _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

https://www.typescriptlang.org/play?#code/LAKAZgrgdgxgLgSwPZQAQIM4EEBOOCGAngBQJQAOEcAXKvlIQNoC6qAPqlBALYBGApjgCUtMpTjoMdBi1ABvUAEgc-OBBxo4OCP1ABfUKAA2q1L3wATAJIUqtDFrIBzANyGQCMKlLY8RYubWthL4UvSEQkLySoE24gB0AFZIZMQA5GlRIAYgoAD0eagAAnAYALQITlBIKqCe3pi4BCSxwZHRIIqtCcmpGVk5QA

💻 Code

function isArray(input: any[] | number): input is any[]
{
	return true
}

let badInput: string;

if (isArray(badInput as any))
{
	badInput.join('')
}

// @ts-ignore
if (isArray(badInput))
{
	badInput.join('')
}

🙁 Actual behavior

🙂 Expected behavior

badInput should know as array when use as any

if (isArray(badInput as any))
{
	badInput.join('')
}

Start with the linked TypeScript Playground and reproduce the difference between isArray(badInput as any) and the @ts-ignore case. Read the type-checking behavior around user-defined type predicates and compare the inferred type inside each branch. Done means the casted call narrows badInput to an array without suppressing the diagnostic.