Union type narrowing with generics not working in TS 3.5 RC strict mode

[ghost]

TypeScript Version: 3.5.0-rc / next

Search Terms: 3.5, 3.5 union, 3.5 strict

Code

test.ts:

function fooT<T = string>(bar: T[] | T | undefined) {
  if (bar !== undefined && !Array.isArray(bar))
    throw new Error("expected array")
  else if (bar !== undefined && bar.length === 0) console.log("empty array")
  else console.log("array or undef.")
}

CLI:

npx [email protected] --strict true test.ts // compiles
npx typescript@next --strict true test.ts // error

Expected behavior:
Compiles and behaves like ts 3.4.5

Actual behavior:
Error:

test.ts:4:37 - error TS2339: Property 'length' does not exist on type 'T | T[]'.
  Property 'length' does not exist on type 'T'.

Related Issues:
Cannot judge, if it is a duplicate of #31380 .
This is some specific case to base further evaluation on.

[ahejlsberg]

This is an effect of #30637. Previously, unconstrained type parameters were treated as if they had the constraint {} and therefore could never be null or undefined. That isn't correct because an unconstrained T can be instantiated at any type, including null or undefined. In 3.5 we now check it correctly. That specifically means that bar !== undefined doesn't imply that T disappears from the type in the false branch.

If you explicitly add a constraint to T you get the expected behavior:

function fooT<T extends {} = string>(bar: T[] | T | undefined) {
    // ...
}

[ghost]

thank you for clarification @ahejlsberg, should have read the changelog more carefully.
The new constraint is a bit more verbose, but makes sense to me logically.
Closing the issue!