Default value seems to be ignored

[Robloche]

Using the following syntax (which has worked until recent flow update), I get an incompatible-call error:

const foo: (a: number, b?: number) => number = (a, b = 100) => Math.min(a, b);

This syntax, however, works:

const foo = (a: number, b?: number = 100): number => Math.min(a, b);

But these two syntaxes should be equivalent, right?

Try:
https://flow.org/try/#0MYewdgzgLgBAZiEMC8MAUBDAXDMBXAWwCMBTAJwBoYiB+HfY8lGARgAY2BKew0slAHwwAshigALAHQEAlmExUinANwAoVaEiwiGMjkw9GlanVy9ynQWaPMF1Zuy5XRE6XLtK1G8NGoYAXvrY1nyKhnyWyEIMfLYYipFCLlKy8vHUKqpAA