TypeScript Version: 3.5.1
Search Terms: parameter, inference, order, generic, function expression, function declaration
Code
type Callback<TFoo, TBar> = (foo: TFoo, bar: TBar) => any
declare function example<TFoo, TBar, TCallback extends Callback<TFoo, TBar>>(
foo: TFoo,
callback: TCallback,
bar: TBar,
): TCallback
// bar is infered to be 'string'
example(42, (foo, bar) => ({
t: () => { }
}), '42')
// bar isn't infered and is 'unknown'
example(42, (foo, bar) => ({
t() { }
}), '42')
Expected behavior:foo and bar should be inferred as number and string respectively in both cases
Actual behavior:
When not using arrow function expression bar is not inferred, even though foo is infered in both cases
Playground Link: click
Related Issues: This code worked in TS 3.3.3 , so #31814 is most definitely related, and so probably #30215 is the root cause. #6627 is somewhat related.
Comment: Type inference stops working for types which appear in arguments after inner call to a generic function when function declaration appears in callback return. Using function expressions instead is limiting because those does not have correct this context.
Is this behavior caused by the same limitations as #31814, or is it something what can be improved?
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