Type inference has failed by parameter order (regression)

[falsandtru]

Intended degradation?

cc @ahejlsberg

TypeScript Version: master

Code

declare function f<a>(as1: a[], as2: a[], cmp: (a1: a, a2: a) => number) 
f(Array.from([0]), [], (a1, a2) => a1 - a2);
f(Array.from([]), [0], (a1, a2) => a1 - a2);

Expected behavior:

no error

Actual behavior:

$ node built/local/tsc.js index.ts -t es6 --strictNullChecks
index.ts(3,36): error TS2362: The left-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.
index.ts(3,41): error TS2363: The right-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.

[mhegazy]

c:\test\sandbox7>type a.ts
declare function f<a>(as1: a[], as2: a[], cmp: (a1: a, a2: a) => number): void;

f(Array.from([0]), [], (a1, a2) => a1 - a2);
f(Array.from([]), [0], (a1, a2) => a1 - a2);

c:\test\sandbox7>tsc --v
Version 2.7.0-dev.20171109

c:\test\sandbox7>tsc a.ts --lib es6

c:\test\sandbox7>echo %ERRORLEVEL%
0

[mhegazy]

I think your example is not correct, and you are looking at a result of #19745.

[falsandtru]

c:\test\sandbox7>tsc a.ts --lib es6

You have to enable strictNullChecks option.

[falsandtru]

you are looking at a result of #19745.

Seems like your are right but this result is bad. @ahejlsberg Is this your intended result?

[gcnew]

#19745 fixes a legitimate issue. However, I do agree that the exhibited behaviour here is also a bug.

The problem is that Array.from([]) should either be:

• contextually typed
• inferred as <T> T[], i.e. an array of a "to be specified" generic type

Currently the call to Array.from is eagerly resolved to Array.from<{}> while before it was incorrectly resolved to Array.from<never>.

In any case, the root cause is #9366.

[ahejlsberg]

It is indeed a regression caused by #19745 where we make no inferences from the never element type of an empty array literal. Another way we could solve #19576 (the issue addressed by #19745) is to give inferences from the element type of empty array literals lower priority than all other inferences. That would still fix #19576 but wouldn't affect the code in this issue.

[falsandtru]

Thanks!