Type inference has failed by parameter order (regression)

Anders Hejlsberg (@ahejlsberg)

Intended degradation?

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.

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

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

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

You have to enable strictNullChecks option.

Anders Hejlsberg (@ahejlsberg)

you are looking at a result of #19745.

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

#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.

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.

Thanks!

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Type inference has failed by parameter order (regression) #19858

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Type inference has failed by parameter order (regression) #19858

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mhegazy commented on Nov 9, 2017

mhegazy commented on Nov 9, 2017

falsandtru commented on Nov 9, 2017

falsandtru commented on Nov 9, 2017

gcnew commented on Nov 10, 2017

ahejlsberg commented on Nov 10, 2017

falsandtru commented on Nov 10, 2017

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