single(()=>false) should have the same behavior on empty and non-empty sequences as first(()=>false) does.

[ghuser]

Bug Report

In the documentation of single I read:

Like first, but emit with error notification if there is more than one value.

The above statement is reasonable but it is not the case in the implementation.

Current Behavior

When using single(()=>false) on an empty sequence there is an error correctly thrown notifying about no elements in sequence. But when using single(()=>false) on a single-element or multiple-element sequence there is one undefined value emitted (unlike first(()=>false) behaviour ).

Reproduction

from([]).pipe(single(()=>false)).pipe(tap((v)=>console.log(v))).subscribe();
//Error [EmptyError]: no elements in sequence]

from(['x']).pipe(single(()=>false)).pipe(tap((v)=>console.log(v))).subscribe();
//undefined

from([]).pipe(first(()=>false)).pipe(tap((v)=>console.log(v))).subscribe();
//[Error [EmptyError]: no elements in sequence]

from(['x']).pipe(first(()=>false)).pipe(tap((v)=>console.log(v))).subscribe();
//[Error [EmptyError]: no elements in sequence]

Expected behavior

In my opinion, the same or some other error should be thrown on the latter case of single also, instead of an undefined value, as it happens with the case of first(()=>false).

Environment

• Runtime: [e.g. Node v12.13.1]
• RxJS version: 6.5.4

Possible Solution

Additional context/Screenshots

[benlesh]

It seems like we're missing an error type for something like "Element Not Found" or whatever. It's not really that the sequence was empty so much as it was nothing matched.

[benlesh]

It looks like single is wrong in a lot of cases.

If it finds two matches, it just completes and does not error, which is incorrect. It should throw. This needs fixed for v7.

[cartant]

Closed by #5325 AFAICT.