False positive for: list((1,2))+[3,]

[colinRawlings]

Environment data

• Language Server version: 2020.10.0
• OS and version: Win 10 Build 20175
• Python version (& distribution if applicable, e.g. Anaconda): Python.org 3.6.8

Expected behaviour

No red squiggle under the expression: list((1,2))+[3,]

Actual behaviour

Red squiggle with message:

 Operator "+" not supported for types "list[Literal[1, 2]]" and "List[int]"
 Operator "+" not supported for types "list[Literal[1, 2]]" and "List[int]"Pylance 
(reportGeneralTypeIssues)

Also not sure why the message is duplicated?

Logs

XXX

Code Snippet / Additional information

XXX

[jakebailey]

I think this is somewhat expected; the first list is inferred to be List[Literal[1, 2]] and that isn't compatible with the other type.

If you write the following, it type checks:

x: List[int] = list((1,2))
y = x + [3,]

[erictraut]

@jakebailey, I'll take a deeper look at this one. The expression (1,2) is inferred to be Tuple[Literal[1], Literal[2]], and I think that's correct. But there's a question (not defined in any of the PEPs) about how a tuple with heterogeneous entry types should be translated into an Iterable. Pyright is currently unioning all of the individual types, which is why it results in an Iterable[Literal[1, 2]] in this case. But arguably, literals should be stripped during this conversion, resulting to Iterable[int]. That would then produce a List[int] when passed through the list constructor. I need to investigate more to see if this would break other things.

[erictraut]

I've improved the bidirectional type inference for tuple expressions. The logic is now smarter about when to honor literals and when to strip them.

This will be included in the next release of pylance.

[jakebailey]

This issue has been fixed in version 2020.10.2, which we've just released. You can find the changelog here: https://github.com/microsoft/pylance-release/blob/master/CHANGELOG.md#2020102-21-october-2020