Pylance generic type error

[pasunx]

Environment data

• Language Server version: 2021.3.4
• OS and version: win32 x64
• Python version (and distribution if applicable, e.g. Anaconda):
• python.analysis.indexing: undefined
• python.analysis.typeCheckingMode: strict

Code Snippet / Additional information

class A:

    class B: ...

    a = list["A.B"]()   # ok but no color
    b: list["A.B"] = [] # A is class but B is Unknown

A.a.append(A.B()) # ok
A.b.append(A.B()) # error:
                  #    Argument of type "B" cannot be assigned to parameter "__object" of type "Literal['A.B']" in function "append"
                  #      "B" cannot be assigned to type "Literal['A.B']" Pylance (reportGeneralTypeIssues)

[erictraut]

Thanks for reporting the problem.

This appears to be a problem in the order of expression evaluation within pyright. The semantic highlighting is causing a different evaluation order than would otherwise occur, and this exposes a bug in the type evaluator. The problem doesn't occur (or is much harder to trigger) when semantic highlighting is disabled. I'll investigate further.

[erictraut]

I found the source of the bug. It was a regression that I introduced about two months ago when adding support for PEP 637. It resulted in inconsistent type evaluations of string literals within type annotations based on the order in which nodes were evaluated. Pylance's semantic highlighting feature caused nodes to be visited in a different order than they otherwise would be, and that exposed the bug.

This will be fixed in the next release of Pylance.

[jakebailey]

This issue has been fixed in version 2021.4.0, which we've just released. You can find the changelog here: https://github.com/microsoft/pylance-release/blob/main/CHANGELOG.md#202140-7-april-2021