Error merging non-generic and generic Protocol classes

[marcinwrochna]

Environment data

• Pylance language server 2021.3.3 (pyright be28f16e)
• Python: 3.9.2
• OS: Fedora 33

Actual behaviour

Defining the merge of a generic Protocol (such as Iterable[T]) and a non-generic Protocol (such as Sized) makes Pylance end with a cryptic error Error: Debug Failure. False expression. raised from canAssignProtocolClassToSelf.

The following example is from PEP-544:

from typing import Iterable, Protocol, Sized, TypeVar

T = TypeVar('T')
class SizedIterable(Iterable[T], Sized, Protocol):
    pass

The same happens if I make T covariant instead, or if I replace Protocol with Protocol[T] or Protocol, Generic[T].

In fact already the following fails:

T = TypeVar('T')

class FirstProtocol(Protocol):
    def first():
        ...
        
class MergedProtocol(FirstProtocol, Protocol[T]):
    def merged() -> T:
        ...

Logs

[Info  - 4:15:07 PM] Pylance language server 2021.3.3 (pyright be28f16e) starting
[Info  - 4:15:07 PM] Server root directory: /home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist
[Info  - 4:15:07 PM] No configuration file found.
[Info  - 4:15:07 PM] Setting pythonPath for service "vscode": "/bin/python"
Search paths found for configured python interpreter:
  /usr/lib64/python3.9
  /usr/lib64/python3.9/lib-dynload
  /home/token0/.local/lib/python3.9/site-packages
  /usr/lib64/python3.9/site-packages
  /usr/lib/python3.9/site-packages
[Warn  - 4:15:07 PM] stubPath /home/token0/tmp/vscode/typings is not a valid directory.
[Info  - 4:15:07 PM] Assuming Python version 3.9
[Info  - 4:15:07 PM] Assuming Python platform Linux
[Info  - 4:15:07 PM] Searching for source files
[Info  - 4:15:07 PM] Found 1 source file
[Info  - 4:15:07 PM] Background analysis(1) root directory: /home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist
[Info  - 4:15:07 PM] Background analysis(1) started
Background analysis message: setConfigOptions
Background analysis message: setTrackedFiles
Background analysis message: markAllFilesDirty
Background analysis message: ensurePartialStubPackages
Background analysis message: setFileOpened
Background analysis message: getSemanticTokens full
[BG(1)] getSemanticTokens full at /home/token0/tmp/vscode/tmp.py ...
[BG(1)]   parsing: /home/token0/tmp/vscode/tmp.py (61ms)
[BG(1)]   parsing: /home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/typeshed-fallback/stdlib/builtins.pyi [fs read 4ms] (135ms)
[BG(1)]   binding: /home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/typeshed-fallback/stdlib/builtins.pyi (38ms)
[BG(1)]   binding: /home/token0/tmp/vscode/tmp.py (0ms)
[BG(1)]   parsing: /home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/typeshed-fallback/stdlib/typing.pyi [fs read 1ms] (43ms)
[BG(1)]   binding: /home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/typeshed-fallback/stdlib/typing.pyi (22ms)
[BG(1)]   parsing: /home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/typeshed-fallback/stdlib/_typeshed/__init__.pyi [fs read 2ms] (23ms)
[BG(1)]   binding: /home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/typeshed-fallback/stdlib/_typeshed/__init__.pyi (2ms)
[BG(1)]   parsing: /home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/typeshed-fallback/stubs/typing-extensions/typing_extensions.pyi [fs read 0ms] (4ms)
[BG(1)]   binding: /home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/typeshed-fallback/stubs/typing-extensions/typing_extensions.pyi (1ms)
[BG(1)]   parsing: /home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/typeshed-fallback/stdlib/abc.pyi [fs read 0ms] (1ms)
[BG(1)]   binding: /home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/typeshed-fallback/stdlib/abc.pyi (1ms)
[BG(1)] getSemanticTokens full at /home/token0/tmp/vscode/tmp.py (369ms)
Background analysis message: getSemanticTokens range
[BG(1)] getSemanticTokens range 0:0 - 9:0 at /home/token0/tmp/vscode/tmp.py (1ms)
Background analysis message: analyze
[Error - 4:15:08 PM] An internal error occurred while type checking file "/home/token0/tmp/vscode/tmp.py": Error: Debug Failure. False expression.
    at canAssignProtocolClassToSelf (/home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/pyright-internal/src/analyzer/typeEvaluator.ts:18276:9)
    at forEach (/home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/pyright-internal/src/analyzer/typeEvaluator.ts:18337:22)
    at Array.forEach (<anonymous>)
    at Object.canAssignProtocolClassToSelf (/home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/pyright-internal/src/analyzer/typeEvaluator.ts:18328:38)
    at callback (/home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/pyright-internal/src/analyzer/typeEvaluatorWithTracker.ts:110:69)
    at i.timeOperation (/home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/pyright-internal/src/common/timing.ts:44:28)
    at run (/home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/pyright-internal/src/analyzer/typeEvaluatorWithTracker.ts:42:46)
    at Object.canAssignProtocolClassToSelf (/home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/pyright-internal/src/analyzer/typeEvaluatorWithTracker.ts:110:13)
    at /home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/pyright-internal/src/analyzer/checker.ts:2555:56
    at Array.forEach (<anonymous>)
[BG(1)] analyzing: /home/token0/tmp/vscode/tmp.py ...
[BG(1)]   checking: /home/token0/tmp/vscode/tmp.py (274ms)
[BG(1)] analyzing: /home/token0/tmp/vscode/tmp.py (275ms)
[Error - 4:15:08 PM] Error performing analysis: Error: Debug Failure. False expression.
    at canAssignProtocolClassToSelf (/home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/pyright-internal/src/analyzer/typeEvaluator.ts:18276:9)
    at forEach (/home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/pyright-internal/src/analyzer/typeEvaluator.ts:18337:22)
    at Array.forEach (<anonymous>)
    at Object.canAssignProtocolClassToSelf (/home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/pyright-internal/src/analyzer/typeEvaluator.ts:18328:38)
    at callback (/home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/pyright-internal/src/analyzer/typeEvaluatorWithTracker.ts:110:69)
    at i.timeOperation (/home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/pyright-internal/src/common/timing.ts:44:28)
    at run (/home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/pyright-internal/src/analyzer/typeEvaluatorWithTracker.ts:42:46)
    at Object.canAssignProtocolClassToSelf (/home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/pyright-internal/src/analyzer/typeEvaluatorWithTracker.ts:110:13)
    at /home/token0/.vscode/extensions/ms-python.vscode-pylance-2021.3.3/dist/pyright-internal/src/analyzer/checker.ts:2555:56
    at Array.forEach (<anonymous>)

[erictraut]

Thanks for the bug report. This will be fixed in the next release of pylance.

[marcinwrochna]

Thanks a lot!

[jakebailey]

This issue has been fixed in version 2021.3.4, which we've just released. You can find the changelog here: https://github.com/microsoft/pylance-release/blob/main/CHANGELOG.md#202134-31-march-2021