Type tracking conditionally extended empty tuple

[Fatsie]

Environment data

• Language Server version: 2021.5.4
• OS and version: Ubuntu 20.04
• Python version (& distribution if applicable, e.g. Anaconda): conda 3.7.10

Reproducer & Actual behaviour

I reduced my problem to following code that can reproduce it:

from typing import Tuple

def f(n: int, check: bool):
    t: Tuple[Tuple[int, float], ...] = tuple()
    for _ in range(n):
        t += ((0, 0.0),)
        if check:
            t += ((1, 1.0), (2, 2.0))

    for (i, f) in t:
        print(i)

This give the following error on the second for line:

{
	"resource": "/home/verhaegs/eda/test/test_emptytuple.py",
	"owner": "_generated_diagnostic_collection_name_#2",
	"code": {
		"value": "reportGeneralTypeIssues",
		"target": {
			"$mid": 1,
			"external": "https://github.com/microsoft/pylance-release/blob/main/DIAGNOSTIC_SEVERITY_RULES.md#diagnostic-severity-rules",
			"path": "/microsoft/pylance-release/blob/main/DIAGNOSTIC_SEVERITY_RULES.md",
			"scheme": "https",
			"authority": "github.com",
			"fragment": "diagnostic-severity-rules"
		}
	},
	"severity": 8,
	"message": "\"object\" is not iterable\n  \"__iter__\" method not defined",
	"source": "Pylance",
	"startLineNumber": 10,
	"startColumn": 9,
	"endLineNumber": 10,
	"endColumn": 15
}

Problem seems related to the fact that on the empty tuple() assignment the type of t is tuple[_T_co@tuple] and not the explicitly specified type. When I move the first t += statement outside the loop all is honky dory.

Expected behaviour

No typing error given.

[erictraut]

Thanks for the bug report. The problem appears to be with the initial assignment of t — an interaction between the tuple() constructor call and the type annotation. I'll investigate further. In the meantime, you can work around the problem by replacing the call to the constructor (tuple()) with an empty tuple expression (()).

[erictraut]

This will be fixed in the next release.

[jakebailey]

This issue has been fixed in version 2021.6.0, which we've just released. You can find the changelog here: https://github.com/microsoft/pylance-release/blob/main/CHANGELOG.md#202160-2-june-2021