Confusing error message when calling a function with incorrect *args

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
38/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
compilers, devtools

Research direction

Reproduce the example in bad_starargs.py and compare mypy's current diagnostics with the proposed output. Trace the handling of the unpacked tuple and update the diagnostics so each mismatched element identifies its own argument, with a note showing the unpacked tuple type.

Written by the indexing model from the issue text.

Description

feature priority-1-normal topic-error-reporting topic-usability

When typechecking this:

def foo(x: int, y: str) -> None: pass
t = ('hi', 12)
foo(*t)

mypy reports

bad_starargs.py:3: error: Argument 1 to "foo" has incompatible type "*Tuple[str, int]"; expected "int"
bad_starargs.py:3: error: Argument 1 to "foo" has incompatible type "*Tuple[str, int]"; expected "str"

This is not great, since Tuple[str, int] isn't being compared to something, and while it appears in argument position 1, the type mismatches don't necessarily.

A better error message might be something like:

bad_starargs.py:3: error: Argument 1 to "foo" has incompatible type "str"; expected "int"
bad_starargs.py:3: note: while unpacking "*Tuple[str, int]"
bad_starargs.py:3: error: Argument 2 to "foo" has incompatible type "int"; expected "str"
bad_starargs.py:3: note: while unpacking "*Tuple[str, int]"
Dominant language
Python
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Merged PRs (30d)
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