Summary

Consider:

class I0: ...
class I1: ...
class I2: ...

def f(x: tuple[I0, *tuple[I1, ...], I2]):
    [a, b, *c] = x
    reveal_type(a)  # I0
    reveal_type(b)  # I1
    reveal_type(c)  # list[I1 | I2]

The types we infer for a and c seem correct here. But the type for b seems incorrect. There are three possible scenarios in which this assignment could succeed; I think we need to consider them all and union the types together:

1. The middle element "materialises" to a 0-length tuple. In this scenario at runtime, a would be of type I0, b would be of type I2 and c would be of type list[Never]
2. The middle element "materialises" to a tuple of length 1. In this scenario at runtime, a would be of type I0, b would be of type I1 and c would be of type list[I2]
3. The middle element "materialises" to a tuple of length >=2. In this scenario at runtime, a would be of type I0, b would be of type I1 and c would be of type list[I1 | I2]

Given these three scenarios in which the assignment could succeed, I think we should be inferring I1 | I2 for b rather than I1.

Version

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