incorrect inference of user-defined variadic tuple subtype instances

[jorenham]

Instances of user-defined variadic tuples subtypes are inferred incorrectly:

class Shape[*Ts](tuple[*Ts]): ...

reveal_type((42, ))         # tuple[Literal[42]?]
reveal_type(Shape((42, )))  # __main__.Shape[Unpack[builtins.tuple[Never, ...]]]

https://mypy-play.net/?mypy=1.15.0&python=3.13&gist=2e2ba990a2112c97ca8d9d071c559a56

It should instead be inferred as Shape[Literal[42]].

This occurs with both mypy 1.15 and the current master branch.

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related: #19093, #19105
ref: #19100 (comment)