Named patterns beginning with underscores are treated as errors

[AviKav]

warning: Unused variable
    ┌─ ./src/lower_your_guards.gleam:121:40
    │
121 │         RefinementType(gamma, _phi) as theta -> RefinementType(gamma, Literal(False))
    │                                        ^^^^^ This variable is never used.

Hint: You can ignore it with an underscore: `_theta`.

After renaming with underscore prefix:

error: Syntax error
    ┌─ ./src/lower_your_guards.gleam:121:40
    │
121 │         RefinementType(gamma, _phi) as _theta -> RefinementType(gamma, Literal(False))
    │                                        ^^^^^^ I'm expecting a lowercase name here.

Hint: Variable and module names start with a lowercase letter, and can
contain a-z, 0-9, or _.

[lpil]

It's not currently supported to use a discard pattern here. The right hand side of the ass operator is not a pattern, it is a variable name.

We could possibly change this. What is the use case for the discard in this instance? It seems like it would do nothing.

Alternatively we could raise a specific error message that says two remove the as entirely.

[lpil]

Converting this to a discussion as the use case isn't clear yet.