Confusing error message when existential parameter used in the body

[rachitnigam]

When trying to compile the following program:

comp Foo<'G:1>() -> () with {
    exists N;
} {
    // Bundle uses N which will not be bound in time during monomorphization.
    bundle a[N]: ['G, 'G+1] 32;
    exists N = 10;
}

comp main<'G:1>() -> () {
    F := new Foo;
}

The compiler says:

rachitnigam ~/git/filament (aetherling-existential !?$)
% cargo run -q -- exist-bad-err.fil
thread 'main' panicked at 'internal error: entered unreachable code: existential parameter `N' occurred in a use location', src/ir_passes/mono/monosig.rs:235:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The actual problem in the program is use of N in bundle a[N]. The problem is that when running the program, N is not yet bound to a value because we have not evaluated the exists binding. In general, existential parameters cannot be used in the body because of this problem. Instead, the correct program should just say bundle a[N].

Our well-formedness pass should emit an error to this effect.