postcheck panic: tag pattern matched non-tag value during build specialization

[lukewilliamboswell]

Summary

On the harden-glue branch, roc build panics during specialization with:

postcheck invariant violated: tag pattern matched a non-tag value

I reduced this from roc-ray/examples/hello_world.roc to a fully standalone three-file app/platform. roc check passes for the same source; only roc build panics.

Version

Observed with:

branch: harden-glue
commit: a4febaa2d63dbe44ff194065f61b19838987f1ac
roc version: debug-6334a2ed

Minimal repro

Create these three files:

mkdir -p repro-tag-pattern/platform
cd repro-tag-pattern

app.roc

app [main!] { pf: platform "platform/main.roc" }

import pf.Draw

main! : () => Try({}, [Exit(I64), ..])
main! = || {
    _offset = Draw.offset(Draw.align_left)
    Ok({})
}

platform/main.roc

platform ""
    requires {} { main! : () => Try({}, [Exit(I64), ..]) }
    exposes [Draw]
    packages {}
    provides {
        "render_for_host": render_for_host!,
    }
    hosted {}
    targets: {
        x64glibc: { inputs: [app] },
    }

import Draw

render_for_host! : () => Try({}, I64)
render_for_host! = || {
    match main!() {
        Ok({}) => Ok({})
        Err(Exit(code)) => Err(code)
        Err(_) => Err(-1)
    }
}

platform/Draw.roc

Draw := [].{
    HAlign : [Left, Center, Right]

    Align : {
        horizontal : HAlign,
    }

    align_left : Align
    align_left = { horizontal: Left }

    offset : Align -> F32
    offset = |align|
        match align.horizontal {
            Left => 0
            Center => 1
            Right => 2
        }
}

Run:

roc check app.roc
roc build app.roc

Expected

roc build should not panic. If this platform is invalid for linking because it only supplies app as a target input, it should fail with a normal linker/platform error after lowering.

Actual

roc check app.roc passes.

roc build app.roc exits 134 and panics:

thread ... panic: postcheck invariant violated: tag pattern matched a non-tag value
src/postcheck/common.zig:82
src/postcheck/monotype_lifted/spec_constr.zig:2927 bindPatToValue
src/postcheck/monotype_lifted/spec_constr.zig:2351 simplifyKnownMatchValue
src/postcheck/monotype_lifted/spec_constr.zig:1609 cloneExprValue
src/postcheck/monotype_lifted/spec_constr.zig:1517 cloneExpr
src/postcheck/monotype_lifted/spec_constr.zig:914 writeSpecialization
src/postcheck/monotype_lifted/spec_constr.zig:493 createSpecializations
src/postcheck/monotype_lifted/spec_constr.zig:421 run
src/lir/checked_pipeline.zig:237 lowerCheckedModulesToLir
src/cli/main.zig:8244 rocBuildLlvm

Reduction notes

• This does not depend on the roc-ray platform package.
• This does not depend on hello_world.roc.
• This does not need hosted effects, raylib libraries, Host, App, Draw.text!, or a model type.
• The minimal trigger appears to be specializing through the platform-provided wrapper while the app calls a function that matches on a tag field from a known record constant.