error: failed when building. moonc version: v0.1.20241218+f4a066f5f

[FlyCloudC]

create project:

moon new --lib project1
cd ./project1

edit ./sec/top.mbt :

///|
pub(open) trait Abelian {
  proof() -> Unit
}

///|
pub impl[A : Abelian] Abelian for A? with proof() {  }

run build command:

moon build

output:

failed: moonc build-package -error-format json D:\Code\moonbit\bug\project1\src\top.mbt -o D:\Code\moonbit\bug\project1\target\wasm-gc\release\build\hello.core -pkg username/hello -std-path C:\Users\FlyCloudC\.moon\lib\core\target\wasm-gc\release\bundle -i D:\Code\moonbit\bug\project1\target\wasm-gc\release\build\lib\lib.mi:lib -pkg-sources username/hello:D:\Code\moonbit\bug\project1\src -target wasm-gc

         --  --
       /  //  / __--------_
      /  //  /_/            \
   ---      -                \ __
  / X        /        ____   /   )
  *_________/__/_____/______/ `--

Oops, the compiler has encountered an unexpected situation.
This is a bug in the compiler.

A bug report containing the error description and relevant code would be
greatly appreciated. You can submit the bug report here:

  https://github.com/moonbitlang/moonbit-docs/issues/new?labels=bug,ICE

Error: Moonc.Hashed_type.Unresolved_type_variable

Compiler args: moonc build-package -error-format json "D:\\Code\\moonbit\\bug\\project1\\src\\top.mbt" -o "D:\\Code\\moonbit\\bug\\project1\\target\\wasm-gc\\release\\build\\hello.core" -pkg username/hello -std-path "C:\\Users\\FlyCloudC\\.moon\\lib\\core\\target\\wasm-gc\\release\\bundle" -i "D:\\Code\\moonbit\\bug\\project1\\target\\wasm-gc\\release\\build\\lib\\lib.mi:lib" -pkg-sources "username/hello:D:\\Code\\moonbit\\bug\\project1\\src" -target wasm-gc

moonc version: v0.1.20241218+f4a066f5f
error: failed when building

[codeworm96]

I found it is also reproducible in the open-sourced v0.1.20241202 version.

https://github.com/moonbitlang/moonbit-compiler/blob/main/src/poly_type.ml#L92 Here a fresh type variable is made for the type parameter A.

Because the type variable is unused, the unification of the method does not bound the type variable: https://github.com/moonbitlang/moonbit-compiler/blob/main/src/toplevel_typer.ml#L2031

Later when we solve the constraints, the unresolved type variable will raise the error because it cannot be hashed.

I think it is a case where the compiler should catch and print the compilation failure.