bug - assert false lowers to unit in non-None return contexts

[dannymeijer]

Thanks for reporting! Please include a minimal reproduction when possible.

Area

• Incan Language (syntax/semantics)
• Compiler (frontend/backend/codegen)

Summary

Expected: assert false, message is a guaranteed failing control-flow path. A function whose body always fails through assert false should typecheck for any declared return type, the same way a typed failure helper can be used to produce an arbitrary T in unreachable paths.

Actual: assert false lowers to a Rust if !(false) { ... } expression with unit type. In a function returning a non-None type, generated Rust fails with E0317: if may be missing an else clause. This makes a direct Incan failure helper impossible without routing through std.testing.fail_t or returning a dummy value after the assertion.

This surfaced while replacing a host-language metadata validator in InQL PR #42 with an Incan script. The current workaround is sound but awkward: the script imports std.testing.fail_t only to express a typed failure path.

Reproduction steps

Minimal non-generic repro:

incan run -c 'def fail_int(message: str) -> int:
    assert false, message


def main() -> None:
    _ = fail_int("boom")'

Motivating generic helper repro:

incan run -c 'def fail_as[T](message: str) -> T:
    assert false, message


def main() -> None:
    _ = fail_as[int]("boom")'

Duplicate searches performed before filing:

gh issue list --repo dannys-code-corner/incan --state open --search "assert false generic return type bottom expected T found ()"
gh issue list --repo dannys-code-corner/incan --state open --search "assert false non-None return E0317"
gh issue list --repo dannys-code-corner/incan --state all --search "assert false expected i64 found unit"
gh issue list --repo dannys-code-corner/incan --state all --search "bottom type Never assert"

No matching issue was found.

Output / logs

For the non-generic repro, generated Rust compilation fails with:

error[E0317]: `if` may be missing an `else` clause
  --> src/main.rs:7:5
   |
 6 |   fn fail_int(message: String) -> i64 {
   |                                   --- expected `i64` because of this return type
 7 | /     if !(false) {
 8 | |         {
 9 | |             let __incan_assert_msg = message;
10 | |             if __incan_assert_msg.is_empty() {
...  |
16 | |     }
   | |_____^ expected `i64`, found `()`
   |
   = note: `if` expressions without `else` evaluate to `()`

For the generic repro, the same lowering fails against the type parameter:

error[E0317]: `if` may be missing an `else` clause
  --> src/main.rs:7:5
   |
 6 |   fn fail_as<T>(message: String) -> T {
   |              -                      - expected `T` because of this return type
   |              |
   |              expected this type parameter
 7 | /     if !(false) {
...
16 | |     }
   | |_____^ expected type parameter `T`, found `()`

Environment

OS: macOS
Rust: local Incan release branch toolchain
Incan: incan 0.3.0-rc10, release/v0.3
Command: incan run -c <repro>
Context: discovered while replacing the InQL RFC 014 metadata checker with an Incan script in InQL PR #42.

[dannymeijer]

Fixed on release/v0.3 in 1634ba48.

What changed:

• assert false, message now lowers as a direct diverging assertion failure instead of a unit-typed if expression.
• The same constant-failure path is handled for canonical assert_false(true, ...) emission.
• Added an end-to-end regression covering both -> int and generic -> T typed failure helpers.
• Updated the v0.3 release notes to include bug - assert false lowers to unit in non-None return contexts #644.

Verification:

• cargo test assert_false_can_satisfy_typed_failure_path
• cargo test -p incan_core
• CLI repros for both issue snippets now build and fail at runtime with AssertionError: boom rather than generated Rust type errors.
• make pre-commit passed, including 2450 nextest tests, clippy, cargo-deny, and smoke-test-fast.

Release PR CI is now queued for the pushed release-branch head; I am monitoring it.

[dannymeijer]

Fixed on release/v0.3 in 1634ba483ae77df2556b3f5bf6566f46b7599836.

Release PR #618 CI is green for the fix: format, clippy, docs, generated reference, Ubuntu/macOS test shards, MSRV shards, cargo-deny, unused-deps, release smoke, and smoke-test shards all completed successfully.