bug - Union model variants fail build/codegen and do not derive Clone

[dannymeijer]

Area

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

Summary

Expected: A Union[...] of model variants should build when constructors are used at values of the union type, and a union whose variants all derive Clone should either support clone() or produce a clear typechecking diagnostic before codegen. Recursive expression trees should also need a clear supported pattern or a frontend diagnostic if direct recursion is not supported.

Actual: Type checking accepts the source, but incan build --lib generates Rust that does not wrap model constructors in the generated union enum. Recursive union payloads also lower to Rust recursive types with infinite size, and expr.clone() on a union reports that Union[...] has no clone(...) method even when all variants derive Clone.

This surfaced while converting InQL's scalar expression tree from a tag/payload model to a v0.3 union-shaped model for PR dannys-code-corner/InQL#29.

Reproduction steps

Repro 1: model union constructors are not wrapped in generated Rust

Create /tmp/union_list_repro:

# incan.toml
[project]
name = "union_list_repro"
version = "0.1.0"

# src/lib.incn
@derive(Clone)
pub model Leaf:
    pub value: int


@derive(Clone)
pub model Pair:
    pub args: list[Expr]


pub type Expr = Union[Leaf, Pair]


pub def pair() -> Expr:
    return Pair(args=[Leaf(value=1), Leaf(value=2)])


pub def sum_expr(expr: Expr) -> int:
    match expr:
        Leaf(leaf) => return leaf.value
        Pair(pair) => return sum_expr(pair.args[0])

Run:

/Users/danny/Development/encero/incan/target/debug/incan build --lib

Repro 2: union of cloneable model variants has no clone()

@derive(Clone)
model Leaf:
    value: int


@derive(Clone)
model Pair:
    left: Expr
    right: Expr


type Expr = Union[Leaf, Pair]


def use_expr(expr: Expr) -> int:
    match expr:
        Leaf(leaf) => return leaf.value
        Pair(pair) => return use_expr(pair.left)


def clone_expr(expr: Expr) -> Expr:
    return expr.clone()

Run:

/Users/danny/Development/encero/incan/target/debug/incan --check repro.incn

Output / logs

Repro 1 fails during Rust build:

error[E0308]: mismatched types
  --> src/lib.rs:60:20
   |
60 |         args: vec![Leaf { value : 1 }, Leaf { value : 2 }],
   |                    ^^^^^^^^^^^^^^^^^^ expected `__IncanUniond44f4244dd4f043b`, found `Leaf`
   |
help: try wrapping the expression in `__IncanUniond44f4244dd4f043b::V0`

error[E0308]: mismatched types
  --> src/lib.rs:59:12
   |
58 |   pub fn pair() -> Expr {
   |                    ---- expected `__IncanUniond44f4244dd4f043b` because of return type
59 |       return Pair {
   |  ____________^
60 | |         args: vec![Leaf { value : 1 }, Leaf { value : 2 }],
61 | |     };
   | |_____^ expected `__IncanUniond44f4244dd4f043b`, found `Pair`

Repro 2 fails during typecheck:

type error: Type 'Union[Leaf, Pair]' has no method 'clone(...)'
  --> repro.incn:22:12
   |
22 |     return expr.clone()
   |            ^^^^^^^^^^^^

A direct recursive enum/union payload also reaches Rust with E0072 infinite-size recursion instead of receiving an Incan diagnostic or supported indirection lowering.

Environment

OS: macOS (local worker)
Rust: local toolchain through incan build --lib
Incan: incan 0.3.0-rc2, commit 8003bdad (bugfix - bind public callable aliases from manifests (#617))
Command: /Users/danny/Development/encero/incan/target/debug/incan build --lib and /Users/danny/Development/encero/incan/target/debug/incan --check repro.incn

Blocking status / workaround

This blocks an honest InQL refactor to Union[ColumnRefExpr, IntLiteralExpr, ...] model variants. InQL can continue with a data-carrying enum plus list-backed recursive operands, but that is a workaround around union/model codegen rather than the intended v0.3 union-model showcase shape.