bug - Decorated functions lose default-argument call-site filling

[dannymeijer]

Area

• Compiler (frontend/backend/codegen)

Summary

Expected: Decorating a function should preserve default-argument call-site behavior. If a plain function can be called without an argument because the parameter has a default, the decorated version should support the same call shape.

Actual: A decorated function with a default argument is emitted as a Rust function that requires the argument. A zero-argument call to the decorated function fails during generated Rust compilation with E0061.

This blocks InQL PR #45 from exposing a clean count() / count(expr) aggregate helper through the existing registry decorator model. The non-decorated helper shape works, but the decorated public helper loses the default argument.

Reproduction steps

1. Create test_decorated_default_probe.incn with:

def identity(func: (int) -> int) -> (int) -> int:
    return func

@identity
def decorated_default(value: int = 7) -> int:
    return value

def plain_default(value: int = 7) -> int:
    return value

def test_decorated_default_probe() -> None:
    assert plain_default() == 7
    assert decorated_default() == 7

2. Run:

incan test test_decorated_default_probe.incn

3. Observe that plain_default() has its default filled, while decorated_default() is emitted as requiring one argument.

Output / logs

error[E0061]: this function takes 1 argument but 0 arguments were supplied
  --> generated test code for decorated_default()

note: function defined here
  generated decorated function has shape similar to:
  fn decorated_default(__incan_arg_0: i64) -> i64

The exact generated file path varies by machine and is intentionally omitted here.

Environment

OS: macOS
Rust: current v0.3 development toolchain
Incan: release/v0.3, observed while testing the rc19-era compiler used by InQL PR #45
Command: incan test test_decorated_default_probe.incn

Related context

Related but not duplicate:

• bug - Decorator callable metadata still fails for imported partial defaults and alias callables #701 covered imported partial defaults and alias callable metadata.
• chore - Unify symbol identity across import and package boundaries #699 is a broader symbol identity cleanup chore.

This bug is narrower: local decorated functions should preserve default-argument call-site filling.

[dannymeijer]

RC20 verification found that #703 is only partially fixed.

Passing cases:

• Decorated primitive default argument from the new CLI regression passes.
• Decorated method default argument from the new CLI regression passes.
• A decorated default argument whose parameter is a concrete model type also passes.

Still failing and still blocking InQL PR #45: decorated default arguments where the parameter type is a Union[...] alias. InQL's ColumnExpr has this shape, so count(expr: ColumnExpr = col("")) is still emitted as requiring one argument.

Minimal repro:

pub model Ref:
    pub name: str


pub model Literal:
    pub value: int


pub type Expr = Union[Ref, Literal]


pub def col(name: str) -> Expr:
    return Ref(name=name)


def identity(func: (Expr) -> int) -> (Expr) -> int:
    return func


@identity
def decorated_default(expr: Expr = col("")) -> int:
    return 1


def plain_default(expr: Expr = col("")) -> int:
    return 1


def test_decorated_default_multi_union_alias_probe() -> None:
    assert plain_default() == 1
    assert decorated_default() == 1

Command:

incan test test_decorated_default_multi_union_alias_probe.incn

Actual result under rc20:

error[E0061]: this function takes 1 argument but 0 arguments were supplied
note: function defined here
fn decorated_default(__incan_arg_0: __IncanUnion...) -> i64

The likely remaining gap is in the wrapper-default shape match: the fix compares the resolved callable surface type against lower_type_with_type_params on the source AST parameter type. For aliases like Expr = Union[...], that appears to compare the resolved union surface against a not-equivalent source alias lowering, so the default plan is discarded and the wrapper parameter default remains None.

[dannymeijer]

Recheck after the latest #703 fixes: union-alias defaults now pass in the same module, but imported decorated default calls still fail, and this is the remaining InQL PR #45 blocker.

Passing now:

• Existing bug - Decorated functions lose default-argument call-site filling #703 CLI regression.
• Original primitive default repro.
• Concrete model default repro.
• Same-module Union[...] alias default repro.

Still failing:

• A decorated function with a Union[...] alias default works in its defining module, but loses the default when imported and called from another module.

Minimal repro:

incan.toml

[project]
name = "imported_decorated_default"
version = "0.1.0"

src/defaults.incn

pub model Ref:
    pub name: str


pub model Literal:
    pub value: int


pub type Expr = Union[Ref, Literal]


pub def col(name: str) -> Expr:
    return Ref(name=name)


def identity(func: (Expr) -> int) -> (Expr) -> int:
    return func


@identity
pub def decorated_default(expr: Expr = col("")) -> int:
    return 1

src/test_consumer.incn

from defaults import decorated_default


def test_imported_decorated_default_call() -> None:
    assert decorated_default() == 1

Command:

incan test src/test_consumer.incn

Actual result:

error[E0061]: this function takes 1 argument but 0 arguments were supplied
note: function defined here
pub fn decorated_default(__incan_arg_0: crate::__IncanUnion...) -> i64

InQL confirmation: with the updated compiler binary, incan test src/functions/aggregates now fails only at the imported call in src/functions/aggregates/count_if.incn:

from functions.aggregates.count import count
...
return count().filter(predicate)

The previous same-module count() failure is gone; the remaining issue is cross-module/default metadata propagation for decorated public callables.

[dannymeijer]

Second recheck after the latest #703 fixes: direct cross-module imports now pass, but re-export/facade imports still lose decorated default-call metadata. This is still the InQL PR #45 blocker because InQL imports count through functions/mod.incn:

pub from functions.aggregates.count import count, count_expr

and tests use:

from functions import count
...
count()

Minimal repro extending the prior package:

src/defaults.incn

pub model Ref:
    pub name: str


pub model Literal:
    pub value: int


pub type Expr = Union[Ref, Literal]


pub def col(name: str) -> Expr:
    return Ref(name=name)


def identity(func: (Expr) -> int) -> (Expr) -> int:
    return func


@identity
pub def decorated_default(expr: Expr = col("")) -> int:
    return 1

src/facade.incn

pub from defaults import decorated_default

src/test_facade_consumer.incn

from facade import decorated_default


def test_reexported_decorated_default_call() -> None:
    assert decorated_default() == 1

Command:

incan test src/test_facade_consumer.incn

Actual result:

error[E0061]: this function takes 1 argument but 0 arguments were supplied
note: function defined here
pub fn decorated_default(__incan_arg_0: crate::__IncanUnion...) -> i64

Verification details from InQL:

• incan test src/functions/aggregates now passes: the direct from functions.aggregates.count import count path used by count_if.incn is fixed.
• incan test tests/test_function_registry.incn still fails because the public facade import path from functions import count drops the default and emits count() as missing the ColumnExpr argument.