fix: apply declared nullability from AnyType return type to resolved concrete type

[moraistejerina]

Summary

Fix AnyType.ReturnType() to apply the declared nullability (? suffix) from the return type definition to the resolved concrete type.

Fixes substrait-io/substrait#943

Problem

When a function declares a nullable return type like any1? (e.g., nullif with nullability: DECLARED_OUTPUT as introduced in substrait spec v0.81.0), the ? nullability is correctly parsed and stored in AnyType.Nullability, but ReturnType() never applies it to the resolved concrete type.

unwrapAnyTypeWithName() returns the matching argument type as-is, discarding the declared nullability. For example:

# From functions_comparison.yaml (substrait spec v0.81.0)
- name: "nullif"
  impls:
    - args:
        - value: any1
        - value: any1
      return: any1?
      nullability: DECLARED_OUTPUT

Calling nullif(string NOT NULL, string NOT NULL) produces string NOT NULL as the return type, instead of the expected string NULL — because NULLIF can always return NULL when both arguments are equal.

Fix

In AnyType.ReturnType(), after resolving the concrete type via unwrapAnyTypeWithName, apply the AnyType's declared nullability when it is nullable:

if m.Nullability == NullabilityNullable {
    typ = typ.WithNullability(NullabilityNullable)
}

Tests

• Added new test case in extensions/variants_test.go: nullif(any1, any1) -> any1? [DECLARED_OUTPUT] — verifies non-nullable arguments still produce nullable output when the function declares return: any1?
• Updated 2 existing test cases in types/any_type_test.go (any, anyOtherName) whose expected results now correctly reflect nullable output when the AnyType declares ?
• Full test suite passes: go test ./...

[benbellick]

Just the one comment about how DECLARED_OUTPUT interacts with the nullability-required case. Thanks!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.24%. Comparing base (8179602) to head (fbc7fec).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #205   +/-   ##
=======================================
  Coverage   68.23%   68.24%           
=======================================
  Files          47       47           
  Lines       10711    10713    +2     
=======================================
+ Hits         7309     7311    +2     
  Misses       3046     3046           
  Partials      356      356           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[benbellick]

LGTM, thanks!

[benbellick]

For those coming back to this in the future, this did not close substrait-io/substrait#943. That was a mistake. That issues is still being resolved upstream.