check: support web bundle idioms so full type-check passes

[ecto]

What

Running loon check on the concatenated web bundle (built by web/build.ts into web/dist/bundle.loon) failed with 1000+ errors — starting with [E0205] no Eq implementation for type Vec. This made npm run build:full (the Rust-enabled build with type-checking, step 6 of web/build.ts) exit non-zero, blocking pre-rendering-from-source. The Vercel deploy only worked because it uses the --no-rust path that skips type-checking entirely.

The interpreter runs all of this code fine — every error was a type-checker gap, not a bug in web/src. So this PR fixes the checker (no web/src changes).

Changes (all in crates/loon-lang/src/check/mod.rs)

• Vec/Map/Set Eq (the original E0205): registered builtin Eq impls for containers, which compare structurally at runtime.
• Variadic & rest params (~1,040 arity errors): split params at &, bind the rest param as Vec elem, and at call sites to known variadic fns collect extra args into the rest Vec (rest elements stay unconstrained — HTML children are heterogeneous).
• Undeclared type names in ADT fields act as implicit type parameters — Props in [El String Props [Vec VNode]] was becoming a rigid nominal type nothing could unify with.
• get: 3-arg [get coll key default] no longer forces field presence; dynamic (non-literal) keys on records return an unconstrained type.
• Union-friendly if/vector literals: branches/elements with distinct concrete types back off to an unconstrained type instead of erroring, and if no longer ties two still-unknown branch types together (which wrongly equated unrelated params in [if [map? x] x y] dispatch code). [if true 1 2] still infers Int.
• Empty {} stays polymorphic instead of forcing Map t t (used as empty props throughout).
• Overloaded builtins: loosened len, contains?, and nth to match runtime polymorphism (strings, tuples, maps).
• Forward references: check_program pre-declares top-level fn names with a ∀a. a placeholder so mutual recursion across definitions (coerce-child ↔ coerce-children) and cross-file use-before-def resolve; the real scheme replaces the placeholder at the definition site. Genuine typos still error.

Trade-off for reviewers

Several of these are deliberate gradual-typing back-offs: heterogeneous unions and forward-referenced calls become unchecked rather than errors. This matches the "invisible types" philosophy and was the only way to type the ui.loon runtime-dispatch style ([if [map? first] ...]) without occurrence typing.

Verification

• npx tsx web/build.ts (full, with type-check) → exit 0, warnings only.
• cargo test --workspace → all pass (700+, zero failures).

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated (UTC)
loon	Ready	Preview, Comment	Jul 7, 2026 6:04am

[chojiai]

What shipped

This is a purely internal change with no user-visible effect. The type-checker used by the build toolchain has been updated to handle patterns common in the web codebase, so the full build process now completes successfully without errors.

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