I ran into this issue using Slate, getting different results during next dev and next build/start, traced back to this issue. In Slate this surfaces by breaking Editor.nodes(editor, { match: <fn> }) between next dev/prod whenever a function predicate is passed to match.

The core issue is that destructured bindings which are later assigned to a function have a param_count of 0 rather than unknown.

Description:

visit_var_decl only records param_count for let x = init style declarators. Destructuring patterns (let { x } = init, let [x] = init, etc.) fall outside the existing if let (Pat::Ident(var), Some(init)) = ... guard with no fallback, so x.param_count stays at the default None. A later assignment to a known-arity arrow then sets param_count via the merge rule (None, Known(N)) => Some(Known(N)), and ignore_unused_args_of_call (added in #11645) trims arguments at every call site of x accordingly.

Minimal repro:

function run(opts) {
    let { match } = opts;
    if (!match) match = () => true;
    return match(\"hello\", \"world\");
}

console.log(run({ match: (a, b) => a + \" \" + b }));

• Source result: \"hello world\"
• Minified result on current main: \"undefined undefined\" (call site becomes match())

Fix:

Mirror what report_assign_pat already does for runtime destructuring assignments: when the binding is anything other than a plain Pat::Ident, mark every bound identifier as having an unknown param count.

} else if !matches!(&decl.name, Pat::Ident(_)) {
    // Destructuring binding (object/array/rest pattern). The
    // bound identifiers are read from runtime properties or
    // elements, so we cannot statically determine the arity
    // of any callable that reaches them.
    for id in find_pat_ids::<_, Id>(&decl.name) {
        self.data
            .var_or_default(id)
            .store_param_count(Value::Unknown);
    }
}

Tests:

• New fixture at tests/fixture/issues/11829/destructure-decl-stale-arity/ exercises the exact pattern above with the same config.json used by the feat(es/minifier): Remove useless arguments for non inlined callee #11645 fixtures (unused: true, keep_fargs: true).
• The new test FAILS on main with diff match(\"hello\", \"world\") → match() and PASSES with this fix applied.
• All 10 sibling tests under tests/fixture/issues/11645/ still pass — including logical-and-assign-stale-arity, child-scope-reassign-merge, for-of-rebind-arity and the eval/with-stmt safety tests.
• Full `cargo test --test compress`: 2855 passed; 0 failed; 27 ignored.

BREAKING CHANGE:

None.

Related issue (if exists):

Closes #11829.