fn maybe_expr(w: *Walk, scope: *Scope, parent_decl: Decl.Index, node: Ast.Node.OptionalIndex) Oom!void
fn maybe_expr(w: *Walk, scope: *Scope, parent_decl: Decl.Index, node: Ast.Node.OptionalIndex) Oom!void { if (node.unwrap()) |n| return expr(w, scope, parent_decl, n); }