From 2caf7f7fca3769adb52f9a086884f478939896b5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jean-Micha=C3=ABl=20Celerier?=
 <jeanmichael.celerier@gmail.com>
Date: Wed, 17 Jun 2026 08:06:02 -0400
Subject: [PATCH] value: model the variant_ish concept (index/valueless/visit)

jk::value wraps the variant in a member, so it satisfied none of the
variant concepts used by generic consumers (e.g. Avendish's back-end
bindings): value.index()/visit(f, value) didn't resolve. Forward the
variant interface (index, valueless_by_exception, and an ADL visit on
the wrapped member) so jk::value is usable directly as a variant value
without exposing .v at every call site.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
---
 include/jk/value.hpp | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/include/jk/value.hpp b/include/jk/value.hpp
index 3974041..289e82c 100644
--- a/include/jk/value.hpp
+++ b/include/jk/value.hpp
@@ -1,6 +1,9 @@
 #pragma once
 #include <jk/config.hpp>
 
+#include <cstddef>
+#include <utility>
+
 namespace jk
 {
 struct value;
@@ -63,7 +66,31 @@ struct value
   operator variant&&() && noexcept { return std::move(v); }
 
   bool operator==(const value& other) const noexcept = default;
+
+  // Model avnd::variant_ish: the Avendish back-end bindings (pd / max / wasm)
+  // treat a value output as a variant (concept check + unqualified visit). Expose
+  // the variant interface so jk::value is usable directly, without libossia.
+  std::size_t index() const noexcept { return v.index(); }
+  bool valueless_by_exception() const noexcept { return v.valueless_by_exception(); }
 };
 
+// ADL hook for the bindings' unqualified visit(f, value): forward to the
+// configured variant's visit on the wrapped member.
+template <typename F>
+inline decltype(auto) visit(F&& f, value& self)
+{
+  return config::variant_ns::visit(std::forward<F>(f), self.v);
+}
+template <typename F>
+inline decltype(auto) visit(F&& f, const value& self)
+{
+  return config::variant_ns::visit(std::forward<F>(f), self.v);
+}
+template <typename F>
+inline decltype(auto) visit(F&& f, value&& self)
+{
+  return config::variant_ns::visit(std::forward<F>(f), std::move(self.v));
+}
+
 // clang-format on
 }