[variant.visit] Add constexpr to as-variant

Author: hewillk

source/utilities.tex

@@ -6215,13 +6215,13 @@
 Let \exposid{as-variant} denote the following exposition-only function templates:
 \begin{codeblock}
 template<class... Ts>
-  auto&& @\exposid{as-variant}@(variant<Ts...>& var) { return var; }
+  constexpr auto&& @\exposid{as-variant}@(variant<Ts...>& var) { return var; }
 template<class... Ts>
-  auto&& @\exposid{as-variant}@(const variant<Ts...>& var) { return var; }
+  constexpr auto&& @\exposid{as-variant}@(const variant<Ts...>& var) { return var; }
 template<class... Ts>
-  auto&& @\exposid{as-variant}@(variant<Ts...>&& var) { return std::move(var); }
+  constexpr auto&& @\exposid{as-variant}@(variant<Ts...>&& var) { return std::move(var); }
 template<class... Ts>
-  auto&& @\exposid{as-variant}@(const variant<Ts...>&& var) { return std::move(var); }
+  constexpr auto&& @\exposid{as-variant}@(const variant<Ts...>&& var) { return std::move(var); }
 \end{codeblock}
 Let $n$ be \tcode{sizeof...(Variants)}.
 For each $0 \leq i < n$, let