Fix identity functors

In order to accept both lvalue and rvalue arguments, their argument must be deduced, not a
class template argument (and therefore concretely specified).

Author: smowton

src/goto-programs/cfg.h

@@ -38,9 +38,10 @@ template <class T>
 class cfg_instruction_to_dense_integert
 {
 public:
-  std::size_t operator()(T &&t) const
+  template <class U>
+  std::size_t operator()(U &&u) const
   {
-    return std::forward<T>(identity_functort<T>{}(t));
+    return std::forward<U>(identity_functort{}(u));
   }
 };
 

src/util/dense_integer_map.h

@@ -19,11 +19,11 @@ Author: Diffblue Ltd
 #include <util/invariant.h>
 
 /// Identity functor. When we use C++20 this can be replaced with std::identity.
-template <typename T>
 class identity_functort
 {
 public:
-  constexpr T &&operator()(T &&t) const
+  template <typename T>
+  constexpr auto operator()(T &&t) const -> decltype(std::forward<T>(t))
   {
     return std::forward<T>(t);
   }
@@ -40,7 +40,7 @@ class identity_functort
 /// cfg_basic_blockst. Due to the vector storage the precise interface of
 /// std::map is hard to achieve, but something close is practically achievable
 /// for the interested developer.
-template <class K, class V, class KeyToDenseInteger = identity_functort<K>>
+template <class K, class V, class KeyToDenseInteger = identity_functort>
 class dense_integer_mapt
 {
 public: