diff --git a/src/hash.hh b/src/hash.hh
index 8e5e319c..35e89e84 100644
--- a/src/hash.hh
+++ b/src/hash.hh
@@ -61,10 +61,12 @@ struct Hash
 // Traits specifying if two types have compatible hashing, that is,
 // if lhs == rhs => hash_value(lhs) == hash_value(rhs)
 template<typename Lhs, typename Rhs>
-constexpr bool HashCompatible = false;
+struct HashCompatible : std::false_type {};
 
-template<typename T>
-constexpr bool HashCompatible<T, T> = true;
+template<typename T> struct HashCompatible<T, T> : std::true_type {};
+
+template<typename Lhs, typename Rhs>
+constexpr bool IsHashCompatible = HashCompatible<Lhs, Rhs>::value;
 
 }
 
diff --git a/src/hash_map.hh b/src/hash_map.hh
index 89c97b00..4263b195 100644
--- a/src/hash_map.hh
+++ b/src/hash_map.hh
@@ -156,7 +156,7 @@ struct HashMap
 
     template<typename KeyType>
     using EnableIfHashCompatible = std::enable_if_t<
-        HashCompatible<Key, std::decay_t<KeyType>>
+        IsHashCompatible<Key, std::decay_t<KeyType>>
     >;
 
     template<typename KeyType, typename = EnableIfHashCompatible<KeyType>>
diff --git a/src/string.hh b/src/string.hh
index 98d78ff6..bb026869 100644
--- a/src/string.hh
+++ b/src/string.hh
@@ -256,8 +256,8 @@ private:
 
 static_assert(std::is_trivial<StringView>::value, "");
 
-template<> constexpr bool HashCompatible<String, StringView> = true;
-template<> constexpr bool HashCompatible<StringView, String> = true;
+template<> struct HashCompatible<String, StringView> : std::true_type {};
+template<> struct HashCompatible<StringView, String> : std::true_type {};
 
 inline String::String(StringView str) : String{str.begin(), str.length()} {}