Introduce hash.lookup.isHashFunction trait

Fix #66.

source/tanya/container/hashtable.d

@@ -386,7 +386,7 @@ struct ByValue(T)
  *  hasher = Hash function for $(D_PARAM Key).
  */
 struct HashTable(Key, Value, alias hasher = hash)
-if (is(typeof(((Key k) => hasher(k))(Key.init)) == size_t))
+if (isHashFunction!(hasher, Key))
 {
     private alias HashArray = .HashArray!(hasher, Key, Value);
     private alias Buckets = HashArray.Buckets;

source/tanya/container/set.d

@@ -154,7 +154,7 @@ struct Range(T)
  *  hasher = Hash function for $(D_PARAM T).
  */
 struct Set(T, alias hasher = hash)
-if (is(typeof(((T x) => hasher(x))(T.init)) == size_t))
+if (isHashFunction!(hasher, T))
 {
     private alias HashArray = .HashArray!(hasher, T);
     private alias Buckets = HashArray.Buckets;
@@ -768,8 +768,8 @@ if (is(typeof(((T x) => hasher(x))(T.init)) == size_t))
     testFunc(set);
 }
 
+// Hasher can take argument by ref
 @nogc nothrow pure @safe unittest
 {
-    // Using hasher that takes argument by ref.
-    Set!(int, (const ref x) => cast(size_t)x) set;
+    static assert(is(Set!(int, (const ref x) => cast(size_t) x)));
 }

source/tanya/hash/lookup.d

@@ -636,3 +636,27 @@ static if (size_t.sizeof == 8) @nogc nothrow pure @safe unittest
     assert(hash(r500!"~") == 0xc1af12bdfe16b5b5UL);
     assert(hash(r500!"\x7f") == 0x39e9f18f2f85e221UL);
 }
+
+/**
+ * Determines whether $(D_PARAM hasher) is hash function for $(D_PARAM T), i.e.
+ * it is callable with a value of type $(D_PARAM T) and returns a
+ * $(D_PSYMBOL size_t) value.
+ *
+ * Params:
+ *  hasher = Hash function candidate.
+ *  T      = Type to test the hash function with.
+ *
+ * Returns: $(D_KEYWORD true) if $(D_PARAM hasher) is a hash function for
+ *          $(D_PARAM T), $(D_KEYWORD false) otherwise.
+ */
+template isHashFunction(alias hasher, T)
+{
+    private alias wrapper = (T x) => hasher(x);
+    enum bool isHashFunction = is(typeof(wrapper(T.init)) == size_t);
+}
+
+///
+@nogc nothrow pure @safe unittest
+{
+    static assert(isHashFunction!(hash, int));
+}