Fix Issue 16744: Add Typeof

This an alternative implementation to #4920

std/traits.d

@@ -129,6 +129,7 @@
  *           $(LREF Unsigned)
  *           $(LREF ValueType)
  *           $(LREF Promoted)
+ *           $(LREF TypeOf)
  * ))
  * $(TR $(TD Misc) $(TD
  *           $(LREF mangledName)
@@ -7284,6 +7285,126 @@ template Promoted(T)
     }
 }
 
+/*
+    A template designed to function in the same manner as the
+    built-in `typeof` utility. As it is a template, Typeof can be
+    passed around like any other symbol, unlike `typeof`.
+
+    Also, whereas `typeof` will not accept symbols that are already
+    types (e.g., `typeof(int)` will not compile), Typeof will accept
+    both values and types.
+
+    Params:
+        T =  The symbol to take the type of. Can be either a type
+             or a value.
+
+    Returns:
+        The type of the given symbol. If the symbol is something that
+        does not have a valid type (such as a module name or an
+        expression that evaluates to the special `__error__` type),
+        an error will be raised at compile time.
+
+    Note:
+        If the given symbol is a template, the result type will be `void`.
+
+*/
+template Typeof(T...)
+if (T.length == 1)
+{
+    static if (__traits(isTemplate, T[0]))
+        alias Typeof = void;
+    else static if (is(T[0] TypeOfT0))
+        alias Typeof = TypeOfT0;
+    else static if (is(typeof(T[0]) TypeOfT0))
+        alias Typeof = TypeOfT0;
+    else
+        static assert(false, "Cannot take the type of " ~ T[0].stringof);
+}
+
+///
+@safe unittest
+{
+    //Types, values and expressions are accepted
+    assert(is(Typeof!1 == typeof(1)));
+    assert(is(Typeof!false == typeof(false)));
+    assert(is(Typeof!"" == string));
+
+    assert(is(Typeof!int == int));
+    assert(is(Typeof!bool == bool));
+    assert(is(Typeof!string == string));
+
+    assert(is(Typeof!(1 + 2 / 3) == typeof(1 + 2 / 3)));
+
+    struct S {}
+    assert(is(Typeof!(S()) == typeof(S())));
+    assert(is(Typeof!S == S));
+
+    class C {}
+    C c;
+    assert(is(Typeof!c == typeof(c)));
+    assert(is(Typeof!C == C));
+
+    //The type of a template is `void`
+    assert(is(Typeof!Typeof == typeof(Typeof)));
+    assert(is(Typeof!Typeof == void));
+
+    //Modules do not have a valid type
+    assert(!__traits(compiles, { alias _ = Typeof!(std.traits); }));
+}
+
+///
+@safe unittest
+{
+    import std.meta: AliasSeq, staticMap;
+
+    alias typesAndValues = AliasSeq!(int, false, char, "test");
+
+    //The built-in typeof cannot be passed to templates.
+    //The following will not compile
+    //alias justTypes = staticMap!(typeof, typeAndValues);
+
+    //But Typeof can
+    assert(is(staticMap!(Typeof, typesAndValues) == AliasSeq!(int, bool, char, string)));
+}
+
+@safe unittest
+{
+    import std.meta: AliasSeq;
+
+    template testMatch(T...)
+    if (T.length == 1)
+    {
+        static if (is(T[0]))
+            enum testMatch = is(Typeof!(T[0]) == T[0]) && is(Typeof!(T[0].init) == typeof(T[0].init));
+        else static if (is(typeof(T[0])))
+            enum testMatch = is(Typeof!(T[0]) == typeof(T[0])) && is(Typeof!(typeof(T[0])) == typeof(T[0]));
+        else
+            enum testMatch = false;
+    }
+
+    struct TestS {}
+    class TestC {}
+
+    foreach (T; AliasSeq!(NumericTypeList,
+                          ImaginaryTypeList,
+                          ComplexTypeList,
+                          CharTypeList,
+                          TestS,
+                          TestC))
+    {
+        static assert(testMatch!T);
+        static assert(testMatch!(T function(T)));
+        static assert(testMatch!(T delegate(T)));
+        static assert(testMatch!(T[]));
+        static assert(testMatch!(T[10]));
+        static assert(testMatch!(T[T]));
+    }
+
+    static assert(testMatch!null);
+    static assert(is(Typeof!void == void));
+    static assert(testMatch!Typeof);
+}
+
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