Add std.typecons.Rebindable2 for internal use.

Rebindable2 is a simplified version of std.typecons.Rebindable that clears up every special case: classes, arrays and structs now have the same struct.
Whichever type you instantiate `Rebindable2` with, you always get the same type out by calling `value.get` on the resulting container.
Also use this type to simplify the parts of Phobos we previously used `Rebindable` for.

std/algorithm/searching.d

@@ -1315,19 +1315,12 @@ in
 }
 do
 {
-    import std.typecons : Rebindable;
+    import std.typecons : Rebindable2;
 
     alias Element = ElementType!Range;
-    Rebindable!Element seed = r.front;
+    auto seed = Rebindable2!Element(r.front);
     r.popFront();
-    static if (is(Rebindable!Element == T[], T))
-    {
-        return extremum!(map, selector)(r, seed);
-    }
-    else
-    {
-        return extremum!(map, selector)(r, seed.get);
-    }
+    return extremum!(map, selector)(r, seed.get);
 }
 
 private auto extremum(alias map, alias selector = "a < b", Range,
@@ -1337,14 +1330,14 @@ if (isInputRange!Range && !isInfinite!Range &&
     !is(CommonType!(ElementType!Range, RangeElementType) == void) &&
      is(typeof(unaryFun!map(ElementType!(Range).init))))
 {
-    import std.typecons : Rebindable;
+    import std.typecons : Rebindable2;
 
     alias mapFun = unaryFun!map;
     alias selectorFun = binaryFun!selector;
 
     alias Element = ElementType!Range;
     alias CommonElement = CommonType!(Element, RangeElementType);
-    Rebindable!CommonElement extremeElement = seedElement;
+    auto extremeElement = Rebindable2!CommonElement(seedElement);
 
     // if we only have one statement in the loop, it can be optimized a lot better
     static if (__traits(isSame, map, a => a))
@@ -1355,7 +1348,7 @@ if (isInputRange!Range && !isInfinite!Range &&
         {
             foreach (const i; 0 .. r.length)
             {
-                if (selectorFun(r[i], extremeElement))
+                if (selectorFun(r[i], extremeElement.get))
                 {
                     extremeElement = r[i];
                 }
@@ -1365,7 +1358,7 @@ if (isInputRange!Range && !isInfinite!Range &&
         {
             while (!r.empty)
             {
-                if (selectorFun(r.front, extremeElement))
+                if (selectorFun(r.front, extremeElement.get))
                 {
                     extremeElement = r.front;
                 }
@@ -1376,7 +1369,7 @@ if (isInputRange!Range && !isInfinite!Range &&
     else
     {
         alias MapType = Unqual!(typeof(mapFun(CommonElement.init)));
-        MapType extremeElementMapped = mapFun(extremeElement);
+        MapType extremeElementMapped = mapFun(extremeElement.get);
 
         // direct access via a random access range is faster
         static if (isRandomAccessRange!Range)
@@ -1405,15 +1398,7 @@ if (isInputRange!Range && !isInfinite!Range &&
             }
         }
     }
-    // Rebindable is an alias to T for arrays
-    static if (is(typeof(extremeElement) == T[], T))
-    {
-        return extremeElement;
-    }
-    else
-    {
-        return extremeElement.get;
-    }
+    return extremeElement.get;
 }
 
 private auto extremum(alias selector = "a < b", Range)(Range r)

std/range/package.d

@@ -3902,24 +3902,17 @@ Returns:
 struct Repeat(T)
 {
 private:
-    //Store a non-qualified T when possible: This is to make Repeat assignable
-    static if ((is(T == class) || is(T == interface)) && (is(T == const) || is(T == immutable)))
-    {
-        import std.typecons : Rebindable;
-        alias UT = Rebindable!T;
-    }
-    else static if (is(T : Unqual!T) && is(Unqual!T : T))
-        alias UT = Unqual!T;
-    else
-        alias UT = T;
-    UT _value;
+    import std.typecons : Rebindable2;
+
+    // Store a rebindable T to make Repeat assignable.
+    Rebindable2!T _value;
 
 public:
     /// Range primitives
-    @property inout(T) front() inout { return _value; }
+    @property inout(T) front() inout { return _value.get; }
 
     /// ditto
-    @property inout(T) back() inout { return _value; }
+    @property inout(T) back() inout { return _value.get; }
 
     /// ditto
     enum bool empty = false;
@@ -3934,7 +3927,7 @@ public:
     @property auto save() inout { return this; }
 
     /// ditto
-    inout(T) opIndex(size_t) inout { return _value; }
+    inout(T) opIndex(size_t) inout { return _value.get; }
 
     /// ditto
     auto opSlice(size_t i, size_t j)
@@ -3959,7 +3952,12 @@ public:
 }
 
 /// Ditto
-Repeat!T repeat(T)(T value) { return Repeat!T(value); }
+Repeat!T repeat(T)(T value)
+{
+    import std.typecons : Rebindable2;
+
+    return Repeat!T(Rebindable2!T(value));
+}
 
 ///
 pure @safe nothrow unittest

std/typecons.d

@@ -2955,6 +2955,140 @@ Rebindable!T rebindable(T)(Rebindable!T obj)
     assert(rebindable(pr3341_aa)[321] == 543);
 }
 
+package(std) struct Rebindable2(T)
+{
+private:
+    static if (isAssignable!(typeof(cast() T.init)))
+    {
+        enum useQualifierCast = true;
+
+        typeof(cast() T.init) data;
+    }
+    else
+    {
+        enum useQualifierCast = false;
+
+        align(T.alignof)
+        static struct Payload
+        {
+            static if (hasIndirections!T)
+            {
+                void[T.sizeof] data;
+            }
+            else
+            {
+                ubyte[T.sizeof] data;
+            }
+        }
+
+        Payload data;
+    }
+
+public:
+
+    static if (!__traits(compiles, { T value; }))
+    {
+        @disable this();
+    }
+
+    /**
+     * Constructs a `Rebindable2` from a given value.
+     */
+    this(T value) @trusted
+    {
+        static if (useQualifierCast)
+        {
+            this.data = cast() value;
+        }
+        else
+        {
+            set(value);
+        }
+    }
+
+    /**
+     * Overwrites the currently stored value with `value`.
+     */
+    void opAssign(this This)(T value) @trusted
+    {
+        clear;
+        set(value);
+    }
+
+    /**
+     * Returns the value currently stored in the `Rebindable2`.
+     */
+    T get(this This)() @property @trusted
+    {
+        static if (useQualifierCast)
+        {
+            return cast(T) this.data;
+        }
+        else
+        {
+            return *cast(T*) &this.data;
+        }
+    }
+
+    /// Ditto
+    inout(T) get() inout @property @trusted
+    {
+        static if (useQualifierCast)
+        {
+            return cast(inout(T)) this.data;
+        }
+        else
+        {
+            return *cast(inout(T)*) &this.data;
+        }
+    }
+
+    static if (!useQualifierCast)
+    {
+        ~this() @trusted
+        {
+            clear;
+        }
+    }
+
+private:
+
+    void set(this This)(T value)
+    {
+        static if (useQualifierCast)
+        {
+            this.data = cast() value;
+        }
+        else
+        {
+            // As we're escaping a copy of `value`, deliberately leak a copy:
+            static union DontCallDestructor
+            {
+                T value;
+            }
+            DontCallDestructor copy = DontCallDestructor(value);
+            this.data = *cast(Payload*) ©
+        }
+    }
+
+    void clear(this This)()
+    {
+        // work around reinterpreting cast being impossible in CTFE
+        if (__ctfe)
+        {
+            return;
+        }
+
+        // call possible struct destructors
+        .destroy!(No.initialize)(*cast(T*) &this.data);
+    }
+}
+
+package(std) Rebindable2!T rebindable2(T)(T value)
+{
+    return Rebindable2!T(value);
+}
+
 /**
     Similar to `Rebindable!(T)` but strips all qualifiers from the reference as
     opposed to just constness / immutability. Primary intended use case is with