Work around bugzilla issue 24415 - only doesn't work with elements with a copy constructor.

Since the compiler is treating the auto-generated copy-constructor for
OnlyResult as private (thus rendering it useless outside of
std.range.package), this commit adds an explicit one and makes it
public. Once the dmd bug has been fixed, the explicit copy constructor
should be removed.

Author: jmdavis

std/internal/test/range.d

@@ -23,3 +23,94 @@ module std.internal.test.range;
     auto r = R().chunks(3);
     assert(r.equal!equal([[ 0, 1, 2 ], [ 3, 4 ]]));
 }
+
+// https://issues.dlang.org/show_bug.cgi?id=24415
+@safe unittest
+{
+    import std.range : only;
+
+    static struct S(T)
+    {
+        T i;
+
+        this(ref return scope inout(S) rhs) scope @safe inout pure nothrow
+        {
+            i = rhs.i;
+        }
+    }
+    {
+        auto a = only(S!int(42));
+        auto b = a;
+        assert(!b.empty);
+        assert(b.front == S!int(42));
+
+        a.popFront();
+        auto c = a;
+        assert(c.empty);
+    }
+    {
+        auto a = only(S!(const int)(42));
+        auto b = a;
+        assert(!b.empty);
+        assert(b.front == S!(const int)(42));
+
+        a.popFront();
+        auto c = a;
+        assert(c.empty);
+    }
+    {
+        auto a = only(S!(immutable int)(42));
+        auto b = a;
+        assert(!b.empty);
+        assert(b.front == S!(immutable int)(42));
+
+        a.popFront();
+        auto c = a;
+        assert(c.empty);
+    }
+    {
+        auto a = only(S!int(42), S!int(192));
+        auto b = a;
+        assert(!b.empty);
+        assert(b.front == S!int(42));
+
+        a.popFront();
+        auto c = a;
+        assert(!c.empty);
+        assert(c.front == S!int(192));
+
+        a.popFront();
+        auto d = a;
+        assert(d.empty);
+    }
+    {
+        auto a = only(S!(const int)(42), S!(const int)(192));
+        auto b = a;
+        assert(!b.empty);
+        assert(b.front == S!(const int)(42));
+
+        a.popFront();
+        auto c = a;
+        assert(!c.empty);
+        assert(c.front == S!(const int)(192));
+
+        a.popFront();
+        auto d = a;
+        assert(d.empty);
+    }
+    {
+        auto a = only(S!(immutable int)(42), S!(immutable int)(192));
+        auto b = a;
+        assert(!b.empty);
+        assert(b.front == S!(immutable int)(42));
+
+        a.popFront();
+        auto c = a;
+        assert(!c.empty);
+        assert(c.front == S!(immutable int)(192));
+
+        a.popFront();
+        auto d = a;
+        assert(d.empty);
+    }
+}

std/range/package.d

@@ -10375,6 +10375,14 @@ private struct OnlyResult(T)
     }
     alias opDollar = length;
 
+    // FIXME Workaround for https://issues.dlang.org/show_bug.cgi?id=24415
+    import std.traits : hasElaborateCopyConstructor;
+    static if (hasElaborateCopyConstructor!T)
+    {
+        private static struct WorkaroundBugzilla24415 {}
+        public this()(WorkaroundBugzilla24415) {}
+    }
+
     private this()(return scope auto ref T value)
     {
         ref @trusted unqual(ref T x){return cast() x;}