Fix bugzilla issue 24809: In some cases, stable sort assigns to unininitialized elements (#9057)

Author: jmdavis

std/algorithm/sorting.d

@@ -2625,11 +2625,21 @@ private template TimSortImpl(alias pred, R)
             // can't use `temp.length` if there's no default constructor
             static if (__traits(compiles, { T defaultConstructed; cast(void) defaultConstructed; }))
             {
-                if (__ctfe) temp.length = newSize;
-                else temp = () @trusted { return uninitializedArray!(T[])(newSize); }();
+
+                static if (hasElaborateAssign!T)
+                    temp.length = newSize;
+                else
+                {
+                    if (__ctfe) temp.length = newSize;
+                    else temp = () @trusted { return uninitializedArray!(T[])(newSize); }();
+                }
             }
             else
             {
+                static assert(!hasElaborateAssign!T,
+                              "Structs which have opAssign but cannot be default-initialized " ~
+                              "do not currently work with stable sort: " ~
+                              "https://issues.dlang.org/show_bug.cgi?id=24810");
                 temp = () @trusted { return uninitializedArray!(T[])(newSize); }();
             }
         }
@@ -3093,6 +3103,65 @@ private template TimSortImpl(alias pred, R)
     array.sort!((a, b) => false, SwapStrategy.stable);
 }
 
+// https://issues.dlang.org/show_bug.cgi?id=24809
+@safe unittest
+{
+    static struct E
+    {
+        int value;
+        int valid = 42;
+
+        ~this()
+        {
+            assert(valid == 42);
+        }
+    }
+
+    import std.array : array;
+    import std.range : chain, only, repeat;
+    auto arr = chain(repeat(E(41), 18),
+                     only(E(39)),
+                     repeat(E(41), 16),
+                     only(E(1)),
+                     repeat(E(42), 33),
+                     only(E(33)),
+                     repeat(E(42), 16),
+                     repeat(E(43), 27),
+                     only(E(33)),
+                     repeat(E(43), 34),
+                     only(E(34)),
+                     only(E(43)),
+                     only(E(63)),
+                     repeat(E(44), 42),
+                     only(E(27)),
+                     repeat(E(44), 11),
+                     repeat(E(45), 64),
+                     repeat(E(46), 3),
+                     only(E(11)),
+                     repeat(E(46), 7),
+                     only(E(4)),
+                     repeat(E(46), 34),
+                     only(E(36)),
+                     repeat(E(46), 17),
+                     repeat(E(47), 36),
+                     only(E(39)),
+                     repeat(E(47), 26),
+                     repeat(E(48), 17),
+                     only(E(21)),
+                     repeat(E(48), 5),
+                     only(E(39)),
+                     repeat(E(48), 14),
+                     only(E(58)),
+                     repeat(E(48), 24),
+                     repeat(E(49), 13),
+                     only(E(40)),
+                     repeat(E(49), 38),
+                     only(E(18)),
+                     repeat(E(49), 11),
+                     repeat(E(50), 6)).array();
+
+    arr.sort!((a, b) => a.value < b.value, SwapStrategy.stable)();
+}
 
 // schwartzSort
 /**