Fix issue 20468 - emplace should forward argument lvalueness correctly

src/core/internal/lifetime.d

@@ -1,5 +1,7 @@
 module core.internal.lifetime;
 
+import core.lifetime : forward;
+
 /+
 emplaceRef is a package function for druntime internal use. It works like
 emplace, but takes its argument by ref (as opposed to "by pointer").
@@ -23,30 +25,27 @@ void emplaceRef(T, UT, Args...)(ref UT chunk, auto ref Args args)
     else static if (
         !is(T == struct) && Args.length == 1 /* primitives, enums, arrays */
         ||
-        Args.length == 1 && is(typeof({T t = args[0];})) /* conversions */
+        Args.length == 1 && is(typeof({T t = forward!(args[0]);})) /* conversions */
         ||
-        is(typeof(T(args))) /* general constructors */)
+        is(typeof(T(forward!args))) /* general constructors */)
     {
         static struct S
         {
             T payload;
-            this(ref Args x)
+            this()(auto ref Args args)
             {
-                static if (Args.length == 1)
-                    static if (is(typeof(payload = x[0])))
-                        payload = x[0];
-                    else
-                        payload = T(x[0]);
+                static if (is(typeof(payload = forward!args)))
+                    payload = forward!args;
                 else
-                    payload = T(x);
+                    payload = T(forward!args);
             }
         }
         if (__ctfe)
         {
-            static if (is(typeof(chunk = T(args))))
-                chunk = T(args);
-            else static if (args.length == 1 && is(typeof(chunk = args[0])))
-                chunk = args[0];
+            static if (is(typeof(chunk = T(forward!args))))
+                chunk = T(forward!args);
+            else static if (args.length == 1 && is(typeof(chunk = forward!(args[0]))))
+                chunk = forward!(args[0]);
             else assert(0, "CTFE emplace doesn't support "
                 ~ T.stringof ~ " from " ~ Args.stringof);
         }
@@ -55,14 +54,14 @@ void emplaceRef(T, UT, Args...)(ref UT chunk, auto ref Args args)
             S* p = () @trusted { return cast(S*) &chunk; }();
             static if (UT.sizeof > 0)
                 emplaceInitializer(*p);
-            p.__ctor(args);
+            p.__ctor(forward!args);
         }
     }
-    else static if (is(typeof(chunk.__ctor(args))))
+    else static if (is(typeof(chunk.__ctor(forward!args))))
     {
         // This catches the rare case of local types that keep a frame pointer
         emplaceInitializer(chunk);
-        chunk.__ctor(args);
+        chunk.__ctor(forward!args);
     }
     else
     {
@@ -82,7 +81,7 @@ static import core.internal.traits;
 void emplaceRef(UT, Args...)(ref UT chunk, auto ref Args args)
 if (is(UT == core.internal.traits.Unqual!UT))
 {
-    emplaceRef!(UT, UT)(chunk, args);
+    emplaceRef!(UT, UT)(chunk, forward!args);
 }
 
 //emplace helper functions

src/core/lifetime.d

@@ -58,7 +58,7 @@ T* emplace(T, Args...)(T* chunk, auto ref Args args)
 {
     import core.internal.lifetime : emplaceRef;
 
-    emplaceRef!T(*chunk, args);
+    emplaceRef!T(*chunk, forward!args);
     return chunk;
 }
 
@@ -606,7 +606,8 @@ version (unittest) private class __conv_EmplaceTestClass
     S1 s1 = void;
     emplace(&s1, 1);
     assert(s1.i == 1);
-    static assert(!__traits(compiles, emplace(&s1, S1.init)));
+    static assert(!__traits(compiles, emplace(&s1, s1))); // copy disabled
+    static assert(__traits(compiles, emplace(&s1, move(s1)))); // move not affected
 
     static struct S2
     {
@@ -624,15 +625,17 @@ version (unittest) private class __conv_EmplaceTestClass
     }
     SS1 ss1 = void;
     emplace(&ss1);
-    static assert(!__traits(compiles, emplace(&ss1, SS1.init)));
+    static assert(!__traits(compiles, emplace(&ss1, ss1))); // copying disabled
+    static assert(__traits(compiles, emplace(&ss1, move(ss1)))); // move unaffected
 
     static struct SS2
     {
         S2 s;
     }
     SS2 ss2 = void;
     emplace(&ss2);
-    static assert(!__traits(compiles, emplace(&ss2, SS2.init)));
+    static assert(!__traits(compiles, emplace(&ss2, ss2))); // copying disabled
+    static assert(__traits(compiles, emplace(&ss2, SS2.init))); // move is OK
 
 
     // SS1 sss1 = s1;      //This doesn't compile
@@ -1135,7 +1138,7 @@ pure nothrow @safe /* @nogc */ unittest
     }
 
     Unsafe unsafe = void;
-    static assert(!__traits(compiles, emplaceRef(unsafe, Unsafe())));
+    static assert(!__traits(compiles, emplaceRef(unsafe, unsafe)));
 
     Unsafe[1] unsafeArr = [Unsafe()];
     Unsafe[1] uninitializedUnsafeArr = void;
@@ -1197,6 +1200,28 @@ pure nothrow @safe /* @nogc */ unittest
     // need ctor args
     static assert(!is(typeof(emplace!A(buf))));
 }
+
+//constructor arguments forwarding
+@system unittest
+{
+    static struct S
+    {
+        this()(auto ref long arg)
+        {
+            // assert that arg is an lvalue
+            static assert(__traits(isRef, arg));
+        }
+        this()(auto ref double arg)
+            // assert that arg is an rvalue
+        {
+            static assert(!__traits(isRef, arg));
+        }
+    }
+    S obj = void;
+    long i;
+    emplace(&obj, i);   // lvalue
+    emplace(&obj, 0.0); // rvalue
+}
 // Bulk of emplace unittests ends here
 
 /**