Fix Issue 23182 - Can't assign struct with opAssign to SumType in CTFE

SumType.opAssign now avoids calling core.lifetime.move or
core.lifetime.forward during CTFE whenever possible.

Author: pbackus

std/sumtype.d

@@ -635,9 +635,19 @@ public:
 
                 this.match!destroyIfOwner;
 
-                mixin("Storage newStorage = { ",
-                    Storage.memberName!T, ": forward!rhs",
-                " };");
+                static if (isCopyable!T)
+                {
+                    // Workaround for https://issues.dlang.org/show_bug.cgi?id=21542
+                    mixin("Storage newStorage = { ",
+                        Storage.memberName!T, ": __ctfe ? rhs : forward!rhs",
+                    " };");
+                }
+                else
+                {
+                    mixin("Storage newStorage = { ",
+                        Storage.memberName!T, ": forward!rhs",
+                    " };");
+                }
 
                 storage = newStorage;
                 tag = tid;
@@ -678,7 +688,17 @@ public:
         {
             import core.lifetime : move;
 
-            rhs.match!((ref value) { this = move(value); });
+            rhs.match!((ref value) {
+                static if (isCopyable!(typeof(value)))
+                {
+                    // Workaround for https://issues.dlang.org/show_bug.cgi?id=21542
+                    this = __ctfe ? value : move(value);
+                }
+                else
+                {
+                    this = move(value);
+                }
+            });
             return this;
         }
     }
@@ -1569,6 +1589,28 @@ version (D_BetterC) {} else
     }
 }
 
+// Assignment of struct with overloaded opAssign in CTFE
+// https://issues.dlang.org/show_bug.cgi?id=23182
+@safe unittest
+{
+    static struct HasOpAssign
+    {
+        void opAssign(HasOpAssign rhs) {}
+    }
+
+    static SumType!HasOpAssign test()
+    {
+        SumType!HasOpAssign s;
+        // Test both overloads
+        s = HasOpAssign();
+        s = SumType!HasOpAssign();
+        return s;
+    }
+
+    // Force CTFE
+    enum result = test();
+}
+
 /// True if `T` is an instance of the `SumType` template, otherwise false.
 private enum bool isSumTypeInstance(T) = is(T == SumType!Args, Args...);