Merge pull request #6832 from FeepingCreature/fix/Issue-19572

Fix issue 19572: Prevent Appender from implicitly calling struct constructors to cast away const.
merged-on-behalf-of: Nicholas Wilson <thewilsonator@users.noreply.github.com>

Author: dlang-bot

std/array.d

@@ -3339,7 +3339,7 @@ if (isDynamicArray!A)
     private template canPutItem(U)
     {
         enum bool canPutItem =
-            isImplicitlyConvertible!(U, T) ||
+            isImplicitlyConvertible!(Unqual!U, Unqual!T) ||
             isSomeChar!T && isSomeChar!U;
     }
     private template canPutConstRange(Range)
@@ -3384,7 +3384,7 @@ if (isDynamicArray!A)
             immutable len = _data.arr.length;
 
             auto bigData = (() @trusted => _data.arr.ptr[0 .. len + 1])();
-            emplaceRef!(Unqual!T)(bigData[len], cast(Unqual!T) item);
+            emplaceRef!(Unqual!T)(bigData[len], cast() item);
             //We do this at the end, in case of exceptions
             _data.arr = bigData;
         }
@@ -3657,6 +3657,26 @@ if (isDynamicArray!A)
     static assert(!__traits(compiles, () pure { test!false(); }));
 }
 
+@system unittest // issue 19572
+{
+    static struct Struct
+    {
+        int value;
+
+        int fun() const { return 23; }
+
+        alias fun this;
+    }
+
+    Appender!(Struct[]) appender;
+
+    appender.put(const(Struct)(42));
+
+    auto result = appender.data[0];
+
+    assert(result.value != 23);
+}
+
 //Calculates an efficient growth scheme based on the old capacity
 //of data, and the minimum requested capacity.
 //arg curLen: The current length