Fix #21020 - Indexing a *cast* AA yields no lvalue anymore (#21029)

Author: kinke

compiler/src/dmd/expression.d

@@ -3632,6 +3632,7 @@ extern (C++) final class CastExp : UnaExp
         if (rvalue || !e1.isLvalue())
             return false;
         return (to.ty == Tsarray && (e1.type.ty == Tvector || e1.type.ty == Tsarray)) ||
+            (to.ty == Taarray && e1.type.ty == Taarray) ||
             e1.type.mutableOf.unSharedOf().equals(to.mutableOf().unSharedOf());
     }
 

compiler/test/runnable/test21020.d

@@ -0,0 +1,11 @@
+// https://github.com/dlang/dmd/issues/21020
+
+shared struct Queue {
+    int[int] map;
+}
+
+void main() {
+    auto queue = Queue();
+    (cast(int[int]) queue.map)[1] = 2;
+    assert(queue.map[1] == 2);
+}