fix #21203 Placement new does not count as initialization (#21243)

compiler/test/runnable/placenew.d

@@ -106,6 +106,32 @@ void test7()
     assert(t.x == 10 && t.y == 20);
 }
 
+/*************************************************/
+// https://github.com/dlang/dmd/issues/21203
+
+struct Y8
+{
+    int a;
+    this() @disable; // default disabled
+    this(int i) { a = i; }
+}
+
+struct S8
+{
+    Y8 m;
+
+    this(int x)
+    {
+        new(m) Y8(x); // initialise `m`
+    }
+}
+
+void test8()
+{
+    S8 s = S8(3);
+    assert(s.m.a == 3);
+}
+
 /*************************************************/
 
 int main()
@@ -117,6 +143,7 @@ int main()
     test5();
     test6();
     test7();
+    test8();
 
     return 0;
 }