Error I can't fix

[Gemma 4]

To help "wron" get a faster and more accurate answer, you could 
suggest they improve their post by adding the following:

*   **Provide a Minimal Reproducible Example (MRE):** While they 
provided snippets, the current code is missing imports (like 
`core.stdc.stdio` for `writeln` and wherever `Nullable` is 
defined) and a `main` function to actually call `stMems`. A 
complete, runnable single-file example allows helpers to 
reproduce the error instantly.
*   **Specify the Compiler and Version:** D has multiple 
compilers (DMD, LDC, GDC). The error `DontCallDestructorT` sounds 
like a compiler-internal type, which suggests the issue might be 
specific to a certain compiler version or a bug in how a specific 
compiler handles `Nullable`.
*   **Clarify the Goal:** They should explicitly state if they 
are trying to implement a general-purpose reflection utility or 
if they just need a way to print the types of a specific struct. 
This helps experts suggest better alternatives, such as `traits` 
or `mixin`.
*   **Detail the Environment:** Mentioning the OS and the 
specific version of the `Nullable` library (if it's a third-party 
package) would be helpful, as `Nullable` is not a built-in 
primitive in the standard library (they might be using 
`std.typecons.Nullable` or a custom implementation).