Can you give a repro for this issue? The option-parsing code does Compiler ??= new CsharpCompiler();; when is it not called?

In other words, it's intended that FullCompileName depends on the compiler; the bug is that Compiler is null, and that's what we should fix.

The method ApplyDefaultOptions() is never called if one builds a DafnyOptions using the constructor.
Previously, it was only depending on whether the compiler target was C# or not, not the full compiler.

Previously, it was only depending on whether the compiler target was C# or not, not the full compiler.

Initializing a compiler instance doesn't really do that much, so it would be fine to initialize the compiler to be the C# one. But isn't it good that you get an error? It tells you that your code may depend on the internals of the compiler, which is good to know?

When this does bug occur? It seems like it occurs if DafnyOptions.O are not properly initialised, like you might forget to do in a test. Isn't initialising them properly the fix then?

Yes indeed. However, previously, parsing would succeed even though DafnyOptions.O is not properly initialized. Users setting themselves the options without depending on our custom initialization faced the problem of having a field that is uninitialized (Compiler), which break their code.