I confirmed this issue with dmd 2.094.0 on my WSL environment and with run.dlang.io (https://run.dlang.io/is/EI2fhc).

- How to reproduce:
```
$ cat sample.d
import std.variant;

void main()
{
    immutable aa = ["0":0];
    auto v = Variant(aa);
}
$ dmd sample.d
```

- Expected behavior: The dmd command successfully compiles the source code and generates an executable file.
- Actual behavior: It fails compiling with the following messages:
```
$ dmd sample.d
/home/ttanjo/dlang/dmd-2.094.0/linux/bin64/../../src/phobos/std/variant.d(490): Error: cannot modify immutable expression (*zis)[args[1].get()]
/home/ttanjo/dlang/dmd-2.094.0/linux/bin64/../../src/phobos/std/variant.d(707): Error: template instance std.variant.VariantN!32LU.VariantN.handler!(immutable(int[string])) error instantiating
/home/ttanjo/dlang/dmd-2.094.0/linux/bin64/../../src/phobos/std/variant.d(604):        instantiated from here: opAssign!(immutable(int[string]))
sample.d(6):        instantiated from here: __ctor!(immutable(int[string]))
```

I guess this issue is the source of the problem of Issue 13930.