Versions: 2.081.1, 2.083.0 (What I had installed)

The following code produces a compiler error when it shouldn't:

struct A {
    M2 stdin;
}

mixin template Handle(T, T invalid_value = T.init) {}

struct M1 { mixin Handle!(size_t); }
struct M2 { mixin Handle!(M1); }

template `foo.Handle(T, T invalid_value = T.init)` recursive template expansion

Not using the struct doesn't produce an error, so clearly expanding M2 works perfectly. Using M1.init instead of relying on the default parameter doesn't produce this issue.