error: generic parameters may not be used in const operations
  --> $DIR/param-in-ct-in-ty-param-default.rs:1:44
   |
LL | struct Foo<T, U = [u8; std::mem::size_of::<T>()]>(T, U);
   |                                            ^ cannot perform const operation using `T`
   |
   = note: type parameters may not be used in const expressions
   = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions

error: aborting due to 1 previous error