x |>
  @(foo)
  class C {}

…would already be an early error. The anti-semicolon developer must explicitly specify:

x |> (
  @(foo)
)
class C {}

…or

x |> (
  @(foo)
  class C { x = @; } // The pipe body must have a topic reference!
)

…Come to think of it, I’m uncertain whether ASI would have happened anyway even without a special early error.

x |> @(foo)
class C {}

I’m…not sure if this fulfills any of the three ASI criteria…

I wonder if records & tuples considered {{ }} and {[ ]}. The second form would require a cover grammar, but it's visually distinct:

const rec = {{ x: 1, y: 2 }};
const tuple = {[1, 2, 3]};

I'd considered something like {+}, {-}, etc. for an "operator functions" proposal, though I'm unsure if it would be well received, but it was heavily predicated on PFA:

sum = ar 
  |> map(##, {*} 2)
  |> reduceLeft(##, {+})