x=x+shl(
  1,
  y
)

Works fine.

x+=shl(
  1,
  y
)

Syntax error: unclosed (

x+=shl(1,
  y
)

Syntax error: unexpected symbol near ')'

Yeah, this is an invisible shortcoming of how the += language extension is implemented. x+=y isn't a part of Lua, so Pico-8 adds it by replacing x+=y with x=x+(y) before giving it to Lua.

In these cases, it sees

x+=shl(

and replaces it with

x=x+(shl()

which produces the "unclosed (" error. When it sees

x+=shl(1,

it replaces it with

x=x+(shl(1,)

which produces the "unexpected symbol near ')'" error.

It'd be a big change to fix it, I think. Pico-8 would have to be much smarter about doing its own parsing around the += operator.

Good to know.

Still a problem with 0.2.3

Probably not a bug per se, but it would be good to note this somewhere in the documentation.

What do you mean? It is a bug in the pico8 pre-processor, as it’s not a desired or documented limitation. Two independent things in the language fail to work as expected when combined.