or c = lambda::[] where the extra ':' indicates that that the function
is auto-called

or c = lambda():[], (now illegal), where () is intended to show that the
default arg is the result of calling the function defined by the
expression.  lambda:[]() (now legal) would mean to (uselessly) call the
function immediately.

Thinking about it, I think those who want a syntax to indicate that the
expression should be compiled into a function and called at runtime
should build on the existing syntax (lambda...) for indicating that an
expression should be compiled into a function, rather than inventing a
replacement for that.

Terry Jan Reedy