
signallers require function call syntax for `what`
==================================================

> deprecate_stop("1.0.0", "foo")
Error: Internal error: `what` must have function call syntax.

  # Good:
  deprecate_stop("foo()")

  # Bad:
  deprecate_stop("foo")

> deprecate_stop("1.0.0", "foo()()")
Error: Internal error: `what` must refer to a function name.

> deprecate_stop("1.0.0", "foo(arg = , arg = )")
Error: Internal error: `what` can't refer to more than one argument.

> deprecate_stop("1.0.0", "foo(arg)")
Error: Internal error: `what` must refer to arguments in the LHS of `=`.

  # Good:
  deprecate_stop("foo(arg = )")

  # Bad:
  deprecate_stop("foo(arg)")

> deprecate_stop("1.0.0", "foo(arg = arg)")
Error: Internal error: `what` must contain reason as a string in the LHS of `=`.

  # Good:
  deprecate_stop("foo(arg = 'must be a string')")

  # Bad:
  deprecate_stop("foo(arg = 42)")

