unary negation operator '-' works only with dist_normal()

[venpopov]

This works:

d1 <- -dist_normal()

These produce an error:

d2 <- -dist_gumbel()
d3 <- -dist_wrap('norm')

Error in Ops.dist_default(X[[i]], ...) : 
  argument "e2" is missing, with no default
Called from: inherits(e2, "dist_default")

Seems like '-' is always treated as a binary operator, expect for dist_normal()

Currently, if you want a negation, you have to do this:

d4 <- (-1) * dist_gumbel()

This is a problem for me, because I am working on a functionality for a package that plots distributions after applying link functions that could be defined by the user or come from another package. For example, this will fail with the same error:

brms::inv_logit_scaled(dist_gumbel())

although this works:

brms::inv_logit_scaled(dist_normal())

it would be great, if you can make a unary '-' operator for all distributions, including custom ones defined by dist_wrap