Show generic signature in error message (RConsortium#482)

* Show generic signature in error message

closes RConsortium#480

* show method formals in error message

* Update R/method-register.R

Co-authored-by: Hadley Wickham <[email protected]>

* update snapshot

---------

Co-authored-by: Hadley Wickham <[email protected]>

Author: t-kalinowski

R/method-register.R

@@ -181,9 +181,14 @@ check_method <- function(method, generic, name = paste0(generic@name, "(???)"))
 
   if (!"..." %in% generic_args && !identical(generic_formals, method_formals)) {
     msg <- sprintf(
-      "%s() lacks `...` so method formals must match generic formals exactly",
+      "%s() generic lacks `...` so method formals must match generic formals exactly.",
       generic@name
     )
+    bullets <- c(
+      sprintf("- generic formals: %s", show_args(generic_formals, name = generic@name)),
+      sprintf("- method formals:  %s", show_args(method_formals, name = generic@name))
+    )
+    msg <- paste0(c(msg, bullets), collapse = "\n")
     stop(msg, call. = FALSE)
   }
 

tests/testthat/_snaps/method-register.md

@@ -81,7 +81,9 @@
       check_method(function(x, y) { }, foo)
     Condition
       Error:
-      ! foo() lacks `...` so method formals must match generic formals exactly
+      ! foo() generic lacks `...` so method formals must match generic formals exactly.
+      - generic formals: foo(x)
+      - method formals:  foo(x, y)
 
 # check_method warn if default arguments don't match