Unregister S3 methods explicitly

This is a workaround for a bug in base R. unloadNamespace does not
unregister any S3 methods, so the promises are invalid if you install a
new version of a package, try to unload and re-load the package.

See https://stat.ethz.ch/pipermail/r-devel/2015-December/072150.html for
more details and a reproducible example.

This fixes the common lazy-load errors people often run into when trying
to use devtools' install functions.

Fixes r-lib#419, r-lib#503, r-lib#942, r-lib#631

Author: jimhester

R/unload.r

@@ -30,14 +30,9 @@
 unload <- function(pkg = ".") {
   pkg <- as.package(pkg)
 
-  # This is a hack to work around unloading devtools itself. The unloading
-  # process normally makes other devtools functions inaccessible,
-  # resulting in "Error in unload(pkg) : internal error -3 in R_decompress1".
-  # If we simply access them here using as.list (without calling them), then
-  # they will remain available for use later.
-  if (pkg$package == "devtools") {
-    as.list(ns_env(pkg))
-  }
+  ns <- asNamespace(pkg$package)
+
+  unregister_S3_methods(ns)
 
   # If the package was loaded with devtools, any s4 classes that were created
   # by the package need to be removed in a special way.
@@ -78,6 +73,38 @@ unload <- function(pkg = ".") {
   unload_dll(pkg)
 }
 
+unregister_S3_methods <- function(ns) {
+  S3_methods <- getNamespaceInfo(ns, "S3methods")
+
+  unregister <- function(name, class, method) {
+    # This code was adapted from the .registerS3method internal function of
+    # base::registerS3methods
+    # https://github.com/wch/r-source/blob/05b76baa411afd3e9d0f3fc3c09a9a252a0a9100/src/library/base/R/namespace.R#L1398-L1426
+    env <-
+      if (!is.na(x <- .knownS3Generics[name])) {
+        asNamespace(x)
+      } else {
+        if(is.null(genfun <- get0(name, envir = ns))) {
+          stop(sprintf("object '%s' not found while unloading namespace '%s'",
+               name, getNamespaceName(ns)), call. = FALSE)
+        }
+        if(.isMethodsDispatchOn() && methods::is(genfun, "genericFunction")) {
+          genfun <- genfun@default  # nearly always, the S3 generic
+        }
+        if (typeof(genfun) == "closure") {
+          environment(genfun)
+        } else {
+          baseenv()
+        }
+      }
+    table <- get(".__S3MethodsTable__.", envir = env, inherits = FALSE)
+    rm(list = method, envir = table)
+  }
+  for (i in seq_len(NROW(S3_methods))) {
+    unregister(S3_methods[i, 1], S3_methods[i, 2], S3_methods[i, 3])
+  }
+}
+
 # This unloads dlls loaded by either library() or load_all()
 unload_dll <- function(pkg = ".") {
   pkg <- as.package(pkg)