Merge master branch from rubygems upstream.

  * It's preparation to release RubyGems 3.0.0.beta2 and Ruby 2.6.0
    preview 3.
  * https://github.com/rubygems/rubygems/compare/v3.0.0.beta1...fad2eb15a282b19dfcb4b48bc95b8b39ebb4511f

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64555 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

1 files changed, 7 insertions, 2 deletions

diff --git a/lib/rubygems/version.rb b/lib/rubygems/version.rb
index fbf75996cd..08f0d1e7a5 100644
--- a/lib/rubygems/version.rb
+++ b/lib/rubygems/version.rb
@@ -170,7 +170,10 @@ class Gem::Version
   # True if the +version+ string matches RubyGems' requirements.
 
   def self.correct? version
-    return false if version.nil?
+    unless Gem::Deprecate.skip
+      warn "nil versions are discouraged and will be deprecated in Rubygems 4" if version.nil?
+    end
+
     !!(version.to_s =~ ANCHORED_VERSION_PATTERN)
   end
 
@@ -325,7 +328,9 @@ class Gem::Version
     segments.pop    while segments.size > 2
     segments.push 0 while segments.size < 2
 
-    "~> #{segments.join(".")}"
+    recommendation = "~> #{segments.join(".")}"
+    recommendation += ".a" if prerelease?
+    recommendation
   end
 
   ##