Auto merge of #6014 - greysteil:failing-pre-release-spec, r=segiddins

Add spec that pre-release versions aren't selected when not in the Gemfile

### What was the end-user problem that led to this PR?

The problem was that a pre-release version was being installed when the user hadn't asked for one, and a non-prerelease install was possible.

### What was your diagnosis of the problem?

My diagnosis was that this was caused by the change to the way pre-releases get selected for resolution when we moved to Molinillo 0.6.0. See the change to `lib/bundler/index.rb` in https://github.com/bundler/bundler/pull/5902.

### What is your fix for the problem, implemented in this PR?

My fix... isn't present yet. Basically we want to replicate the `wants_prerelease || only_prerelease` behaviour in `Bundler::Resolver#requirement_satisfied_by?`, but it's late and I haven't thought about how to do that yet. Instead, here's a failing spec.

### Why did you choose this fix out of the possible options?

I chose this fix because it's late and I haven't thought about how to fix this yet, but I at least wanted it flagged.

1 files changed, 11 insertions, 0 deletions

diff --git a/lib/bundler/resolver.rb b/lib/bundler/resolver.rb
index 78152d71cd..7962ff40f8 100644
--- a/lib/bundler/resolver.rb
+++ b/lib/bundler/resolver.rb
@@ -42,6 +42,9 @@ module Bundler
     end
 
     def start(requirements)
+      @prerelease_specified = {}
+      requirements.each {|dep| @prerelease_specified[dep.name] ||= dep.prerelease? }
+
       verify_gemfile_dependencies_are_found!(requirements)
       dg = @resolver.resolve(requirements, @base_dg)
       dg.
@@ -103,6 +106,14 @@ module Bundler
       search = @search_for[dependency] ||= begin
         index = index_for(dependency)
         results = index.search(dependency, @base[dependency.name])
+
+        unless @prerelease_specified[dependency.name]
+          # Move prereleases to the beginning of the list, so they're considered
+          # last during resolution.
+          pre, results = results.partition {|spec| spec.version.prerelease? }
+          results = pre + results
+        end
+
         if vertex = @base_dg.vertex_named(dependency.name)
           locked_requirement = vertex.payload.requirement
         end