Add better conditional messaging to check_yarn task

Author: jakeNiemiec

Gemfile

@@ -5,6 +5,7 @@ gemspec
 gem "rails"
 gem "rake", ">= 11.1"
 gem "rack-proxy", require: false
+gem "semantic_range", require: false
 
 group :test do
   gem "minitest", "~> 5.0"

Gemfile.lock

@@ -5,6 +5,7 @@ PATH
       activesupport (>= 4.2)
       rack-proxy (>= 0.6.1)
       railties (>= 4.2)
+      semantic_range (>= 2.3.0)
 
 GEM
   remote: https://rubygems.org/
@@ -136,6 +137,7 @@ GEM
     rubocop-performance (1.3.0)
       rubocop (>= 0.68.0)
     ruby-progressbar (1.10.1)
+    semantic_range (2.3.0)
     sprockets (4.0.0)
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
@@ -165,6 +167,7 @@ DEPENDENCIES
   rake (>= 11.1)
   rubocop (< 0.69)
   rubocop-performance
+  semantic_range
   webpacker!
 
 BUNDLED WITH

lib/tasks/webpacker/check_yarn.rake

@@ -1,3 +1,4 @@
+require "semantic_range"
 namespace :webpacker do
   desc "Verifies if Yarn is installed"
   task :check_yarn do
@@ -6,19 +7,25 @@ namespace :webpacker do
       raise Errno::ENOENT if yarn_version.blank?
 
       pkg_path = Pathname.new("#{__dir__}/../../../package.json").realpath
-      yarn_requirement = JSON.parse(pkg_path.read)["engines"]["yarn"]
+      yarn_range = JSON.parse(pkg_path.read)["engines"]["yarn"]
+      is_valid = SemanticRange.satisfies?(yarn_version, yarn_range) rescue false
+      is_unsupported = SemanticRange.satisfies?(yarn_version, ">=2.0.0") rescue false
 
-      requirement = Gem::Requirement.new(yarn_requirement)
-      version = Gem::Version.new(yarn_version)
-
-      unless requirement.satisfied_by?(version)
-        $stderr.puts "Webpacker requires Yarn #{requirement} and you are using #{version}"
-        $stderr.puts "Please upgrade Yarn https://yarnpkg.com/lang/en/docs/install/"
-        $stderr.puts "Exiting!" && exit!
+      unless is_valid
+        $stderr.puts "Webpacker requires Yarn \"#{yarn_range}\" and you are using #{yarn_version}"
+        if is_unsupported
+          $stderr.puts "This version of Webpacker does not support Yarn #{yarn_version}. Please downgrade to a supported version of Yarn https://yarnpkg.com/lang/en/docs/install/"
+          $stderr.puts "For information on using Webpacker with Yarn 2.0, see https://github.com/rails/webpacker/issues/2112"
+        else
+          $stderr.puts "Please upgrade Yarn https://yarnpkg.com/lang/en/docs/install/"
+        end
+        $stderr.puts "Exiting!"
+        exit!
       end
     rescue Errno::ENOENT
       $stderr.puts "Yarn not installed. Please download and install Yarn from https://yarnpkg.com/lang/en/docs/install/"
-      $stderr.puts "Exiting!" && exit!
+      $stderr.puts "Exiting!"
+      exit!
     end
   end
 end

webpacker.gemspec

@@ -17,9 +17,10 @@ Gem::Specification.new do |s|
 
   s.required_ruby_version = ">= 2.3.0"
 
-  s.add_dependency "activesupport", ">= 4.2"
-  s.add_dependency "railties",      ">= 4.2"
-  s.add_dependency "rack-proxy",    ">= 0.6.1"
+  s.add_dependency "activesupport",  ">= 4.2"
+  s.add_dependency "railties",       ">= 4.2"
+  s.add_dependency "rack-proxy",     ">= 0.6.1"
+  s.add_dependency "semantic_range", ">= 2.3.0"
 
   s.add_development_dependency "bundler", ">= 1.3.0"
   s.add_development_dependency "rubocop", "< 0.69"