Switch compilers when no build is specified

Given the current state of OS X compilers, the original fails_with
behavior is becoming less useful, mostly resulting in build failures
each time the compiler is updated. So make the following changes:

When a build is specified, we retain the old behavior: switch compilers
if the available compiler is <= the build, don't switch if it is > the
build.

When no build is specified, unconditionally switch compilers, and don't
output the advice message. This allows us to mark formulae as
perpetually failing, avoiding the need to update formulae each time a
new compiler build is made available.

As a bonus, this makes the logic much easier to reason about.

Closes Homebrew/homebrew#18175.

Library/Homebrew/build.rb

@@ -114,11 +114,7 @@ def install f
     end
   end
 
-  if f.fails_with? ENV.compiler
-    cs = CompilerSelector.new f
-    cs.select_compiler
-    cs.advise
-  end
+  CompilerSelector.new(f).select_compiler if f.fails_with? ENV.compiler
 
   f.brew do
     if ARGV.flag? '--git'

Library/Homebrew/compilers.rb

@@ -68,6 +68,7 @@ class CompilerFailure
   def initialize compiler, &block
     @compiler = compiler
     instance_eval(&block) if block_given?
+    @build ||= 9999
   end
 
   def build val=nil
@@ -100,8 +101,7 @@ class CompilerSelector
     # the failing build is >= the currently installed version of foo.
     @compilers = @compilers.reject do |cc|
       failure = @f.fails_with? cc
-      next unless failure
-      failure.build >= cc.build or not ARGV.homebrew_developer?
+      failure && failure.build >= cc.build
     end
 
     return if @compilers.empty? or @compilers.include? ENV.compiler
@@ -126,37 +126,4 @@ class CompilerSelector
       end
     end
   end
-
-  def advise
-    failure = @f.fails_with? @old_compiler
-    return unless failure
-
-    # If we're still using the original ENV.compiler, then the formula did not
-    # declare a specific failing build, so we continue and print some advice.
-    # Otherwise, tell the user that we're switching compilers.
-    if @old_compiler == ENV.compiler
-      cc = Compiler.new(ENV.compiler)
-      subject = "#{@f.name}-#{@f.version}: builds with #{NAMES[cc.name]}-#{cc.build}-#{MACOS_VERSION}"
-      warning = "Using #{NAMES[cc.name]}, but this formula is reported to fail with #{NAMES[cc.name]}."
-      warning += "\n\n#{failure.cause.strip}\n" unless failure.cause.nil?
-      warning += <<-EOS.undent
-
-        We are continuing anyway so if the build succeeds, please open a ticket with
-        the subject
-
-          #{subject}
-
-        so that we can update the formula accordingly. Thanks!
-        EOS
-
-      viable = @compilers.reject { |c| @f.fails_with? c }
-      unless viable.empty?
-        warning += "\nIf it fails you can use "
-        options = viable.map { |c| "--use-#{c.name}" }
-        warning += "#{options*' or '} to try a different compiler."
-      end
-
-      opoo warning
-    end
-  end
 end

Library/Homebrew/formula.rb

@@ -200,10 +200,8 @@ class Formula
   def fails_with? cc
     return false if self.class.cc_failures.nil?
     cc = Compiler.new(cc) unless cc.is_a? Compiler
-    return self.class.cc_failures.find do |failure|
-      next unless failure.compiler == cc.name
-      failure.build.zero? or \
-        (failure.build >= cc.build or not ARGV.homebrew_developer?)
+    self.class.cc_failures.find do |failure|
+      failure.compiler == cc.name && failure.build >= cc.build
     end
   end
 

Library/Homebrew/test/test_compilers.rb

@@ -94,9 +94,9 @@ class CompilerTests < Test::Unit::TestCase
 
     cs.select_compiler
 
-    assert_equal case MacOS.clang_build_version
-      when 0..210 then :gcc
-      else :clang
+    assert_equal case MacOS.gcc_42_build_version
+      when nil then :llvm
+      else :gcc
       end, ENV.compiler
   end
 
@@ -110,6 +110,6 @@ class CompilerTests < Test::Unit::TestCase
 
     cs.select_compiler
 
-    assert_equal MacOS.default_compiler, ENV.compiler
+    assert_not_equal :clang, ENV.compiler
   end
 end