CompilerSelector: raise when no compatible compiler

This replaces the old behaviour of falling back to the original
compiler with no messaging.

Fixes Homebrew/homebrew#19170.
Fixes mistydemeo/tigerbrew#45.

Library/Homebrew/build.rb

@@ -115,7 +115,11 @@ def install f
   end
 
   if f.fails_with? ENV.compiler
-    ENV.send CompilerSelector.new(f, ENV.compiler).compiler
+    begin
+      ENV.send CompilerSelector.new(f, ENV.compiler).compiler
+    rescue CompilerSelectionError => e
+      raise e.message
+    end
   end
 
   f.brew do

Library/Homebrew/compilers.rb

@@ -53,11 +53,18 @@ class CompilerSelector
     end
   end
 
+  # Attempts to select an appropriate alternate compiler, but
+  # if none can be found raises CompilerError instead
   def compiler
     begin
       cc = @compilers.pop
     end while @f.fails_with?(cc)
-    cc.nil? ? @old_compiler : cc.name
+
+    if cc.nil?
+      raise CompilerSelectionError
+    else
+      cc.name
+    end
   end
 
   private

Library/Homebrew/exceptions.rb

@@ -160,6 +160,26 @@ class BuildError < Homebrew::InstallationError
   end
 end
 
+# raised by CompilerSelector if the formula fails with all of
+# the compilers available on the user's system
+class CompilerSelectionError < StandardError
+  def message
+    if MacOS.version > :tiger then <<-EOS.undent
+      This formula cannot be built with any available compilers.
+      To install this formula, you may need to:
+        brew tap homebrew/dupes
+        brew install apple-gcc42
+      EOS
+    # tigerbrew has a separate apple-gcc42 for Xcode 2.5
+    else <<-EOS.undent
+      This formula cannot be built with any available compilers.
+      To install this formula, you need to:
+        brew install apple-gcc42
+      EOS
+    end
+  end
+end
+
 # raised in CurlDownloadStrategy.fetch
 class CurlDownloadStrategyError < RuntimeError
 end

Library/Homebrew/test/test_compiler_selector.rb

@@ -31,7 +31,7 @@ class CompilerSelectorTests < Test::Unit::TestCase
 
   def test_all_compiler_failures
     @f << :clang << :llvm << :gcc
-    assert_equal @cc, actual_cc
+    assert_raise(CompilerSelectionError) { actual_cc }
   end
 
   def test_no_compiler_failures
@@ -77,13 +77,13 @@ class CompilerSelectorTests < Test::Unit::TestCase
   def test_missing_gcc
     MacOS.stubs(:gcc_build_version).returns(nil)
     @f << :clang << :llvm
-    assert_equal @cc, actual_cc
+    assert_raise(CompilerSelectionError) { actual_cc }
   end
 
   def test_missing_llvm_and_gcc
     MacOS.stubs(:gcc_build_version).returns(nil)
     MacOS.stubs(:llvm_build_version).returns(nil)
     @f << :clang
-    assert_equal @cc, actual_cc
+    assert_raise(CompilerSelectionError) { actual_cc }
   end
 end