Support core GCC formula as a GCC compiler.

It is activated by the same mechanism as the Homebrew/versions compilers
which now check if the GCC formula uses the same, correct version.

References Homebrew#28418.

Library/Homebrew/extend/ENV/shared.rb

@@ -185,11 +185,29 @@ def ld64
     append "LDFLAGS", "-B#{ld64.bin}/"
   end
 
+  def gcc_version_formula(version)
+    gcc_formula = Formulary.factory("gcc")
+    return gcc_formula if gcc_formula.version.to_s.include?(version)
+
+    gcc_name = 'gcc' + version.delete('.')
+    Formulary.factory(gcc_name)
+  end
+
   def warn_about_non_apple_gcc(gcc)
+    gcc_name = 'gcc' + gcc.delete('.')
+
     begin
-      gcc_name = 'gcc' + gcc.delete('.')
-      gcc = Formulary.factory(gcc_name)
-      if !gcc.opt_prefix.exist?
+      gcc_formula = gcc_version_formula(gcc)
+      if gcc_formula.name == "gcc"
+        return if gcc_formula.opt_prefix.exist?
+        raise <<-EOS.undent
+        The Homebrew GCC was not installed.
+        You must:
+          brew install gcc
+        EOS
+      end
+
+      if !gcc_formula.opt_prefix.exist?
         raise <<-EOS.undent
         The requested Homebrew GCC, #{gcc_name}, was not installed.
         You must:

Library/Homebrew/extend/ENV/std.rb

@@ -73,9 +73,8 @@ def setup_build_environment(formula=nil)
 
     if cc =~ GNU_GCC_REGEXP
       warn_about_non_apple_gcc($1)
-      gcc_name = 'gcc' + $1.delete('.')
-      gcc = Formulary.factory(gcc_name)
-      self.append_path('PATH', gcc.opt_prefix/'bin')
+      gcc_formula = gcc_version_formula($1)
+      self.append_path('PATH', gcc_formula.opt_prefix/'bin')
     end
 
     # Add lib and include etc. from the current macosxsdk to compiler flags:

Library/Homebrew/extend/ENV/super.rb

@@ -135,9 +135,8 @@ def determine_path
     end
 
     if self['HOMEBREW_CC'] =~ GNU_GCC_REGEXP
-      gcc_name = 'gcc' + $1.delete('.')
-      gcc = Formulary.factory(gcc_name)
-      paths << gcc.opt_prefix/'bin'
+      gcc_formula = gcc_version_formula($1)
+      paths << gcc_formula.opt_prefix/'bin'
     end
 
     paths.to_path_s

Library/Homebrew/os/mac.rb

@@ -131,8 +131,9 @@ def gcc_40_build_version
     def gcc_42_build_version
       @gcc_42_build_version ||=
         begin
-          gcc = MacOS.locate('gcc-4.2') || Formula.factory('apple-gcc42').opt_prefix/'bin/gcc-4.2'
-          raise unless gcc.exist?
+          gcc = MacOS.locate('gcc-4.2')
+          gcc ||= Formula.factory('apple-gcc42').opt_prefix/'bin/gcc-4.2' rescue nil
+          raise if gcc.nil? || !gcc.exist?
         rescue
           gcc = nil
         end
@@ -167,13 +168,16 @@ def clang_build_version
     end
 
     def non_apple_gcc_version(cc)
-      return unless path = locate(cc)
+      path = Formula.factory("gcc").opt_prefix/"bin/#{cc}"
+      path = nil unless path.exist?
+
+      return unless path ||= locate(cc)
 
       ivar = "@#{cc.gsub(/(-|\.)/, '')}_version"
       return instance_variable_get(ivar) if instance_variable_defined?(ivar)
 
-      `#{path} --version` =~ /gcc-\d.\d \(GCC\) (\d\.\d\.\d)/
-      instance_variable_set(ivar, $1)
+      `#{path} --version` =~ /gcc(-\d\.\d \(GCC\))? (\d\.\d\.\d)/
+      instance_variable_set(ivar, $2)
     end
 
     # See these issues for some history:

Library/Homebrew/test/test_stdlib.rb

@@ -23,6 +23,7 @@ def test_apple_libstdcxx_intercompatibility
   end
 
   def test_compatibility_same_compilers_and_type
+    assert @gcc.compatible_with?(@gcc)
     assert @gcc48.compatible_with?(@gcc48)
     assert @clang.compatible_with?(@clang)
   end
@@ -33,8 +34,8 @@ def test_apple_gnu_libstdcxx_incompatibility
   end
 
   def test_gnu_cross_version_incompatibility
-    assert !@clang.compatible_with?(@gcc48)
-    assert !@gcc48.compatible_with?(@clang)
+    assert !@gcc48.compatible_with?(@gcc49)
+    assert !@gcc49.compatible_with?(@gcc48)
   end
 
   def test_libstdcxx_libcxx_incompatibility