ENV: mark gcc-6 as supporting C++11 (#349)

Add SharedEnvExtension#gcc_with_cxx11_support? to centralise the
logic for checking whether a compiler is known to support C++11.

Update logic to accept GCC 4.8 and above (including 6). Thereby also
address oversight in #163 where support for GCC 6 was added without
updating the C++11 compiler whitelist.

Add tests for Superenv#cxx11.

Closes #346.

3 files changed, 8 insertions, 4 deletions

diff --git a/Library/Homebrew/extend/ENV/shared.rb b/Library/Homebrew/extend/ENV/shared.rb
index eeba5bbf9..2debbae58 100644
--- a/Library/Homebrew/extend/ENV/shared.rb
+++ b/Library/Homebrew/extend/ENV/shared.rb
@@ -322,4 +322,9 @@ module SharedEnvExtension
       raise "Non-Apple GCC can't build universal binaries"
     end
   end
+
+  def gcc_with_cxx11_support?(cc)
+    version = cc[/^gcc-(\d+(?:\.\d+)?)$/, 1]
+    version && Version.new(version) >= Version.new("4.8")
+  end
 end
diff --git a/Library/Homebrew/extend/ENV/std.rb b/Library/Homebrew/extend/ENV/std.rb
index 87eebb753..8be0b48ce 100644
--- a/Library/Homebrew/extend/ENV/std.rb
+++ b/Library/Homebrew/extend/ENV/std.rb
@@ -287,7 +287,7 @@ module Stdenv
     if compiler == :clang
       append "CXX", "-std=c++11"
       append "CXX", "-stdlib=libc++"
-    elsif compiler =~ /gcc-(4\.(8|9)|5)/
+    elsif gcc_with_cxx11_support?(compiler)
       append "CXX", "-std=c++11"
     else
       raise "The selected compiler doesn't support C++11: #{compiler}"
diff --git a/Library/Homebrew/extend/ENV/super.rb b/Library/Homebrew/extend/ENV/super.rb
index 7f335acb2..1ef5c465d 100644
--- a/Library/Homebrew/extend/ENV/super.rb
+++ b/Library/Homebrew/extend/ENV/super.rb
@@ -303,11 +303,10 @@ module Superenv
   end
 
   def cxx11
-    case homebrew_cc
-    when "clang"
+    if homebrew_cc == "clang"
       append "HOMEBREW_CCCFG", "x", ""
       append "HOMEBREW_CCCFG", "g", ""
-    when /gcc-(4\.(8|9)|5)/
+    elsif gcc_with_cxx11_support?(homebrew_cc)
       append "HOMEBREW_CCCFG", "x", ""
     else
       raise "The selected compiler doesn't support C++11: #{homebrew_cc}"