C++11 support.

* Add options and ENV method to specify building in C++11 mode.
  - Set C++ compiler flags to enable C++11 mode.
  - To add options to support C++11 mode, a formula can now use

      option :cxx11

    to provide "--c++11" option, and detect and enable C++11 support in
    install method using

      ENV.cxx11 if build.cxx11?

Closes Homebrew/homebrew#22453.

Library/ENV/4.3/cc

@@ -45,13 +45,21 @@ class Cmd
     if @arg0 == 'cpp' or @arg0 == 'ld'
       @arg0.to_sym
     elsif @args.include? '-c'
+      if @arg0 =~ /(?:c|g|clang)\+\+/
+        :cxx
+      else
         :cc
+      end
     elsif @args.include? '-E'
       :ccE
+    else
+      if @arg0 =~ /(?:c|g|clang)\+\+/
+        :cxxld
       else
         :ccld
       end
     end
+  end
   def tool
     @tool ||= case @arg0
     when 'ld' then 'ld'
@@ -83,9 +91,9 @@ class Cmd
       args << "-syslibroot" << $sdkroot
     end if nclt?
     allflags = case mode
-    when :ccld
+    when :ccld, :cxxld
       cflags + args + cppflags + ldflags
-    when :cc
+    when :cc, :cxx
       cflags + args + cppflags
     when :ccE
       args + cppflags
@@ -149,9 +157,15 @@ class Cmd
     args
   end
   def cflags
-    return [] unless cccfg? 'O'
+    args = []
+    if mode == :cxx
+      args << '-std=c++11' if cccfg? 'x'
+      args << '-stdlib=libc++' if cccfg? 'g'
+    end
 
-    args = %w{-pipe -w -Os}
+    return args unless cccfg? 'O'
+
+    args << '-pipe' << '-w' << '-Os'
 
     # When bottling use the oldest supported CPU type.
     if cccfg? 'bc'
@@ -199,6 +213,9 @@ class Cmd
     case mode
     when :ld   then args << '-headerpad_max_install_names'
     when :ccld then args << '-Wl,-headerpad_max_install_names'
+    when :cxxld
+      args << '-Wl,-headerpad_max_install_names'
+      args << '-stdlib=libc++' if cccfg? 'g'
     end
     args
   end

Library/Homebrew/build_options.rb

@@ -7,6 +7,7 @@ class BuildOptions
 
   attr_accessor :args
   attr_accessor :universal
+  attr_accessor :cxx11
   attr_reader :options
   protected :options
 
@@ -25,6 +26,7 @@ class BuildOptions
     description ||= case name.to_s
       when "universal" then "Build a universal binary"
       when "32-bit" then "Build 32-bit only"
+      when "c++11" then "Build using C++11 mode"
       end.to_s
 
     @options << Option.new(name, description)
@@ -94,6 +96,11 @@ class BuildOptions
     universal || args.include?('--universal') && has_option?('universal')
   end
 
+  # True if the user requested to enable C++11 mode.
+  def cxx11?
+    cxx11 || args.include?('--c++11') && has_option?('c++11')
+  end
+
   # Request a 32-bit only build.
   # This is needed for some use-cases though we prefer to build Universal
   # when a 32-bit version is needed.

Library/Homebrew/extend/ENV/std.rb

@@ -322,6 +322,17 @@ module Stdenv
     end
   end
 
+  def cxx11
+    if compiler == :clang
+      append 'CXX', '-std=c++11'
+      append 'CXX', '-stdlib=libc++'
+    elsif compiler =~ /gcc-4\.(8|9)/
+      append 'CXX', '-std=c++11'
+    else
+      raise "The selected compiler doesn't support C++11: #{compiler}"
+    end
+  end
+
   def replace_in_cflags before, after
     CC_FLAG_VARS.each do |key|
       self[key] = self[key].sub(before, after) if has_key?(key)

Library/Homebrew/extend/ENV/super.rb

@@ -97,6 +97,8 @@ module Superenv
     # A - Installing from a bottle on PPC with Altivec
     # O - Enables argument refurbishing. Only active under the
     #     make/bsdmake wrappers currently.
+    # x - Enable C++11 mode.
+    # g - Enable "-stdlib=libc++" for clang.
     #
     # On 10.8 and newer, these flags will also be present:
     # s - apply fix for sed's Unicode support
@@ -110,6 +112,17 @@ module Superenv
     append 'HOMEBREW_CCCFG', "u", ''
   end
 
+  def cxx11
+    if self['HOMEBREW_CC'] == 'clang'
+      append 'HOMEBREW_CCCFG', "x", ''
+      append 'HOMEBREW_CCCFG', "g", ''
+    elsif self['HOMEBREW_CC'] =~ /gcc-4\.(8|9)/
+      append 'HOMEBREW_CCCFG', "x", ''
+    else
+      raise "The selected compiler doesn't support C++11: #{self['HOMEBREW_CC']}"
+    end
+  end
+
   # m32 on superenv does not add any CC flags. It prevents "-m32" from being erased.
   def m32
     append 'HOMEBREW_CCCFG', "3", ''

Library/Homebrew/software_spec.rb

@@ -50,6 +50,7 @@ class SoftwareSpec
   end
 
   def option name, description=nil
+    name = 'c++11' if name == :cxx11
     name = name.to_s if Symbol === name
     raise "Option name is required." if name.empty?
     raise "Options should not start with dashes." if name[0, 1] == "-"