Prefer brewed Clang over gcc-6 for needs :openmp

Clang has fully implemented OpenMP support as of LLVM 3.7, so if OpenMP
is required by a formula gcc is no longer the only choice of compiler.

Clang should be preferred over gcc because using gcc meant linking
against libstdc++, which is ABI incompatible with libc++. This may be
unnoticeable for some users, but it causes other builds to fail, e.g.
pstoedit when imagemagick was built with OpenMP. pstoedit is required
for the octave formula, so for some users this could be a significant
problem.

Library/Homebrew/compilers.rb

@@ -3,9 +3,10 @@ module CompilerConstants
   GNU_GCC_VERSIONS = %w[4.3 4.4 4.5 4.6 4.7 4.8 4.9 5 6 7].freeze
   GNU_GCC_REGEXP = /^gcc-(4\.[3-9]|[5-7])$/
   COMPILER_SYMBOL_MAP = {
-    "gcc-4.0"  => :gcc_4_0,
+    "gcc-4.0"    => :gcc_4_0,
-    "gcc-4.2"  => :gcc,
+    "gcc-4.2"    => :gcc,
-    "clang"    => :clang,
+    "clang"      => :clang,
+    "llvm_clang" => :llvm_clang,
   }.freeze
 
   COMPILERS = COMPILER_SYMBOL_MAP.values +
@@ -86,7 +87,7 @@ class CompilerSelector
   Compiler = Struct.new(:name, :version)
 
   COMPILER_PRIORITY = {
-    clang: [:clang, :gcc, :gnu, :gcc_4_0],
+    clang: [:clang, :gcc, :gnu, :gcc_4_0, :llvm_clang],
     gcc: [:gcc, :gnu, :clang, :gcc_4_0],
     gcc_4_0: [:gcc_4_0, :gcc, :gnu, :clang],
   }.freeze

Library/Homebrew/development_tools.rb

@@ -86,6 +86,13 @@ class DevelopmentTools
         end
     end
 
+    def llvm_clang_build_version
+      @llvm_clang_build_version ||= if Tab.for_name "llvm"
+        path = Formulary.factory("llvm").opt_prefix/"bin/clang"
+        `#{path} --version`[/clang version (\d\.\d\.\d)/, 1]
+      end
+    end
+
     def non_apple_gcc_version(cc)
       (@non_apple_gcc_version ||= {}).fetch(cc) do
         path = HOMEBREW_PREFIX.join("opt", "gcc", "bin", cc)

Library/Homebrew/shims/super/cc

@@ -58,6 +58,8 @@ class Cmd
     @tool ||= case @arg0
     when "ld" then "ld"
     when "cpp" then "cpp"
+    when /llvm_(clang(\+\+)?)/
+     "#{ENV["HOMEBREW_PREFIX"]}/opt/llvm/bin/#{$1}"
     when /\w\+\+(-\d(\.\d)?)?$/
       case ENV["HOMEBREW_CC"]
       when /clang/
@@ -154,7 +156,7 @@ class Cmd
       /^-O[0-9zs]?$/, "-fast", "-no-cpp-precomp",
       "-pedantic", "-pedantic-errors", "-Wno-long-double",
       "-Wno-unused-but-set-variable"
-    when "-fopenmp", "-lgomp", "-mno-fused-madd", "-fforce-addr", "-fno-defer-pop",
+    when "-mno-fused-madd", "-fforce-addr", "-fno-defer-pop",
       "-mno-dynamic-no-pic", "-fearly-inlining", /^-f(?:no-)?inline-functions-called-once/,
       /^-finline-limit/, /^-f(?:no-)?check-new/, "-fno-delete-null-pointer-checks",
       "-fcaller-saves", "-fthread-jumps", "-fno-reorder-blocks", "-fcse-skip-blocks",
@@ -163,6 +165,8 @@ class Cmd
       "-fuse-linker-plugin", "-frounding-math"
       # clang doesn't support these flags
       args << arg unless tool =~ /^clang/
+    when "-fopenmp", "-lgomp"
+      args << arg if tool =~ /^llvm_clang/
     when "--fast-math"
       arg = "-ffast-math" if tool =~ /^clang/
       args << arg

Library/Homebrew/shims/super/llvm_clang

@@ -0,0 +1 @@
+cc

Library/Homebrew/shims/super/llvm_clang++

@@ -0,0 +1 @@
+cc