compilers: prioritise LLVM clang over GCC on macOS

However, when the formula has a GCC dependency, then try GCC first

Author: cho-m

Library/Homebrew/compilers.rb

@@ -102,12 +102,18 @@ class CompilerSelector
   Compiler = Struct.new(:type, :name, :version)
 
   COMPILER_PRIORITY = {
-    clang: [:clang, :gnu, :llvm_clang],
+    clang: [:clang, :llvm_clang, :gnu],
     gcc:   [:gnu, :gcc, :llvm_clang, :clang],
   }.freeze
 
-  def self.select_for(formula, compilers = self.compilers)
-    new(formula, DevelopmentTools, compilers).compiler
+  def self.select_for(formula, compilers = nil, testing_formula: false)
+    if compilers.nil? && DevelopmentTools.default_compiler == :clang
+      deps = formula.deps.filter_map do |dep|
+        dep.name if dep.required? || (testing_formula && dep.test?) || (!testing_formula && dep.build?)
+      end
+      compilers = [:clang, :gnu, :llvm_clang] if deps.none?("llvm") && deps.any?(/^gcc(@\d+)?$/)
+    end
+    new(formula, DevelopmentTools, compilers || self.compilers).compiler
   end
 
   def self.compilers

Library/Homebrew/extend/ENV/shared.rb

@@ -53,6 +53,7 @@ def setup_build_environment(formula: nil, cc: nil, build_bottle: false, bottle_a
     @build_bottle = build_bottle
     @bottle_arch = bottle_arch
     @debug_symbols = debug_symbols
+    @testing_formula = testing_formula
     reset
   end
 
@@ -221,12 +222,12 @@ def compiler
 
       if @formula
         compilers = [compiler] + CompilerSelector.compilers
-        compiler = CompilerSelector.select_for(@formula, compilers)
+        compiler = CompilerSelector.select_for(@formula, compilers, testing_formula: @testing_formula)
       end
 
       compiler
     elsif @formula
-      CompilerSelector.select_for(@formula)
+      CompilerSelector.select_for(@formula, testing_formula: @testing_formula)
     else
       DevelopmentTools.default_compiler
     end