compilers: prefer gcc 5 on linux

Fixes #10170 by preferring gcc@5 on linux
This makes sure ENV.cc and ENV.cxx is correctly set:

If a formula does not explicitely depend on a brewed gcc,
ENV.cc is set to gcc-5 (system gcc-5 or brewed gcc-5) with this change,
even if other gcc versions are installed on the system.

Library/Homebrew/compilers.rb

@@ -78,6 +78,7 @@ end
 #
 # @api private
 class CompilerSelector
+  extend T::Sig
   include CompilerConstants
 
   Compiler = Struct.new(:name, :version)
@@ -111,9 +112,14 @@ class CompilerSelector
 
   private
 
+  sig { returns(String) }
+  def preferred_gcc
+    "gcc"
+  end
+
   def gnu_gcc_versions
     # prioritize gcc version provided by gcc formula.
-    v = Formulary.factory("gcc").version.to_s.slice(/\d+/)
+    v = Formulary.factory(preferred_gcc).version.to_s.slice(/\d+/)
     GNU_GCC_VERSIONS - [v] + [v] # move the version to the end of the list
   rescue FormulaUnavailableError
     GNU_GCC_VERSIONS
@@ -150,3 +156,5 @@ class CompilerSelector
     end
   end
 end
+
+require "extend/os/compilers"

Library/Homebrew/extend/os/compilers.rb

@@ -0,0 +1,4 @@
+# typed: strict
+# frozen_string_literal: true
+
+require "extend/os/linux/compilers" if OS.linux?

Library/Homebrew/extend/os/linux/compilers.rb

@@ -0,0 +1,11 @@
+# typed: strict
+# frozen_string_literal: true
+
+class CompilerSelector
+  sig { returns(String) }
+  def preferred_gcc
+    # gcc-5 is the lowest gcc version we support on Linux.
+    # gcc-5 is the default gcc in Ubuntu 16.04 (used for our CI)
+    "gcc@5"
+  end
+end

Library/Homebrew/test/compiler_selector_spec.rb

@@ -22,6 +22,7 @@ describe CompilerSelector do
       case name
       when "gcc-7" then Version.create("7.1")
       when "gcc-6" then Version.create("6.1")
+      when "gcc-5" then Version.create("5.1")
       else Version::NULL
       end
     end
@@ -42,16 +43,31 @@ describe CompilerSelector do
       expect(selector.compiler).to eq("gcc-7")
     end
 
-    it "returns gcc-6 if gcc formula offers gcc-6" do
+    it "returns gcc-6 if gcc formula offers gcc-6 on mac", :needs_macos do
       software_spec.fails_with(:clang)
       allow(Formulary).to receive(:factory).with("gcc").and_return(double(version: "6.0"))
       expect(selector.compiler).to eq("gcc-6")
     end
 
+    it "returns gcc-5 if gcc formula offers gcc-5 on linux", :needs_linux do
+      software_spec.fails_with(:clang)
+      allow(Formulary).to receive(:factory).with("gcc@5").and_return(double(version: "5.0"))
+      expect(selector.compiler).to eq("gcc-5")
+    end
+
+    it "returns gcc-6 if gcc formula offers gcc-6 and fails with gcc-5 and gcc-7 on linux", :needs_linux do
+      software_spec.fails_with(:clang)
+      software_spec.fails_with(gcc: "5")
+      software_spec.fails_with(gcc: "7")
+      allow(Formulary).to receive(:factory).with("gcc@5").and_return(double(version: "5.0"))
+      expect(selector.compiler).to eq("gcc-6")
+    end
+
     it "raises an error when gcc or llvm is missing" do
       software_spec.fails_with(:clang)
       software_spec.fails_with(gcc: "7")
       software_spec.fails_with(gcc: "6")
+      software_spec.fails_with(gcc: "5")
 
       expect { selector.compiler }.to raise_error(CompilerSelectionError)
     end