Cleanup use of CxxStdlib

- remove usage on macOS as we don't care about it there
- don't error out on incompatibility but still store stdlib on Linux
- remove (now) unused methods

Library/Homebrew/build.rb

@@ -8,7 +8,6 @@ old_trap = trap("INT") { exit! 130 }
 
 require_relative "global"
 require "build_options"
-require "cxxstdlib"
 require "keg"
 require "extend/ENV"
 require "debrew"
@@ -174,7 +173,7 @@ class Build
             "#{formula.full_name} #{formula.build.used_options.sort.join(" ")}".strip
           formula.install
 
-          stdlibs = detect_stdlibs(ENV.compiler)
+          stdlibs = detect_stdlibs
           tab = Tab.create(formula, ENV.compiler, stdlibs.first)
           tab.write
 
@@ -186,9 +185,8 @@ class Build
     end
   end
 
-  def detect_stdlibs(compiler)
+  def detect_stdlibs
     keg = Keg.new(formula.prefix)
-    CxxStdlib.check_compatibility(formula, deps, keg, compiler)
 
     # The stdlib recorded in the install receipt is used during dependency
     # compatibility checks, so we only care about the stdlib that libraries

Library/Homebrew/cxxstdlib.rb

@@ -22,52 +22,14 @@ class CxxStdlib
   def self.create(type, compiler)
     raise ArgumentError, "Invalid C++ stdlib type: #{type}" if type && [:libstdcxx, :libcxx].exclude?(type)
 
-    apple_compiler = !compiler.to_s.match?(GNU_GCC_REGEXP)
-    CxxStdlib.new(type, compiler, apple_compiler)
-  end
-
-  def self.check_compatibility(formula, deps, keg, compiler)
-    return if formula.skip_cxxstdlib_check?
-
-    stdlib = create(keg.detect_cxx_stdlibs.first, compiler)
-
-    begin
-      stdlib.check_dependencies(formula, deps)
-    rescue CompatibilityError => e
-      opoo e.message
-    end
+    CxxStdlib.new(type, compiler)
   end
 
   attr_reader :type, :compiler
 
-  def initialize(type, compiler, apple_compiler)
+  def initialize(type, compiler)
     @type = type
     @compiler = compiler.to_sym
-    @apple_compiler = apple_compiler
-  end
-
-  # If either package doesn't use C++, all is well.
-  # libstdc++ and libc++ aren't ever intercompatible.
-  # libstdc++ is compatible across Apple compilers, but
-  # not between Apple and GNU compilers, nor between GNU compiler versions.
-  def compatible_with?(other)
-    return true if type.nil? || other.type.nil?
-
-    return false unless type == other.type
-
-    apple_compiler? && other.apple_compiler? ||
-      !other.apple_compiler? && compiler.to_s[4..6] == other.compiler.to_s[4..6]
-  end
-
-  def check_dependencies(formula, deps)
-    deps.each do |dep|
-      # Software is unlikely to link against libraries from build-time deps, so
-      # it doesn't matter if they link against different C++ stdlibs.
-      next if dep.build?
-
-      dep_stdlib = Tab.for_formula(dep.to_formula).cxxstdlib
-      raise CompatibilityError.new(formula, dep, dep_stdlib) unless compatible_with? dep_stdlib
-    end
   end
 
   def type_string
@@ -78,8 +40,4 @@ class CxxStdlib
   def inspect
     "#<#{self.class.name}: #{compiler} #{type}>"
   end
-
-  def apple_compiler?
-    @apple_compiler
-  end
 end

Library/Homebrew/extend/os/mac/keg_relocate.rb

@@ -16,7 +16,7 @@ class Keg
     end
   end
 
-  undef relocate_dynamic_linkage, detect_cxx_stdlibs
+  undef relocate_dynamic_linkage
 
   def relocate_dynamic_linkage(relocation)
     mach_o_files.each do |file|
@@ -39,25 +39,6 @@ class Keg
     end
   end
 
-  # Detects the C++ dynamic libraries in-place, scanning the dynamic links
-  # of the files within the keg.
-  # Note that this doesn't attempt to distinguish between libstdc++ versions,
-  # for instance between Apple libstdc++ and GNU libstdc++.
-  def detect_cxx_stdlibs(options = {})
-    skip_executables = options.fetch(:skip_executables, false)
-    results = Set.new
-
-    mach_o_files.each do |file|
-      next if file.mach_o_executable? && skip_executables
-
-      dylibs = file.dynamically_linked_libraries
-      results << :libcxx unless dylibs.grep(/libc\+\+.+\.dylib/).empty?
-      results << :libstdcxx unless dylibs.grep(/libstdc\+\+.+\.dylib/).empty?
-    end
-
-    results.to_a
-  end
-
   def fix_dynamic_linkage
     mach_o_files.each do |file|
       file.ensure_writable do

Library/Homebrew/formula_installer.rb

@@ -1,7 +1,6 @@
 # typed: false
 # frozen_string_literal: true
 
-require "cxxstdlib"
 require "formula"
 require "keg"
 require "tab"
@@ -1137,13 +1136,6 @@ class FormulaInstaller
     skip_linkage = formula.bottle_specification.skip_relocation?
     keg.replace_placeholders_with_locations tab.changed_files, skip_linkage: skip_linkage
 
-    unless ignore_deps?
-      CxxStdlib.check_compatibility(
-        formula, formula.recursive_dependencies,
-        Keg.new(formula.prefix), tab.compiler
-      )
-    end
-
     # fill in missing/outdated parts of the tab
     # keep in sync with Tab#to_bottle_json
     tab.used_options = []

Library/Homebrew/test/cxxstdlib_spec.rb

@@ -6,50 +6,7 @@ require "cxxstdlib"
 
 describe CxxStdlib do
   let(:clang) { described_class.create(:libstdcxx, :clang) }
-  let(:gcc6) { described_class.create(:libstdcxx, "gcc-6") }
-  let(:gcc7) { described_class.create(:libstdcxx, "gcc-7") }
   let(:lcxx) { described_class.create(:libcxx, :clang) }
-  let(:purec) { described_class.create(nil, :clang) }
-
-  describe "#compatible_with?" do
-    specify "compatibility with itself" do
-      expect(gcc7).to be_compatible_with(gcc7)
-      expect(clang).to be_compatible_with(clang)
-    end
-
-    specify "Apple/GNU libstdcxx incompatibility" do
-      expect(clang).not_to be_compatible_with(gcc7)
-      expect(gcc7).not_to be_compatible_with(clang)
-    end
-
-    specify "GNU cross-version incompatibility" do
-      expect(gcc6).not_to be_compatible_with(gcc7)
-      expect(gcc7).not_to be_compatible_with(gcc6)
-    end
-
-    specify "libstdcxx and libcxx incompatibility" do
-      expect(clang).not_to be_compatible_with(lcxx)
-      expect(lcxx).not_to be_compatible_with(clang)
-    end
-
-    specify "compatibility for non-cxx software" do
-      expect(purec).to be_compatible_with(clang)
-      expect(clang).to be_compatible_with(purec)
-      expect(purec).to be_compatible_with(purec)
-      expect(purec).to be_compatible_with(gcc7)
-      expect(gcc7).to be_compatible_with(purec)
-    end
-  end
-
-  describe "#apple_compiler?" do
-    it "returns true for Apple compilers" do
-      expect(clang).to be_an_apple_compiler
-    end
-
-    it "returns false for non-Apple compilers" do
-      expect(gcc7).not_to be_an_apple_compiler
-    end
-  end
 
   describe "#type_string" do
     specify "formatting" do