Check version conflicts using linkage.

Instead of refusing to install software preemptively by assuming
multiple linkage to differing versions of the same library we now make
`brew linkage --test` verify that we don't have two versions of the
same library linked at the same time.

This will be considerably more permissive whilst checking the actual
problem that we're worried about.

Library/Homebrew/formula_installer.rb

@@ -155,8 +155,6 @@ class FormulaInstaller
 
     recursive_deps = formula.recursive_dependencies
     recursive_formulae = recursive_deps.map(&:to_formula)
-    recursive_runtime_formulae =
-      formula.runtime_formula_dependencies(undeclared: false)
 
     recursive_dependencies = []
     recursive_formulae.each do |dep|
@@ -182,25 +180,6 @@ class FormulaInstaller
       EOS
     end
 
-    version_hash = {}
-    version_conflicts = Set.new
-    recursive_runtime_formulae.each do |f|
-      name = f.name
-      unversioned_name, = name.split("@")
-      next if unversioned_name == "python"
-      version_hash[unversioned_name] ||= Set.new
-      version_hash[unversioned_name] << name
-      next if version_hash[unversioned_name].length < 2
-      version_conflicts += version_hash[unversioned_name]
-    end
-    unless version_conflicts.empty?
-      raise CannotInstallFormulaError, <<~EOS
-        #{formula.full_name} contains conflicting version recursive dependencies:
-          #{version_conflicts.to_a.join ", "}
-        View these with `brew deps --tree #{formula.full_name}`.
-      EOS
-    end
-
     pinned_unsatisfied_deps = recursive_deps.select do |dep|
       dep.to_formula.pinned? && !dep.satisfied?(inherited_options_for(dep))
     end

Library/Homebrew/linkage_checker.rb

@@ -19,6 +19,7 @@ class LinkageChecker
     @indirect_deps    = []
     @undeclared_deps  = []
     @unnecessary_deps = []
+    @version_conflict_deps = []
 
     check_dylibs(rebuild_cache: rebuild_cache)
   end
@@ -50,11 +51,14 @@ class LinkageChecker
   def display_test_output(puts_output: true)
     display_items "Missing libraries", @broken_dylibs, puts_output: puts_output
     display_items "Broken dependencies", @broken_deps, puts_output: puts_output
+    display_items "Conflicting libraries", @version_conflict_deps, puts_output: puts_output
     puts "No broken library linkage" unless broken_library_linkage?
   end
 
   def broken_library_linkage?
-    !@broken_dylibs.empty? || !@broken_deps.empty?
+    !@broken_dylibs.empty? ||
+      !@broken_deps.empty? ||
+      !@version_conflict_deps.empty?
   end
 
   private
@@ -127,8 +131,8 @@ class LinkageChecker
     end
 
     if formula
-      @indirect_deps, @undeclared_deps, @unnecessary_deps =
+      @indirect_deps, @undeclared_deps, @unnecessary_deps,
-        check_undeclared_deps
+        @version_conflict_deps = check_formula_deps
     end
 
     return unless keg_files_dylibs_was_empty
@@ -136,7 +140,7 @@ class LinkageChecker
     store&.update!(keg_files_dylibs: keg_files_dylibs)
   end
 
-  def check_undeclared_deps
+  def check_formula_deps
     filter_out = proc do |dep|
       next true if dep.build?
       next false unless dep.optional? || dep.recommended?
@@ -170,13 +174,25 @@ class LinkageChecker
     unnecessary_deps = declared_deps_full_names.reject do |full_name|
       next true if Formula[full_name].bin.directory?
       name = full_name.split("/").last
-      @brewed_dylibs.keys.map { |x| x.split("/").last }.include?(name)
+      @brewed_dylibs.keys.map { |l| l.split("/").last }.include?(name)
     end
 
     missing_deps = @broken_deps.values.flatten.map { |d| dylib_to_dep(d) }
     unnecessary_deps -= missing_deps
 
-    [indirect_deps, undeclared_deps, unnecessary_deps]
+    version_hash = {}
+    version_conflict_deps = Set.new
+    @brewed_dylibs.keys.each do |l|
+      name = l.split("/").last
+      unversioned_name, = name.split("@")
+      version_hash[unversioned_name] ||= Set.new
+      version_hash[unversioned_name] << name
+      next if version_hash[unversioned_name].length < 2
+      version_conflict_deps += version_hash[unversioned_name]
+    end
+
+    [indirect_deps, undeclared_deps,
+     unnecessary_deps, version_conflict_deps.to_a]
   end
 
   def sort_by_formula_full_name!(arr)