Handle conflicts where links point at symlinks

Fixes Homebrew/homebrew#30664.

Library/Homebrew/keg.rb

@@ -322,11 +322,12 @@ class Keg
   private
 
   def resolve_any_conflicts dst, mode
-    # if it isn't a directory then a severe conflict is about to happen. Let
-    # it, and the exception that is generated will message to the user about
-    # the situation
-    if dst.symlink? and dst.directory?
-      src = dst.resolved_path
+    src = dst.resolved_path
+    # src itself may be a symlink, so check lstat to ensure we are dealing with
+    # a directory, and not a symlink pointing at a directory (which needs to be
+    # treated as a file). In other words, we onlly want to resolve one symlink.
+    # If it isn't a directory, make_relative_symlink will raise an exception.
+    if dst.symlink? && src.lstat.directory?
       keg = Keg.for(src)
       dst.unlink unless mode.dry_run
       keg.link_dir(src, mode) { :mkpath }

Library/Homebrew/test/test_keg.rb

@@ -217,4 +217,20 @@ class LinkTests < Homebrew::TestCase
     assert_predicate link.resolved_path, :symlink?
     assert_predicate link.lstat, :symlink?
   end
+
+  def test_links_to_symlinks_are_not_removed
+    a = HOMEBREW_CELLAR.join("a", "1.0")
+    b = HOMEBREW_CELLAR.join("b", "1.0")
+
+    a.join("lib", "example").mkpath
+    a.join("lib", "example2").make_symlink "example"
+    b.join("lib", "example2").mkpath
+
+    Keg.new(a).link
+
+    lib = HOMEBREW_PREFIX.join("lib")
+    assert_equal 2, lib.children.length
+    assert_raises(Keg::ConflictError) { Keg.new(b).link }
+    assert_equal 2, lib.children.length
+  end
 end