install_symlink_p: Fix when dest includes a symlink

install_symlink_p does not work as intended when dst includes a symlink in its path.
relative_path_from requires that both src and dst be real paths without symlinks.

From https://ruby-doc.org/stdlib-2.3.7/libdoc/pathname/rdoc/Pathname.html#method-i-relative_path_from
This method doesn't access the filesystem. It assumes no symlinks.

Library/Homebrew/extend/pathname.rb

@@ -135,10 +135,11 @@ class Pathname
   end
 
   def install_symlink_p(src, new_basename)
-    src = Pathname(src).expand_path(self)
-    dst = join(new_basename)
     mkpath
-    FileUtils.ln_sf(src.relative_path_from(dst.parent), dst)
+    dstdir = realpath
+    src = Pathname(src).expand_path(dstdir)
+    src = src.dirname.realpath/src.basename if src.dirname.exist?
+    FileUtils.ln_sf(src.relative_path_from(dstdir), dstdir/new_basename)
   end
   private :install_symlink_p
 

Library/Homebrew/test/pathname_spec.rb

@@ -245,6 +245,14 @@ describe Pathname do
       dst.install_symlink "foo" => "bar"
       expect((dst/"bar").readlink).to eq(described_class.new("foo"))
     end
+
+    it "can install relative symlinks in a symlinked directory" do
+      mkdir_p dst/"1/2"
+      dst.install_symlink "1/2" => "12"
+      expect((dst/"12").readlink).to eq(described_class.new("1/2"))
+      (dst/"12").install_symlink dst/"foo"
+      expect((dst/"12/foo").readlink).to eq(described_class.new("../../foo"))
+    end
   end
 
   describe InstallRenamed do