Handle case-insensitive filenames for load-path shadows (Bug#5845)

* lisp/emacs-lisp/shadow.el (load-path-shadows-find): Check for
shadowing with case-insensitive matching for files of case-insensitive
directories (as determined by `file-name-case-insensitive-p').
* test/lisp/emacs-lisp/shadow-tests.el: New test.
* test/lisp/emacs-lisp/shadow-resources/p1/foo.el:
* test/lisp/emacs-lisp/shadow-resources/p2/FOO.el: New test files.

lisp/emacs-lisp/shadow.el

@@ -78,6 +78,7 @@ See the documentation for `list-load-path-shadows' for further information."
 	shadows				; List of shadowings, to be returned.
 	files				; File names ever seen, with dirs.
 	dir				; The dir being currently scanned.
+        dir-case-insensitive            ; `file-name-case-insentive-p' for dir.
 	curr-files			; This dir's Emacs Lisp files.
 	orig-dir			; Where the file was first seen.
 	files-seen-this-dir		; Files seen so far in this dir.
@@ -104,6 +105,9 @@ See the documentation for `list-load-path-shadows' for further information."
 	     (message "Checking %d files in %s..." (length curr-files) dir))
 
 	(setq files-seen-this-dir nil)
+        ;; We assume that case sensitivity of a directory applies to
+        ;; its files.
+        (setq dir-case-insensitive (file-name-case-insensitive-p dir))
 
 	(dolist (file curr-files)
 
@@ -123,10 +127,12 @@ See the documentation for `list-load-path-shadows' for further information."
 	    ;; XXX.elc (or vice-versa) when they are in the same directory.
 	    (setq files-seen-this-dir (cons file files-seen-this-dir))
 
-	    (if (setq orig-dir (assoc file files))
+            (if (setq orig-dir (assoc file files
+                                      (when dir-case-insensitive
+                                        (lambda (f1 f2) (eq (compare-strings f1 nil nil f2 nil nil t) t)))))
 		;; This file was seen before, we have a shadowing.
 		;; Report it unless the files are identical.
-		(let ((base1 (concat (cdr orig-dir) "/" file))
+                (let ((base1 (concat (cdr orig-dir) "/" (car orig-dir)))
 		      (base2 (concat dir "/" file)))
 		  (if (not (and load-path-shadows-compare-text
 				(load-path-shadows-same-file-or-nonexistent

test/lisp/emacs-lisp/shadow-resources/p1/foo.el

@@ -0,0 +1 @@
+;;; This file intentionally left blank.

test/lisp/emacs-lisp/shadow-resources/p2/FOO.el

@@ -0,0 +1 @@
+;;; This file intentionally left blank.

test/lisp/emacs-lisp/shadow-tests.el

@@ -0,0 +1,49 @@
+;;; shadow-tests.el --- Test suite for shadow.  -*- lexical-binding: t -*-
+
+;; Copyright (C) 2018 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Code:
+
+(require 'ert)
+(require 'shadow)
+(eval-when-compile (require 'cl-lib))
+
+(defconst shadow-tests-data-directory
+  (expand-file-name "lisp/emacs-lisp/shadow-resources"
+                    (or (getenv "EMACS_TEST_DIRECTORY")
+                        (expand-file-name "../../.."
+                                          (or load-file-name
+                                              buffer-file-name))))
+  "Directory for shadow test files.")
+
+(ert-deftest shadow-case-insensitive ()
+  "Test shadowing for case insensitive filenames."
+  ;; Override `file-name-case-insentive-p' so we test the same thing
+  ;; regardless of what file system we're running on.
+  (cl-letf (((symbol-function 'file-name-case-insensitive-p) (lambda (_f) t)))
+    (should (equal (list (expand-file-name "p1/foo" shadow-tests-data-directory)
+                         (expand-file-name "p2/FOO" shadow-tests-data-directory))
+                   (load-path-shadows-find
+                    (list (expand-file-name "p1/" shadow-tests-data-directory)
+                          (expand-file-name "p2/" shadow-tests-data-directory))))))
+  (cl-letf (((symbol-function 'file-name-case-insensitive-p) (lambda (_f) nil)))
+    (should-not (load-path-shadows-find
+                 (list (expand-file-name "p1/" shadow-tests-data-directory)
+                       (expand-file-name "p2/" shadow-tests-data-directory))))))
+
+;;; shadow-tests.el ends here.