Fix 'internal--c-header-file-path'

* lisp/subr.el (internal--c-header-file-path): Fix for MS-Windows:
don't prepend the (usually non-existent) "/usr/include", and run
each directory through 'expand-file-name' to remove the many ".."
elements and mirror any backslashes.  Invoke "clang" if "gcc" is
not available or is actually clang.

* test/lisp/subr-tests.el
(subr-tests-internal--c-header-file-path): Fix for MS-Windows:
test the path by looking for stdio.h, and expand all directory
names to compare to expected results.

lisp/subr.el

@@ -7642,14 +7642,20 @@ and return the value found in PLACE instead."
   ;; cc-search-directories, semantic-c-dependency-system-include-path,
   ;; semantic-gcc-setup
   (delete-dups
-   (let ((base '("/usr/include" "/usr/local/include")))
-     (cond ((or (internal--gcc-is-clang-p)
-                (and (executable-find "clang")
-                     (not (executable-find "gcc"))))
-            ;; This is either macOS, or a system with clang only.
+   ;; We treat MS-Windows/MS-DOS specially, since there's no
+   ;; widely-accepted canonical directory for C include files.
+   (let ((base (if (not (memq system-type '(windows-nt ms-dos)))
+                   '("/usr/include" "/usr/local/include")))
+         (call-clang-p (or (internal--gcc-is-clang-p)
+                           (and (executable-find "clang")
+                                (not (executable-find "gcc"))))))
+     (cond ((or call-clang-p
+                (memq system-type '(windows-nt ms-dos)))
+            ;; This is either macOS, or MS-Windows/MS-DOS, or a system
+            ;; with clang only.
             (with-temp-buffer
               (ignore-errors
-                (call-process (if (internal--gcc-is-clang-p) "gcc" "clang")
+                (call-process (if call-clang-p "clang" "gcc")
                               nil t nil
                               "-v" "-E" "-"))
               (goto-char (point-min))
@@ -7663,10 +7669,15 @@ and return the value found in PLACE instead."
                  (pos-bol)))
               (while (search-forward "(framework directory)" nil t)
                 (delete-line))
-              (append base
-                      (reverse
-                       (split-string (buffer-substring-no-properties
-                                      (point-min) (point-max)))))))
+              ;; "gcc -v" reports file names with many "..", so we
+              ;; normalize it.
+              (or (mapcar #'expand-file-name
+                          (append base
+                                  (split-string (buffer-substring-no-properties
+                                                 (point-min) (point-max)))))
+                  ;; Fallback for whedn the compiler is not available.
+                  (list (expand-file-name "/usr/include")
+                        (expand-file-name "/usr/local/include")))))
            ;; Prefer GCC.
            ((let ((arch (with-temp-buffer
                           (when (eq 0 (ignore-errors

test/lisp/subr-tests.el

@@ -1465,14 +1465,17 @@ final or penultimate step during initialization."))
 
 (ert-deftest subr-tests-internal--c-header-file-path ()
   (should (seq-every-p #'stringp (internal--c-header-file-path)))
-  (should (member "/usr/include" (internal--c-header-file-path)))
+  (should (locate-file "stdio.h" (internal--c-header-file-path)))
+  (or (memq system-type '(windows-nt ms-dos))
+      (should (member "/usr/include" (internal--c-header-file-path))))
   (should (equal (internal--c-header-file-path)
                  (delete-dups (internal--c-header-file-path))))
   ;; Return a meaningful result even if calling some compiler fails.
   (cl-letf (((symbol-function 'call-process)
              (lambda (_program &optional _infile _destination _display &rest _args) 1)))
     (should (seq-every-p #'stringp (internal--c-header-file-path)))
-    (should (member "/usr/include" (internal--c-header-file-path)))
+    (should (member (expand-file-name "/usr/include")
+                    (internal--c-header-file-path)))
     (should (equal (internal--c-header-file-path)
                    (delete-dups (internal--c-header-file-path))))))
 
@@ -1482,13 +1485,16 @@ final or penultimate step during initialization."))
              (lambda (_program &optional _infile _destination _display &rest args)
                (when (equal (car args) "-print-multiarch")
                  (insert "\n") 0))))
-    (should (member "/usr/include" (internal--c-header-file-path))))
+    (should (member (expand-file-name "/usr/include")
+                    (internal--c-header-file-path))))
   ;; Handle single values of "gcc -print-multiarch".
-  (cl-letf (((symbol-function 'call-process)
+  (cl-letf ((system-type 'foo)
+            ((symbol-function 'call-process)
              (lambda (_program &optional _infile _destination _display &rest args)
                (when (equal (car args) "-print-multiarch")
                  (insert "x86_64-linux-gnu\n") 0))))
-    (should (member "/usr/include/x86_64-linux-gnu" (internal--c-header-file-path)))))
+    (should (member (expand-file-name "/usr/include/x86_64-linux-gnu")
+                    (internal--c-header-file-path)))))
 
 (ert-deftest subr-tests-internal--c-header-file-path/clang-mocked ()
   ;; Handle clang 15.0.0 output on macOS 15.2.
@@ -1519,8 +1525,7 @@ End of search list.
 # 1 \"<built-in>\" 2
 # 1 \"<stdin>\" 2")
                0)))
-    (should (member "/usr/include" (internal--c-header-file-path)))
-    (should (member "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/15.0.0/include"
+    (should (member (expand-file-name "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/15.0.0/include")
                     (internal--c-header-file-path)))))
 
 (provide 'subr-tests)