Support expansions in directory completions as well

Also fixes some issues with duplicate results in __fish_complete_suffix.

CHANGELOG.md

@@ -57,6 +57,7 @@ This section is for changes merged to the `major` branch that are not also merge
 - Variables set in `if` and `while` conditions are available outside the block (#4820).
 - The universal variables file no longer contains the MAC address. It is now at the fixed location `.config/fish/fish_universal_variables` (#1912).
 - `alias` now has a `-s` and `--save` option to save the function generated by the alias using `funcsave` (#4878).
+- Path completions now support expansions, meaning expressions like `python ~/<TAB>` now provides file suggestions just like any other relative or absolute path. (This includes support for other expansions, too.)
 
 ## Other significant changes
 - Command substitution output is now limited to 10 MB by default (#3822).

share/functions/__fish_complete_directories.fish

@@ -12,7 +12,10 @@ function __fish_complete_directories -d "Complete directory prefixes" --argument
         set comp (commandline -ct)
     end
 
-    set -l dirs $comp*/
+    # Inherit support for completing expansions from __fish_complete_suffix
+    # Supports completing expressions like ~/foo or x$bar, etc.
+    set -l dirs (__fish_complete_suffix $comp /)
+
     if set -q dirs[1]
         printf "%s\t$desc\n" $dirs
     end

share/functions/__fish_complete_suffix.fish

@@ -35,16 +35,16 @@ function __fish_complete_suffix -d "Complete using files"
 
     # Strip leading ./ as it confuses the detection of base and suffix
     # It is conditionally re-added below.
-    set -l base_temp (string replace -r '^\./' '' -- $comp)
+    set -l base_temp (string replace -r '^\\./' '' -- $comp)
 
-    set base (string replace -r '\.[^.]*$' '' -- $base_temp | string trim -c '\'"') # " make emacs syntax highlighting happy
+    set base (string replace -r '\\.[^.]*$' '' -- $base_temp | string trim -c '\'"') # " make emacs syntax highlighting happy
     # echo "base: $base" > /dev/tty
     # echo "suffix: $suff" > /dev/tty
 
     # If $comp is "./ma" and the file is "main.py", we'll catch that case here,
     # but complete.cpp will not consider it a match, so we have to output the
     # correct form.
-    if string match -qr '^\./' -- $comp
+    if string match -qr '^\\./' -- $comp
         # Also do directory completion, since there might be files
         # with the correct suffix in a subdirectory
         eval "set files ./$base*{$suff,/}"
@@ -60,7 +60,11 @@ function __fish_complete_suffix -d "Complete using files"
     if string match -qr '[${}*~]' $comp
         set -l expanded
         eval "set expanded $comp"
-        set files (string replace -- $expanded $comp $files)
+        # It's very unfortunate to do all this work in-process and have to shell out here,
+        # but unfortunately at this time expressions like "foo{t,te}*" applied against
+        # "footer" will result in "footer" being reported twice. Not sure if this can be
+        # term a "bug" per-se.
+        set files (string replace -- $expanded $comp $files | sort -u)
     end
 
     if test $files[1]