complete: Don't load completions if command isn't in $PATH

This stops us from loading the completions for e.g. `./foo` if there
is no `foo` in path.

This is because the completion scripts will call an unqualified `foo`,
and then error out.

This of course means if the script would work because it never calls
the command, we still don't load it.

Pathed completions via `complete --path` should be unaffected because
they aren't autoloaded anyway.

Workaround for #3117
Fixes #9133

src/complete.cpp

@@ -821,9 +821,11 @@ bool completer_t::complete_param_for_command(const wcstring &cmd_orig, const wcs
     wcstring cmd, path;
     parse_cmd_string(cmd_orig, &path, &cmd, ctx.vars);
 
-    // Use cmd_orig here for paths, as it is potentially pathed.
+    // Don't use cmd_orig here for paths. It's potentially pathed,
+    // so that command might exist, but the completion script
+    // won't be using it.
     bool cmd_exists = builtin_exists(cmd) || function_exists_no_autoload(cmd) ||
-                      path_get_path(cmd_orig, ctx.vars).has_value();
+                      path_get_path(cmd, ctx.vars).has_value();
     if (!cmd_exists) {
         // Do not load custom completions if the command does not exist
         // This prevents errors caused during the execution of completion providers for

tests/checks/complete.fish

@@ -1,4 +1,4 @@
-# RUN: %fish %s
+#RUN: %fish -C 'set -l fish %fish' %s
 function complete_test_alpha1
     echo $argv
 end
@@ -499,3 +499,32 @@ complete -C'oooops '
 # CHECK: oops
 echo $oops
 # CHECK: 1
+
+
+# See that we load completions only if the command exists in $PATH,
+# as a workaround for #3117.
+
+# We need a completion script that runs the command,
+# and prints something simple, and isn't already used above.
+#
+# Currently, tr fits the bill (it does `tr --version` to check for GNUisms)
+begin
+    $fish -c "complete -C'tr -'" | string match -- '-d*'
+    # CHECK: -d{{\t.*}}
+end
+
+set -l tmpdir (mktemp -d)
+cd $tmpdir
+begin
+    touch tr
+    chmod +x tr
+    set -l PATH
+    complete -C'./tr -' | string match -- -d
+    or echo No match for relative
+    # CHECK: No match for relative
+    complete -c tr | string length -q
+    or echo Empty completions
+    # CHECK: Empty completions
+end
+
+rm -r $tmpdir