patch 9.1.0605: internal error with fuzzy completion

Problem:  internal error with fuzzy completion
          (techntools)
Solution: only fuzzy complete the pattern after directory separator
          (glepnir)

fixes: #15287
closes: #15291

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/insexpand.c b/src/insexpand.c
index fa4ac7d..4ad1f41 100644
--- a/src/insexpand.c
+++ b/src/insexpand.c
@@ -3537,11 +3537,41 @@
     int		in_fuzzy = ((get_cot_flags() & COT_FUZZY) != 0 && leader_len > 0);
     char_u	**sorted_matches;
     int		*fuzzy_indices_data;
+    char_u	*last_sep = NULL;
+    size_t	path_with_wildcard_len;
+    char_u	*path_with_wildcard;
 
     if (in_fuzzy)
     {
-	vim_free(compl_pattern);
-	compl_pattern = vim_strsave((char_u *)"*");
+	last_sep = vim_strrchr(leader, PATHSEP);
+	if (last_sep == NULL)
+	{
+	    // No path separator or separator is the last character,
+	    // fuzzy match the whole leader
+	    vim_free(compl_pattern);
+	    compl_pattern = vim_strsave((char_u *)"*");
+	    compl_patternlen = STRLEN(compl_pattern);
+	}
+	else if (*(last_sep + 1) == '\0')
+	    in_fuzzy = FALSE;
+	else
+	{
+	    // Split leader into path and file parts
+	    int path_len = last_sep - leader + 1;
+	    path_with_wildcard_len = path_len + 2;
+	    path_with_wildcard = alloc(path_with_wildcard_len);
+	    if (path_with_wildcard != NULL)
+	    {
+		vim_strncpy(path_with_wildcard, leader, path_len);
+		vim_strcat(path_with_wildcard, (char_u *)"*", path_with_wildcard_len);
+		vim_free(compl_pattern);
+		compl_pattern = path_with_wildcard;
+		compl_patternlen = STRLEN(compl_pattern);
+
+		// Move leader to the file part
+		leader = last_sep + 1;
+	    }
+	}
     }
 
     if (expand_wildcards(1, &compl_pattern, &num_matches, &matches,