diff --git a/src/goto-diff/unified_diff.cpp b/src/goto-diff/unified_diff.cpp
index aefcdc4c91c..6cd5b556f3e 100644
--- a/src/goto-diff/unified_diff.cpp
+++ b/src/goto-diff/unified_diff.cpp
@@ -54,7 +54,7 @@ unified_difft::get_diff(const irep_idt &function) const
 unified_difft::goto_program_difft unified_difft::get_diff(
   const goto_programt &old_goto_program,
   const goto_programt &new_goto_program,
-  const differencest &differences) const
+  const differencest &differences)
 {
   goto_programt::instructionst::const_iterator old_it =
     old_goto_program.instructions.begin();
@@ -148,7 +148,7 @@ void unified_difft::lcss(
   const irep_idt &identifier,
   const goto_programt &old_goto_program,
   const goto_programt &new_goto_program,
-  differencest &differences) const
+  differencest &differences)
 {
   std::size_t old_count = old_goto_program.instructions.size();
   std::size_t new_count = new_goto_program.instructions.size();
diff --git a/src/goto-diff/unified_diff.h b/src/goto-diff/unified_diff.h
index e8a6e227e46..1b785881542 100644
--- a/src/goto-diff/unified_diff.h
+++ b/src/goto-diff/unified_diff.h
@@ -61,16 +61,16 @@ class unified_difft
     const goto_programt &old_goto_program,
     const goto_programt &new_goto_program);
 
-  void lcss(
+  static void lcss(
     const irep_idt &identifier,
     const goto_programt &old_goto_program,
     const goto_programt &new_goto_program,
-    differencest &differences) const;
+    differencest &differences);
 
-  goto_program_difft get_diff(
+  static goto_program_difft get_diff(
     const goto_programt &old_goto_program,
     const goto_programt &new_goto_program,
-    const differencest &differences) const;
+    const differencest &differences);
 
   void output_diff(
     const irep_idt &identifier,