diff --git a/src/util/StringBuffer.hxx b/src/util/StringBuffer.hxx
index fdb250287..5e4498aa8 100644
--- a/src/util/StringBuffer.hxx
+++ b/src/util/StringBuffer.hxx
@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2018 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2010-2022 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -62,7 +62,7 @@ public:
 		return front() == SENTINEL;
 	}
 
-	void clear() noexcept {
+	constexpr void clear() noexcept {
 		the_data[0] = SENTINEL;
 	}
 
@@ -70,7 +70,7 @@ public:
 		return the_data.data();
 	}
 
-	pointer data() noexcept {
+	constexpr pointer data() noexcept {
 		return the_data.data();
 	}
 
@@ -81,14 +81,14 @@ public:
 	/**
 	 * Returns one character.  No bounds checking.
 	 */
-	value_type operator[](size_type i) const noexcept {
+	constexpr value_type operator[](size_type i) const noexcept {
 		return the_data[i];
 	}
 
 	/**
 	 * Returns one writable character.  No bounds checking.
 	 */
-	reference operator[](size_type i) noexcept {
+	constexpr reference operator[](size_type i) noexcept {
 		return the_data[i];
 	}