diff --git a/CHANGES.current b/CHANGES.current
index 028adcf74..b52bf0bf1 100644
--- a/CHANGES.current
+++ b/CHANGES.current
@@ -7,6 +7,12 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.1.0 (in progress)
 ===========================
 
+2022-01-25: olly
+	    #2091 Support most cases of `sizeof` applied to an expression
+	    in constant expressions.  Previously there was only support for
+	    `sizeof(<type>)` and expressions which syntactically look like a
+	    type (such as `sizeof(foo)`).
+
 2022-01-25: olly
 	    #80 #635 https://sourceforge.net/p/swig/bugs/1139/
 	    Add support for parsing common cases of `<` and `>` comparisons
diff --git a/Examples/test-suite/constant_expr_c.i b/Examples/test-suite/constant_expr_c.i
index b7431c1f1..aae0b953c 100644
--- a/Examples/test-suite/constant_expr_c.i
+++ b/Examples/test-suite/constant_expr_c.i
@@ -32,4 +32,25 @@ double d_array[12 % 9];
 #define TEST_I 0
 #endif
 
+/* sizeof didn't work on an expression before SWIG 4.1.0 except for cases where
+ * the expression was in parentheses and looked syntactically like a type (so
+ * sizeof(X) worked because X could be a type syntactically).
+ */
+const int s1a = sizeof(X); /* worked before 4.1.0 */
+//const int s1b = sizeof X; /* not currently supported */
+const int s2a = sizeof("a string" );
+const int s2b = sizeof "a string";
+const int s3a = sizeof('c');
+const int s3b = sizeof('c');
+const int s4a = sizeof(L"a wstring");
+const int s4b = sizeof L"a wstring";
+const int s5a = sizeof(L'C');
+const int s5b = sizeof L'C';
+const int s6a = sizeof(sizeof(X));
+const int s6b = sizeof sizeof(X);
+const int s7a = sizeof(3.14);
+const int s7b = sizeof 3.14;
+const int s8a = sizeof(2.1e-6);
+const int s8b = sizeof 2.1e-6;
+
 %}
diff --git a/Source/CParse/parser.y b/Source/CParse/parser.y
index 562c51269..458ef1a7b 100644
--- a/Source/CParse/parser.y
+++ b/Source/CParse/parser.y
@@ -6554,6 +6554,25 @@ exprsimple     : exprnum {
 		  $$.val = NewStringf("sizeof...(%s)",SwigType_str($6,0));
 		  $$.type = T_ULONG;
                }
+	       /* We don't support all valid expressions here currently - e.g.
+		* sizeof(<unaryop> x) doesn't work - but those are unlikely to
+		* be seen in real code.
+		*
+		* Note: sizeof(x) is not handled here, but instead by the rule
+		* for sizeof(<type>) because it matches that syntactically.
+		*/
+	       | SIZEOF LPAREN exprsimple RPAREN {
+		  $$.val = NewStringf("sizeof(%s)", $3.val);
+		 $$.type = T_ULONG;
+	       }
+	       /* `sizeof expr` without parentheses is valid for an expression,
+		* but not for a type.  This doesn't support `sizeof x` in
+		* addition to the case not supported above.
+		*/
+	       | SIZEOF exprsimple {
+		  $$.val = NewStringf("sizeof(%s)", $2.val);
+		  $$.type = T_ULONG;
+	       }
 	       | wstring {
 		    $$.val = $1;
 		    $$.rawval = NewStringf("L\"%s\"", $$.val);