Fix operator precedence in preprocessor expressions

Examples/test-suite/common.mk

@@ -746,6 +746,7 @@ C_TEST_CASES += \
 	preproc \
 	preproc_constants_c \
 	preproc_defined \
+	preproc_expr \
 	preproc_gcc_output \
 	preproc_include \
 	preproc_line_file \

Examples/test-suite/preproc_expr.i

@@ -0,0 +1,34 @@
+%module preproc_expr
+
+// Check expressions which suffered from incorrect operator precedence prior
+// to SWIG 4.1.0.
+
+%inline %{
+// `!` should have higher precedence than binary `+`.
+#if !0 + 1
+#else
+# error Bad preprocessor expression operator precedence
+#endif
+
+// `!=` should have higher precedence than bitwise and.
+#if 1 & 2 != 0
+#else
+# error Bad preprocessor expression operator precedence
+#endif
+
+// `==` should have higher precedence than bitwise or.
+#if (2 | 1 == 3) != 2
+# error Bad preprocessor expression operator precedence
+#endif
+
+// `!=` should have higher precedence than bitwise xor.
+#if 1 ^ 2 != 4
+# error Bad preprocessor expression operator precedence
+#endif
+
+// `<` should have higher precedence than '=='.
+#if 2 == 2 < 2
+# error Bad preprocessor expression operator precedence
+#endif
+
+%}