-DFOO on the SWIG command line now sets FOO to 1

This is consistent with C/C++ compiler preprocessors.  Previously
SWIG set FOO to an empty value.

Fixes #2193

CHANGES.current

@@ -7,6 +7,11 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.1.0 (in progress)
 ===========================
 
+2022-02-06: olly
+	    #2193 -DFOO on the SWIG command line now sets FOO to 1 for
+	    consistency with C/C++ compiler preprocessors.  Previously
+	    SWIG set FOO to an empty value.
+
 2022-02-06: sethrj
 	    #2194 Classes that are non-assignable due to const data or const
 	    reference members are now automatically detected.

Examples/test-suite/command_line_define.i

@@ -0,0 +1,7 @@
+%module command_line_define
+
+// Test handling of -D without a value specified.
+
+#if FOO-0 != 1
+# error "-DFOO didn't set FOO to 1"
+#endif

Examples/test-suite/common.mk

@@ -692,6 +692,7 @@ C_TEST_CASES += \
 	c_delete \
 	c_delete_function \
 	char_constant \
+	command_line_define \
 	const_const \
 	constant_expr_c \
 	default_args_c \
@@ -761,6 +762,7 @@ MULTI_CPP_TEST_CASES += \
 # Custom tests - tests with additional commandline options
 wallkw.cpptest: SWIGOPT += -Wallkw
 preproc_include.ctest: SWIGOPT += -includeall
+command_line_define.ctest: SWIGOPT += -DFOO
 
 # Allow modules to define temporarily failing tests.
 C_TEST_CASES := $(filter-out $(FAILING_C_TESTS),$(C_TEST_CASES))

Source/Modules/main.cxx

@@ -477,7 +477,10 @@ static void getoptions(int argc, char *argv[]) {
 	Swig_mark_arg(i);
       } else if (strncmp(argv[i], "-D", 2) == 0) {
 	String *d = NewString(argv[i] + 2);
-	Replace(d, "=", " ", DOH_REPLACE_FIRST);
+	if (Replace(d, "=", " ", DOH_REPLACE_FIRST) == 0) {
+	  // Match C preprocessor behaviour whereby -DFOO sets FOO=1.
+	  Append(d, " 1");
+	}
 	Preprocessor_define((DOH *) d, 0);
 	Delete(d);
 	// Create a symbol