Warn and ignore %constant with unsupported implicit type

SWIG 4.2.0 reported an assertion failure.  SWIG 4.1.1 generated
C/C++ code which failed to compile.

CHANGES.current

@@ -7,6 +7,10 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.2.1 (in progress)
 ===========================
 
+2024-02-14: olly
+	    SWIG now warns and ignores if %constant is used with an implicit
+	    type which SWIG can't deduce.
+
 2024-02-13: olly
 	    Fix type deduction for certain cases involving C-style casts, or
 	    which are syntactically like a C-style cast applied to an unary

Doc/Manual/Warnings.html

@@ -394,6 +394,7 @@ example.i(4) : Syntax error in input(1).
 <li>301. <tt>class</tt> keyword used, but not in C++ mode.
 <li>302. Redefinition of identifier '<em>name</em>' as <em>decl</em> ignored.
 <li>303. <tt>%extend</tt> defined for an undeclared class '<em>name</em>'.
+<li>304. Unsupported constant value (ignored).
 <li>305. Bad constant value (ignored).
 <li>308. Namespace alias '<em>name</em>' not allowed here. Assuming '<em>name</em>'
 <li>309. [private | protected] inheritance ignored.

Examples/test-suite/constant_directive.i

@@ -52,3 +52,13 @@ Type1 getType1Instance() { return Type1(111); }
 /* Regular constant */
 %constant int TYPE_INT = 0;
 %constant enum EnumType newValue = enumValue;
+
+/* Test handling of %constant with an implicit type which SWIG can't handle. */
+#pragma SWIG nowarn=SWIGWARN_PARSE_UNSUPPORTED_VALUE
+%ignore ignored_int_variable;
+%inline %{
+int ignored_int_variable = 42;
+%}
+%constant unsupported_constant_value1 = &ignored_int_variable;
+%constant unsupported_constant_value2 = getType1Instance;
+%constant unsupported_constant_value3 = &getType1Instance;

Source/CParse/parser.y

@@ -2045,15 +2045,20 @@ clear_directive : CLEAR tm_list SEMI {
 
 constant_directive :  CONSTANT identifier EQUAL definetype SEMI {
 		 SwigType *type = NewSwigType($4.type);
-		 $$ = new_node("constant");
-		 Setattr($$, "name", $2);
-		 Setattr($$, "type", type);
-		 Setattr($$, "value", $4.val);
-		 if ($4.rawval) Setattr($$, "rawval", $4.rawval);
-		 Setattr($$, "storage", "%constant");
-		 SetFlag($$, "feature:immutable");
-		 add_symbols($$);
-		 Delete(type);
+		 if (Len(type) > 0) {
+		   $$ = new_node("constant");
+		   Setattr($$, "name", $2);
+		   Setattr($$, "type", type);
+		   Setattr($$, "value", $4.val);
+		   if ($4.rawval) Setattr($$, "rawval", $4.rawval);
+		   Setattr($$, "storage", "%constant");
+		   SetFlag($$, "feature:immutable");
+		   add_symbols($$);
+		   Delete(type);
+		 } else {
+		   Swig_warning(WARN_PARSE_UNSUPPORTED_VALUE, cparse_file, cparse_line, "Unsupported constant value (ignored)\n");
+		   $$ = 0;
+		 }
 	       }
                | CONSTANT type declarator def_args SEMI {
 		 SwigType_push($2, $3.type);

Source/Include/swigwarn.h

@@ -70,7 +70,7 @@
 #define WARN_PARSE_CLASS_KEYWORD      301
 #define WARN_PARSE_REDEFINED          302
 #define WARN_PARSE_EXTEND_UNDEF       303
-/* Unused since 4.2.0: #define WARN_PARSE_UNSUPPORTED_VALUE  304 */
+#define WARN_PARSE_UNSUPPORTED_VALUE  304
 #define WARN_PARSE_BAD_VALUE          305
 /* Unused since 1.3.32: #define WARN_PARSE_PRIVATE            306 */
 /* Unused since 4.2.0: #define WARN_PARSE_BAD_DEFAULT        307 */

Source/Swig/error.c

@@ -22,7 +22,7 @@
  * The warning filter is a string of numbers prefaced by (-) or (+) to
  * indicate whether or not a warning message is displayed.  For example:
  *
- *      "-301-201-140+210+201"
+ *      "-304-201-140+210+201"
  *
  * The filter string is scanned left to right and the first occurrence
  * of a warning number is used to determine printing behavior.