From de4e3b454e23af3f26f8c2ea670482bef738e983 Mon Sep 17 00:00:00 2001
From: Olly Betts <olly@survex.com>
Date: Fri, 21 Apr 2023 07:38:52 +1200
Subject: Fix #ifdef and #ifndef to work inside a %define

Previously they were silently ignored in this context (but #if defined
already worked here if you need a workaround which works for older
versions).

Fixes #2183
---
 CHANGES.current                       |  6 ++++++
 Examples/test-suite/preproc_defined.i | 35 ++++++++++++++++++++++++++++++++++
 Source/Preprocessor/cpp.c             | 36 ++++++++++++++++++++++++++++-------
 3 files changed, 70 insertions(+), 7 deletions(-)

diff --git a/CHANGES.current b/CHANGES.current
index ed80e145e..5dfc249e9 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 4.2.0 (in progress)
 ===========================
 
+2023-04-21: olly
+	    #2183 Fix #ifdef and #ifndef to work inside a %define.  Previously
+	    they were silently ignored in this context (but #if defined already
+	    worked here if you need a workaround which works for older
+	    versions).
+
 2023-04-20: erezgeva
 	    [Go] #2533 Implement argcargv.i library for Go.
 
diff --git a/Examples/test-suite/preproc_defined.i b/Examples/test-suite/preproc_defined.i
index 9ae884691..f000446c4 100644
--- a/Examples/test-suite/preproc_defined.i
+++ b/Examples/test-suite/preproc_defined.i
@@ -151,3 +151,38 @@ const int y = 0;
 
 #undef y
 #undef x
+
+/* Regression test for #ifdef and #ifndef not working inside a %define.
+ * https://github.com/swig/swig/issues/2183
+ */
+#undef THISMACROISNOTDEFINED /* Make sure! */
+#define THISMACROISDEFINED
+
+%define %test()
+
+#ifdef THISMACROISNOTDEFINED
+# error #ifdef inside percent-define failed
+#endif
+#ifndef THISMACROISDEFINED
+# error #ifndef inside percent-define failed
+#endif
+/* Check pre-defined macro too. */
+#ifndef SWIG
+# error #ifndef inside percent-define failed
+#endif
+
+/* These cases already worked, but should still have test coverage. */
+#if defined THISMACROISNOTDEFINED
+# error #if defined inside percent-define failed
+#endif
+#if !defined THISMACROISDEFINED
+# error #if !defined inside percent-define failed
+#endif
+/* Check pre-defined macro too. */
+#if !defined SWIG
+# error #if !defined inside percent-define failed
+#endif
+
+%enddef
+
+%test()
diff --git a/Source/Preprocessor/cpp.c b/Source/Preprocessor/cpp.c
index 55058fc13..ae1e2ecb5 100644
--- a/Source/Preprocessor/cpp.c
+++ b/Source/Preprocessor/cpp.c
@@ -1191,14 +1191,36 @@ static DOH *Preprocessor_replace(DOH *s, DOH *line_file) {
 	} else if (Equal(kpp_hash_if, id) || Equal(kpp_hash_elif, id)) {
 	  expand_defined_operator = 1;
 	  Append(ns, id);
-	  /*
-	} else if (Equal("%#if", id) || Equal("%#ifdef", id)) {
-	  Swig_warning(998, Getfile(s), Getline(s), "Found: %s preprocessor directive.\n", id);
-	  Append(ns, id);
 	} else if (Equal("#ifdef", id) || Equal("#ifndef", id)) {
-	  Swig_warning(998, Getfile(s), Getline(s), "The %s preprocessor directive does not work in macros, try #if instead.\n", id);
-	  Append(ns, id);
-	  */
+	  /* Evaluate the defined-ness check and substitute `#if 0` or `#if 1`. */
+	  int allow = 0;
+	  skip_whitespace(s, 0);
+	  c = Getc(s);
+	  if (isidchar(c)) {
+	    DOH *arg = NewStringEmpty();
+	    Putc(c, arg);
+	    while (((c = Getc(s)) != EOF)) {
+	      if (!isidchar(c)) {
+		Ungetc(c, s);
+		break;
+	      }
+	      Putc(c, arg);
+	    }
+	    if (Equal("#ifdef", id)) {
+	      if (Getattr(symbols, arg)) allow = 1;
+	    } else {
+	      if (!Getattr(symbols, arg)) allow = 1;
+	    }
+	    Delete(arg);
+	  } else {
+	    Ungetc(c, s);
+	    Swig_error(Getfile(s), Getline(s), "Missing identifier for %s.\n", id);
+	  }
+	  if (allow) {
+	    Append(ns, "#if 1");
+	  } else {
+	    Append(ns, "#if 0");
+	  }
 	} else if ((m = Getattr(symbols, id))) {
 	  /* See if the macro is defined in the preprocessor symbol table */
 	  DOH *args = 0;
-- 
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