From 661f13e0facb9195061ba6bce18e8f9603a8585a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ludvig=20Gunne=20Lindstr=C3=B6m?=
 <ludviggunnelindstrom@gmail.com>
Date: Tue, 1 Jul 2025 15:43:04 +0200
Subject: [PATCH 1/2] fix #454

---
 main.cpp      | 1 +
 simplecpp.cpp | 4 ++--
 simplecpp.h   | 3 ++-
 3 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/main.cpp b/main.cpp
index 424ef6fa..baf006ee 100644
--- a/main.cpp
+++ b/main.cpp
@@ -60,6 +60,7 @@ int main(int argc, char **argv)
             case 's':
                 if (std::strncmp(arg, "-std=",5)==0) {
                     dui.std = arg + 5;
+                    dui.gnu = dui.std.rfind("gnu", 0) != std::string::npos;
                     found = true;
                 }
                 break;
diff --git a/simplecpp.cpp b/simplecpp.cpp
index 599ffdfe..bb076c10 100755
--- a/simplecpp.cpp
+++ b/simplecpp.cpp
@@ -2760,7 +2760,7 @@ static std::string extractRelativePathFromAbsolute(const std::string& absoluteSi
 static std::string openHeader(std::ifstream &f, const simplecpp::DUI &dui, const std::string &sourcefile, const std::string &header, bool systemheader);
 static void simplifyHasInclude(simplecpp::TokenList &expr, const simplecpp::DUI &dui)
 {
-    if (!isCpp17OrLater(dui))
+    if (!isCpp17OrLater(dui) && !dui.gnu)
         return;
 
     for (simplecpp::Token *tok = expr.front(); tok; tok = tok->next) {
@@ -3483,7 +3483,7 @@ void simplecpp::preprocess(simplecpp::TokenList &output, const simplecpp::TokenL
     // use a dummy vector for the macros because as this is not part of the file and would add an empty entry - e.g. /usr/include/poll.h
     std::vector<std::string> dummy;
 
-    const bool hasInclude = isCpp17OrLater(dui);
+    const bool hasInclude = isCpp17OrLater(dui) || dui.gnu;
     MacroMap macros;
     for (std::list<std::string>::const_iterator it = dui.defines.begin(); it != dui.defines.end(); ++it) {
         const std::string &macrostr = *it;
diff --git a/simplecpp.h b/simplecpp.h
index f5c69593..0d199977 100755
--- a/simplecpp.h
+++ b/simplecpp.h
@@ -337,12 +337,13 @@ namespace simplecpp {
      * On the command line these are configured by -D, -U, -I, --include, -std
      */
     struct SIMPLECPP_LIB DUI {
-        DUI() : clearIncludeCache(false), removeComments(false) {}
+        DUI() : clearIncludeCache(false), removeComments(false), gnu(false) {}
         std::list<std::string> defines;
         std::set<std::string> undefined;
         std::list<std::string> includePaths;
         std::list<std::string> includes;
         std::string std;
+        bool gnu;
         bool clearIncludeCache;
         bool removeComments; /** remove comment tokens from included files */
     };

From 332ce916180e72d1a0b1edcf1b8cc0fe19ace331 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ludvig=20Gunne=20Lindstr=C3=B6m?=
 <ludviggunnelindstrom@gmail.com>
Date: Wed, 2 Jul 2025 09:23:35 +0200
Subject: [PATCH 2/2] add test

---
 test.cpp | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/test.cpp b/test.cpp
index cec253b8..81d1e46f 100644
--- a/test.cpp
+++ b/test.cpp
@@ -1563,6 +1563,22 @@ static void has_include_5()
     ASSERT_EQUALS("", preprocess(code));
 }
 
+static void has_include_6()
+{
+    const char code[] = "#if defined( __has_include)\n"
+                        "  #if !__has_include(<testsuite/unrealFileName2.abcdef>)\n"
+                        "    A\n"
+                        "  #else\n"
+                        "    B\n"
+                        "  #endif\n"
+                        "#endif";
+    simplecpp::DUI dui;
+    dui.std = "gnu99";
+    dui.gnu = true;
+    ASSERT_EQUALS("\n\nA", preprocess(code, dui));
+    ASSERT_EQUALS("", preprocess(code));
+}
+
 static void ifdef1()
 {
     const char code[] = "#ifdef A\n"
@@ -3123,6 +3139,7 @@ int main(int argc, char **argv)
     TEST_CASE(has_include_3);
     TEST_CASE(has_include_4);
     TEST_CASE(has_include_5);
+    TEST_CASE(has_include_6);
 
     TEST_CASE(ifdef1);
     TEST_CASE(ifdef2);