From 3b4f3d0d46dd278fbe4ffa68b1b6e14e3ea3b17f Mon Sep 17 00:00:00 2001
From: Tim Ruffing <crypto@timruffing.de>
Date: Thu, 17 Mar 2022 22:26:19 +0100
Subject: [PATCH] build: Reject C++ compilers in the preprocessor

---
 .cirrus.yml     |  7 +++----
 src/secp256k1.c | 11 +++++++++++
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/.cirrus.yml b/.cirrus.yml
index 271cbe5f..94d1811b 100644
--- a/.cirrus.yml
+++ b/.cirrus.yml
@@ -312,10 +312,9 @@ task:
   name: "C++ -fpermissive"
   << : *LINUX_CONTAINER
   env:
-    # ./configure correctly errors out when given CC=g++.
-    # We hack around this by passing CC=g++ only to make.
-    CC: gcc
-    MAKEFLAGS: -j4 CC=g++ CFLAGS=-fpermissive\ -g
+    CC: g++
+    CFLAGS: -fpermissive -g
+    CPPFLAGS: -DSECP256K1_CPLUSPLUS_TEST_OVERRIDE
     WERROR_CFLAGS:
     EXPERIMENTAL: yes
     ECDH: yes
diff --git a/src/secp256k1.c b/src/secp256k1.c
index 8f34c352..96102d36 100644
--- a/src/secp256k1.c
+++ b/src/secp256k1.c
@@ -4,6 +4,17 @@
  * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
  ***********************************************************************/
 
+/* This is a C project. It should not be compiled with a C++ compiler,
+ * and we error out if we detect one.
+ *
+ * We still want to be able to test the project with a C++ compiler
+ * because it is still good to know if this will lead to real trouble, so
+ * there is a possibility to override the check. But be warned that
+ * compiling with a C++ compiler is not supported. */
+#if defined(__cplusplus) && !defined(SECP256K1_CPLUSPLUS_TEST_OVERRIDE)
+#error Trying to compile a C project with a C++ compiler.
+#endif
+
 #define SECP256K1_BUILD
 
 #include "../include/secp256k1.h"