diff --git a/.cirrus.yml b/.cirrus.yml
index aca8e0b52..bf71a7083 100644
--- a/.cirrus.yml
+++ b/.cirrus.yml
@@ -320,3 +320,23 @@ task:
     - ./ci/cirrus.sh
   << : *CAT_LOGS
 
+task:
+  name: "C++ -fpermissive"
+  container:
+    dockerfile: ci/linux-debian.Dockerfile
+    cpu: 1
+    memory: 1G
+  env:
+    # ./configure correctly errors out when given CC=g++.
+    # We hack around this by passing CC=g++ only to make.
+    CC: gcc
+    MAKEFLAGS: -j2 CC=g++ CFLAGS=-fpermissive
+    WERROR_CFLAGS:
+    EXPERIMENTAL: yes
+    ECDH: yes
+    RECOVERY: yes
+    SCHNORRSIG: yes
+  << : *MERGE_BASE
+  test_script:
+    - ./ci/cirrus.sh
+  << : *CAT_LOGS
diff --git a/ci/linux-debian.Dockerfile b/ci/linux-debian.Dockerfile
index 6def91333..2c02ed69d 100644
--- a/ci/linux-debian.Dockerfile
+++ b/ci/linux-debian.Dockerfile
@@ -13,6 +13,7 @@ RUN apt-get install --no-install-recommends --no-upgrade -y \
         git ca-certificates \
         make automake libtool pkg-config dpkg-dev valgrind qemu-user \
         gcc clang llvm libc6-dbg \
+        g++ \
         gcc-i686-linux-gnu libc6-dev-i386-cross libc6-dbg:i386 libubsan1:i386 libasan5:i386 \
         gcc-s390x-linux-gnu libc6-dev-s390x-cross libc6-dbg:s390x \
         gcc-arm-linux-gnueabihf libc6-dev-armhf-cross libc6-dbg:armhf \
diff --git a/include/secp256k1_schnorrsig.h b/include/secp256k1_schnorrsig.h
index d68bba62c..74cbcac45 100644
--- a/include/secp256k1_schnorrsig.h
+++ b/include/secp256k1_schnorrsig.h
@@ -85,7 +85,7 @@ typedef struct {
     void* ndata;
 } secp256k1_schnorrsig_extraparams;
 
-#define SECP256K1_SCHNORRSIG_EXTRAPARAMS_MAGIC "\xda\x6f\xb3\x8c"
+#define SECP256K1_SCHNORRSIG_EXTRAPARAMS_MAGIC { 0xda, 0x6f, 0xb3, 0x8c }
 #define SECP256K1_SCHNORRSIG_EXTRAPARAMS_INIT {\
     SECP256K1_SCHNORRSIG_EXTRAPARAMS_MAGIC,\
     NULL,\
diff --git a/src/ecmult.h b/src/ecmult.h
index 7ab617e20..84537bbfe 100644
--- a/src/ecmult.h
+++ b/src/ecmult.h
@@ -17,7 +17,6 @@ typedef struct {
     secp256k1_ge_storage (*pre_g_128)[]; /* odd multiples of 2^128*generator */
 } secp256k1_ecmult_context;
 
-static const size_t SECP256K1_ECMULT_CONTEXT_PREALLOCATED_SIZE;
 static void secp256k1_ecmult_context_init(secp256k1_ecmult_context *ctx);
 static void secp256k1_ecmult_context_build(secp256k1_ecmult_context *ctx, void **prealloc);
 static void secp256k1_ecmult_context_finalize_memcpy(secp256k1_ecmult_context *dst, const secp256k1_ecmult_context *src);
diff --git a/src/ecmult_gen.h b/src/ecmult_gen.h
index 539618dcb..05cf4d52c 100644
--- a/src/ecmult_gen.h
+++ b/src/ecmult_gen.h
@@ -35,7 +35,6 @@ typedef struct {
     secp256k1_gej initial;
 } secp256k1_ecmult_gen_context;
 
-static const size_t SECP256K1_ECMULT_GEN_CONTEXT_PREALLOCATED_SIZE;
 static void secp256k1_ecmult_gen_context_init(secp256k1_ecmult_gen_context* ctx);
 static void secp256k1_ecmult_gen_context_build(secp256k1_ecmult_gen_context* ctx, void **prealloc);
 static void secp256k1_ecmult_gen_context_finalize_memcpy(secp256k1_ecmult_gen_context *dst, const secp256k1_ecmult_gen_context* src);
diff --git a/src/modules/schnorrsig/main_impl.h b/src/modules/schnorrsig/main_impl.h
index e6de73b8a..693b78f03 100644
--- a/src/modules/schnorrsig/main_impl.h
+++ b/src/modules/schnorrsig/main_impl.h
@@ -47,6 +47,8 @@ static void secp256k1_nonce_function_bip340_sha256_tagged_aux(secp256k1_sha256 *
  * by using the correct tagged hash function. */
 static const unsigned char bip340_algo[13] = "BIP0340/nonce";
 
+static const unsigned char schnorrsig_extraparams_magic[4] = SECP256K1_SCHNORRSIG_EXTRAPARAMS_MAGIC;
+
 static int nonce_function_bip340(unsigned char *nonce32, const unsigned char *msg, size_t msglen, const unsigned char *key32, const unsigned char *xonly_pk32, const unsigned char *algo, size_t algolen, void *data) {
     secp256k1_sha256 sha;
     unsigned char masked_key[32];
@@ -194,7 +196,7 @@ int secp256k1_schnorrsig_sign_custom(const secp256k1_context* ctx, unsigned char
 
     if (extraparams != NULL) {
         ARG_CHECK(secp256k1_memcmp_var(extraparams->magic,
-                                       SECP256K1_SCHNORRSIG_EXTRAPARAMS_MAGIC,
+                                       schnorrsig_extraparams_magic,
                                        sizeof(extraparams->magic)) == 0);
         noncefp = extraparams->noncefp;
         ndata = extraparams->ndata;
diff --git a/src/modules/schnorrsig/tests_impl.h b/src/modules/schnorrsig/tests_impl.h
index 0405d46f4..59357afa9 100644
--- a/src/modules/schnorrsig/tests_impl.h
+++ b/src/modules/schnorrsig/tests_impl.h
@@ -122,7 +122,7 @@ void test_schnorrsig_api(void) {
     secp256k1_xonly_pubkey zero_pk;
     unsigned char sig[64];
     secp256k1_schnorrsig_extraparams extraparams = SECP256K1_SCHNORRSIG_EXTRAPARAMS_INIT;
-    secp256k1_schnorrsig_extraparams invalid_extraparams = { 0 };
+    secp256k1_schnorrsig_extraparams invalid_extraparams = {{ 0 }, NULL, NULL};
 
     /** setup **/
     secp256k1_context *none = secp256k1_context_create(SECP256K1_CONTEXT_NONE);
@@ -219,7 +219,7 @@ void test_schnorrsig_api(void) {
 /* Checks that hash initialized by secp256k1_schnorrsig_sha256_tagged has the
  * expected state. */
 void test_schnorrsig_sha256_tagged(void) {
-    char tag[17] = "BIP0340/challenge";
+    unsigned char tag[17] = "BIP0340/challenge";
     secp256k1_sha256 sha;
     secp256k1_sha256 sha_optimized;