diff --git a/sci-mathematics/giac/files/giac-1.9.0.21-pari-2.15.patch b/sci-mathematics/giac/files/giac-1.9.0.21-pari-2.15.patch
new file mode 100644
index 0000000000000..c1df76ea70489
--- /dev/null
+++ b/sci-mathematics/giac/files/giac-1.9.0.21-pari-2.15.patch
@@ -0,0 +1,17 @@
+--- a/src/pari.cc
++++ b/src/pari.cc
+@@ -39,6 +39,14 @@ using namespace std;
+ #endif
+ 
+ #ifdef HAVE_LIBPARI
++#if !defined ANYARG
++// Anyarg disappeared from PARI 2.15.0
++#ifdef __cplusplus
++#  define ANYARG ...
++#else
++#  define ANYARG
++#endif
++#endif
+ 
+ #ifdef HAVE_PTHREAD_H
+ #include <pthread.h>
diff --git a/sci-mathematics/giac/giac-1.9.0.21.ebuild b/sci-mathematics/giac/giac-1.9.0.21.ebuild
index bf7ed2bdb24c1..a079e6b8a5109 100644
--- a/sci-mathematics/giac/giac-1.9.0.21.ebuild
+++ b/sci-mathematics/giac/giac-1.9.0.21.ebuild
@@ -49,6 +49,7 @@ BDEPEND="dev-tex/hevea
 PATCHES=(
 	"${FILESDIR}/${PN}-1.7.0.1-gsl_lapack.patch"
 	"${FILESDIR}/${PN}-1.6.0-pari-2.11.patch"
+	"${FILESDIR}/${PN}-1.9.0.21-pari-2.15.patch"
 )
 
 REQUIRED_USE="test? ( gui )"
@@ -74,6 +75,11 @@ src_configure() {
 
 		# Get the big-L flags from fltk's LDFLAGS.
 		append-ldflags $(fltk-config --ldflags | sed -e 's/\(-L\S*\)\s.*/\1/')
+	else
+		# see https://trac.sagemath.org/ticket/31563#comment:91 onwards.
+		# Unless this variable is defined a non existent function will be requested.
+		# The spelling is correct - upstream is French.
+		append-cppflags -DUSE_OBJET_BIDON=1
 	fi
 
 	# Using libsamplerate is currently broken