ICE in gfc_enforce_clean_symbol_state, at fortran/symbol.c:4273

The following f90 file :

$ cat test.f90
module test
contains
function matimul(a,b)
    implicit none
    real(kind=4), dimension(:,:),intent(in) :: a,b
    real(kind=4), dimension(size(a,1),size(b,2)) :: matimul
    matimul=matmul(a,b)
end function
end module test

compiled with flags '-O2 -fexternal-blas' give the following ICE :

$ gfortran -c test.f90 -O2 -fexternal-blas -o test.o
f951: internal compiler error: in gfc_enforce_clean_symbol_state, at fortran/symbol.c:4273
Please submit a full bug report,
with preprocessed source if appropriate.
See <file:///usr/share/doc/gcc-9/README.Bugs> for instructions.

It is the combination of a matmul used in a module and compiled with both flags '-O2' and '-fexternal-blas' which trigger this error. This error did not occur with previous gcc versions.

$ lsb_release -rd
Description: Ubuntu 19.10
Release: 19.10

$ gfortran -v
Using built-in specs.
COLLECT_GCC=gfortran
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/9/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 9.2.1-9ubuntu2' --with-bugurl=file:///usr/share/doc/gcc-9/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,gm2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-9 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none,hsa --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 9.2.1 20191008 (Ubuntu 9.2.1-9ubuntu2)