circular dependency when using a seperately-built gcc

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

After failing to build with clang on -8, I'm trying to use GCC 6.

To evade the problems that different packages built with different compilers 
may cause, I built gcc6 (actually as an indirect dependency of clang) in 
/usr/pkg/compiler_boot (e.g. with a different LOCALBASE, PKG_DBDIR etc.).
So there is a GCC 6 in there, but for the now default LOCALBASE etc., 
there's no gcc6 package installed.

In mk.conf, I have:
	PKGSRC_COMPILER=        gcc
	CC=                     gcc
	CXX=                    g++
	CPP=                    ${CC} -E
	GCCBASE=                /usr/pkg/compiler_boot/gcc6
	USE_NATIVE_GCC=         yes
	USE_PKGSRC_GCC=         no

But now, when I try to build pkgtools/cwrappers, I get a circular dependency:

	===> Installing dependencies for cwrappers-20220403
	=> Build dependency gcc6>=6.5.0: NOT found
	=> Verifying package-install for ../../lang/gcc6
	=> Bootstrap dependency digest>=20211023: NOT found
	=> Verifying package-install for ../../pkgtools/digest
	===> Installing dependencies for digest-20220214
	=> Tool dependency cwrappers>=20150314: NOT found
	=> Verifying package-install for ../../pkgtools/cwrappers
	ERROR: This package has set PKG_FAIL_REASON:
	ERROR: Circular dependency detected

I'm unsure who is setting GCC_REQD+=6.5.0 (the value is 2.8.0 3.0 6.5.0) 
but I do have GCC 6.5.0 in /usr/pkg/compiler_boot/gcc6, so why isn't that 
being used?

Shouldn't this Just Work like FooBariX with a native GCC 6.5?

Or am I doing something stupidely wrong?