_GCC_DEPENDS and _GCC_DEPENDENCY

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

I'm trying to understand compiler/gcc.mk.

I can see _GCC_DEPENDENCY being set, but I can't see it being used anwhere in 
gcc.mk nor in any other pkgsrc Makefile.

There's also _GCC_DEPENDS, but I fail to understand what this and 
_NEED_NEWER_GCC are supposed to do.
Suppose we arrive at _GCC_REQD=6.0. Then we have _GCC_PKGBASE=gcc6, right? So _GCC_DEPENDS is set to gcc6-6.0.
We also have _GCC_DIST_NAME (my impression is that there ought to be some "HIGHEST" in the name)=gcc7, and (via lang/gcc7/version.mk GCC7_DIST_VERSION=7.1.0, so _GCC_DIST_VERSION (also missing "HIGHEST")=7.1.0.
This (supposing _NEED_NEWER_GCC has not already been set to NO because we need Ada and thus "AUX" GCC) leads to _PKGSRC_GCC_VERSION (missing "HIGHEST")=gcc7-7.0.1
But then the test executed is pkg_admin pmatch gcc6-6.0 gcc7-7.0.1, which fails.

How is this supposed to work?