Can not choose clang version on FreeBSD

Commonly there are several different versions of clang installed on the FreeBSD system, but b2 seems can only use the default clang:

This works:
b2 toolset=clang

This works:
using clang ;

But these all fail:
b2 toolset=clang10
b2 toolset=clang-10
b2 toolset=clang11
b2 toolset=clang-11
b2 toolset=clang-devel

using clang : 10 ;

using clang : 11 ;

using clang : devel ;

It just can not use alt clang except the default.
If I try them and find,
b2 toolset=clang-11 -n
gives me screen information that it still invokes clang++,
the real c++ compiler command should be clang++11

Start by reproducing the reported commands with b2 on FreeBSD, especially toolset=clang-11 and the using clang : 11 form, and inspect the -n output. Trace how B2 resolves clang toolset names and versions; done means selecting the requested alternate compiler such as clang++11 instead of the default clang++.

Written by the indexing model from the issue text.