I ran into a problem using the <file> feature for indicating a prebuilt library file.  After building a variant of lib bar of examples/libraries/util/foo/Jamfile, I changed the Jamfile as follows just to demonstrate the problem:

#lib bar : bar.cpp ;
lib bar : : <file>bin/hptnsr3d/libbar.so ;

The indicated file is the result of a build before changing Jamfile.  With the change to Jamfile, I get the following error trace:

/cygdrive/c/mse/proj/boost-build/build/generators.jam:918: in ensure-type from module generators
error: target { bin/hptnsr3d/libbar.so. } has no type
/cygdrive/c/mse/proj/boost-build/build/generators.jam:1138: in generators.construct from module generators
/cygdrive/c/mse/proj/boost-build/build/targets.jam:1299: in construct from module object(typed-target)@44
/cygdrive/c/mse/proj/boost-build/build/targets.jam:1150: in object(typed-target)@44.generate from module object(typed-target)@44
/cygdrive/c/mse/proj/boost-build/build/targets.jam:763: in generate-really from module object(main-target)@53
/cygdrive/c/mse/proj/boost-build/build/targets.jam:736: in object(main-target)@53.generate from module object(main-target)@53
/cygdrive/c/mse/proj/boost-build/build/targets.jam:251: in object(project-target)@43.generate from module object(project-target)@43
/cygdrive/c/mse/proj/boost-build/build-system.jam:276: in load from module build-system
/cygdrive/c/mse/proj/boost-build/example/../kernel/modules.jam:261: in import from module modules
/cygdrive/c/mse/proj/boost-build/example/../kernel/bootstrap.jam:186: in boost-build from module
/cygdrive/c/mse/proj/boost-build/example/boost-build.jam:2: in module scope from module

Is this a bonified problem or cockpit error in writing the Jamfile?