Using GHC 8.2.1:

> ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.2.1

And compiling with --backpack as follows:

> ghc --backpack foo.bkp
[1 of 4] Processing foo-indef
  [1 of 2] Compiling Str[sig]         ( foo-indef\Str.hsig, nothing )
  [2 of 2] Compiling Foo              ( foo-indef\Foo.hs, nothing )
[2 of 4] Processing foo-string
  Instantiating foo-string
  [1 of 1] Compiling Str              ( foo-string\Str.hs, foo-string\Str.o )
[3 of 4] Processing foo-int
  Instantiating foo-int
  [1 of 1] Compiling Str              ( foo-int\Str.hs, foo-int\Str.o )
[4 of 4] Processing main
ghc.EXE: panic! (the 'impossible' happened)
  (GHC version 8.2.1 for x86_64-unknown-mingw32):
        no package name
CallStack (from HasCallStack):
  error, called at compiler\backpack\DriverBkp.hs:573:32 in ghc:DriverBkp

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

make: *** [foo-indef/Foo.hi] Error 1

See attached foo.bkp file. While the Backpack file is invalid, in that the main unit mentions the foo dependency, which does not exist, this shouldn't lead to a GHC panic.