bug#48993: why error "unable to infer tagged configuration"?

[Bruno Haible]

Hi,

In a Makefile of mine, I want to use libtool to compile a .asm file with
the Microsoft assembler, for use in a libtool library:

  /bin/sh ../libtool --mode=compile ml64 -c -nologo avcall-x86_64.asm
  libtool: compile: unable to infer tagged configuration
  libtool:   error: specify a tag with '--tag'

The documentation [1] says:

  "libtool tries to automatically infer what tag to use from the compiler
   command being used to compile or link. If it can’t infer a tag, then it
   defaults to the configuration for the C language."

If it defaults to --tag=C, why does libtool report an error then?

       Bruno

[1] https://www.gnu.org/software/libtool/manual/html_node/Tags.html