Using of parse tree externally

[Richard Kenner]

    I'm sure the GCC license could be modified in such a way to say, in
    effect, "In addition to re-use of GCC source code, any code which
    utilizes, in any way, any intermediate output of the compiler, such as
    the parse tree or RTL, or any code which links against sections of
    GCC, shall fall under the GPL".

What is your legal basis that such a clause would be enforeable?