repro:

# assigning a varargs... with the result of an if causes an assertion failure:
# scopes: ../src/quote.cpp:292: scopes::ValueRef scopes::Quoter::quote_typed(const scopes::TypedValueRef &): Assertion `result' failed.
let include-options... =
    if (operating-system == 'windows)
        _ "-I/usr/include/SDL2" "-D_REENTRANT"
    else
        _ "" ""
# but we have to run a stage!
run-stage;
# the condition of the if doesn't seem to matter, I just used that because it was an actual
# example.