Default parameters not supported in derived functions

[comnik]

Adding the following derived function on MyQueryGroup to test/Salsa.jl

# Verify that derived functions can take default parameters.
Salsa.@derived function default_source(db::MyQueryGroup, name::String="stdlib")
    return db.source_text[name]
end

and including the following testset

@testset "Default parameters in derived functions" begin
    db = MyQueryGroup()
    db.source_text["stdlib"] = "..."
    @test default_source(db) == default_source(db, "stdlib")
end

fails with

Default parameters in derived functions: Error During Test at /Users/niko/Projects/raicode/packages/Salsa/test/Salsa.jl:279
  Test threw exception
  Expression: default_source(db) == default_source(db, "stdlib")
  MethodError: no method matching default_source(::Main.SalsaTest.MyQueryGroup)
  Closest candidates are:
    default_source(::Main.SalsaTest.MyQueryGroup, !Matched::String) at /Users/niko/Projects/raicode/packages/Salsa/src/Salsa.jl:481
  Stacktrace:
   [1] top-level scope at /Users/niko/Projects/raicode/packages/Salsa/test/Salsa.jl:279
   [2] top-level scope at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.4/Test/src/Test.jl:1113
   [3] top-level scope at /Users/niko/Projects/raicode/packages/Salsa/test/Salsa.jl:277

Would it be possible to support default parameters on derived functions? Or is there a conceptual reason not to do so?

Thanks!