PrecompilableError thrown outside of __precompile__(false) cases

[IanButterworth]

PrecompilableError is reserved for when packages use __precompile__(false) which seems quite rare in the ecosystem now.

julia/base/loading.jl

Lines 1688 to 1692 in 7b88c58

	# we throw PrecompilableError when a module doesn't want to be precompiled
	struct PrecompilableError <: Exception end
	function show(io::IO, ex::PrecompilableError)
	print(io, "Declaring __precompile__(false) is not allowed in files that are being precompiled.")
	end

However it can also get thrown when the package cache gets into a bad state.

This means Pkg.precompile can't handle these cases properly (marked here with ?)

Precompiling project...
  Progress [=====================================>   ]  99/108
  ✓ Revise
  ? Symbolics
  ? DiffEqBase → DiffEqBaseUnitfulExt
  ? SteadyStateDiffEq
  ◐ NonlinearSolve

In the case above it was also reported that simply restarting the session fixed the issue.