interpret Expr(:isdefined, ::QuoteNode) correctly

[simeonschaub]

discovered in JuliaDebug/JuliaInterpreter.jl#477

[vtjnash]

This sounds like someone generated corrupt IR, which is not permitted. Using lowering from an AST should prevent this from reaching the backend, and any compiler passes in Core.Compiler are written to avoid generating it later.

Can you add a test to julia/base/compiler/validation.jl instead to reject this?

[simeonschaub]

Would it be problematic to consider this valid? Many Cassette style code transformations use Meta.partially_inline! to replace slots or static parameters with QuoteNodes, the specific example here was Zygote: JuliaDebug/JuliaInterpreter.jl#476.

[vtjnash]

Hm, perhaps we should eliminate that code, but in the meantime, it should be taught how to correctly inline a :isdefined node

[JeffBezanson]

Right; inlining into isdefined is not valid since isdefined is not referentially transparent --- it operates on a variable, not its value, so you can't replace the variable with its value.