invalid construction, Vector{Tuple{:a, :b}}, creates a traceback rather than an error

[clarkevans]

This is illegal construction, but rather than being an error, it creates a traceback.

julia> versioninfo()
Julia Version 1.6.1
Commit 6aaedecc44 (2021-04-23 05:59 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Celeron(R) CPU J3455 @ 1.50GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, goldmont)

julia> T = Vector{Tuple{:a, :b}}
Vector{Tuple{:a, :b}} (alias for Array{Tuple{:a, :b}, 1})

julia> T()
Internal error: encountered unexpected error in runtime:
MethodError(f=Core.Compiler.widenconst, args=(:a,), world=0x00000000000010a8)
jl_method_error_bare at /buildworker/worker/package_linux64/build/src/gf.c:1812
jl_method_error at /buildworker/worker/package_linux64/build/src/gf.c:1830
jl_lookup_generic_ at /buildworker/worker/package_linux64/build/src/gf.c:2400 [inlined]
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2415
...

[simeonschaub]

reproduces on master as well

[simeonschaub]

Reduced to:

julia> f() = getfield(Ref{Tuple{:a}}()[], 1)
f (generic function with 1 method)

julia> @code_warntype f()
ERROR: MethodError: no method matching widenconst(::Symbol)
Closest candidates are:
  widenconst(::Union{Core.Compiler.Conditional, Core.InterConditional}) at compiler/typelattice.jl:270
  widenconst(::Type) at compiler/typelattice.jl:285
  widenconst(::Core.Const) at compiler/typelattice.jl:271
  ...
Stacktrace:
  [1] widenreturn(rt::Any, bestguess::Any, nslots::Int64, slottypes::Vector{Any}, changes::Vector{Any})
    @ Core.Compiler ./compiler/abstractinterpretation.jl:1629
  [2] typeinf_local(interp::Core.Compiler.NativeInterpreter, frame::Core.Compiler.InferenceState)
    @ Core.Compiler ./compiler/abstractinterpretation.jl:1715
  [3] typeinf_nocycle(interp::Core.Compiler.NativeInterpreter, frame::Core.Compiler.InferenceState)
    @ Core.Compiler ./compiler/abstractinterpretation.jl:1875
  [4] _typeinf(interp::Core.Compiler.NativeInterpreter, frame::Core.Compiler.InferenceState)
    @ Core.Compiler ./compiler/typeinfer.jl:226
  [5] typeinf(interp::Core.Compiler.NativeInterpreter, frame::Core.Compiler.InferenceState)
    @ Core.Compiler ./compiler/typeinfer.jl:209
  [6] typeinf_code(interp::Core.Compiler.NativeInterpreter, method::Method, atypes::Any, sparams::Core.SimpleVector, run_optimizer::Bool)
    @ Core.Compiler ./compiler/typeinfer.jl:847
[...]

Does it make sense for getindex(::Ref{Tuple{:a}}) to infer to Tuple{:a} or should that be Union{} instead? Or alternatively, should getfield_tfunc just be made more robust? (probably related to #40067, although it does not fix this error)

[vtjnash]

Ah, that seems to make this a duplicate of #39988