The following arguments should be traced but were not:

[dominic-chang]

The following code:

function temp(m::T) where {T}
    oneT = one(T)
    a = oneT
    b = sqrt(oneT - m)
    sum_c2 = m 
    pow2 = oneT

    Reactant.@trace for _ = 1:8
        c = (a - b) 
        next_a = (a + b) 
        next_b = sqrt(a * b)
        sum_c2 += pow2 * c * c
        pow2 += pow2
        a = next_a
        b = next_b
    end

    return a
end

 m_r = Reactant.to_rarray(ms)

 Float64[0.01, 0.1, 0.5, 0.9]

Results in the error:

ERROR: Types do not match between function arguments and results.
The following arguments should be traced but were not: pow2 (path=(7, 1), type=Reactant.TracedRNumber{Float64})
The following arguments should be returned but were not: 
argprefix = ##loop_bodyarg#3367
resprefix = ##loop_bodyres#3368
verify_arg_names = (Symbol("##i#3337"), Symbol("##start#3341"), Symbol("##step#3342"), Symbol("##limit#3343"), :a, :b, :pow2, :sum_c2, :next_b, :next_a, :c)
argtys = DataType[Base.RefValue{Reactant.TracedRNumber{Int64}}, Base.RefValue{Reactant.TracedRNumber{Int64}}, Base.RefValue{Reactant.TracedRNumber{Int64}}, Base.RefValue{Reactant.TracedRNumber{Int64}}, Base.RefValue{Reactant.TracedRNumber{Float64}}, Base.RefValue{Reactant.TracedRNumber{Float64}}, Base.RefValue{Reactant.TracedRNumber{Float64}}, Base.RefValue{Reactant.TracedRNumber{Float64}}, Base.RefValue{Nothing}, Base.RefValue{Nothing}, Base.RefValue{Nothing}]
Traced Arg Paths: 
(Ptr{Nothing} @0x0000000372064a10, ((Symbol("##loop_bodyarg#3367"), 1, 1),))
(Ptr{Nothing} @0x0000000372064b10, ((Symbol("##loop_bodyarg#3367"), 2, 1), (Symbol("##loop_bodyresarg#3369"), 2, 1)))
(Ptr{Nothing} @0x0000000372064c10, ((Symbol("##loop_bodyarg#3367"), 3, 1), (Symbol("##loop_bodyresarg#3369"), 3, 1)))
(Ptr{Nothing} @0x0000000372064d30, ((Symbol("##loop_bodyarg#3367"), 4, 1), (Symbol("##loop_bodyresarg#3369"), 4, 1)))
(Ptr{Nothing} @0x0000000372064e50, ((Symbol("##loop_bodyarg#3367"), 5, 1),))
(Ptr{Nothing} @0x0000000372064f50, ((Symbol("##loop_bodyarg#3367"), 6, 1),))
(Ptr{Nothing} @0x0000000372065150, ((Symbol("##loop_bodyarg#3367"), 8, 1),))

Traced Res Paths: 
(Ptr{Nothing} @0x00000003720286b0, ((Symbol("##loop_bodyresarg#3369"), 6, 1),))
(Ptr{Nothing} @0x00000003720294b0, ((Symbol("##loop_bodyresarg#3369"), 8, 1),))
(Ptr{Nothing} @0x0000000372029930, ((Symbol("##loop_bodyresarg#3369"), 7, 1),))
(Ptr{Nothing} @0x0000000372064b10, ((Symbol("##loop_bodyarg#3367"), 2, 1), (Symbol("##loop_bodyresarg#3369"), 2, 1)))
(Ptr{Nothing} @0x0000000372064c10, ((Symbol("##loop_bodyarg#3367"), 3, 1), (Symbol("##loop_bodyresarg#3369"), 3, 1)))
(Ptr{Nothing} @0x0000000372064d30, ((Symbol("##loop_bodyarg#3367"), 4, 1), (Symbol("##loop_bodyresarg#3369"), 4, 1)))
(Ptr{Nothing} @0x00000003720673f0, ((Symbol("##loop_bodyresarg#3369"), 1, 1),))
(Ptr{Nothing} @0x0000000372067d30, ((Symbol("##loop_bodyresarg#3369"), 5, 1),))

Traced Arg NI Paths: 
(Ptr{Nothing} @0x0000000372064a10, ((Symbol("##loop_bodyarg#3367"), 1, 1),))
(Ptr{Nothing} @0x0000000372064e50, ((Symbol("##loop_bodyarg#3367"), 5, 1),))
(Ptr{Nothing} @0x0000000372064f50, ((Symbol("##loop_bodyarg#3367"), 6, 1),))
(Ptr{Nothing} @0x0000000372065150, ((Symbol("##loop_bodyarg#3367"), 8, 1),))

Traced Res NI Paths: 
(Ptr{Nothing} @0x00000003720286b0, ((Symbol("##loop_bodyresarg#3369"), 6, 1),))
(Ptr{Nothing} @0x00000003720294b0, ((Symbol("##loop_bodyresarg#3369"), 8, 1),))
(Ptr{Nothing} @0x0000000372029930, ((Symbol("##loop_bodyresarg#3369"), 7, 1),))
(Ptr{Nothing} @0x00000003720673f0, ((Symbol("##loop_bodyresarg#3369"), 1, 1),))
(Ptr{Nothing} @0x0000000372067d30, ((Symbol("##loop_bodyresarg#3369"), 5, 1),))

traced_result : Nothing

Stacktrace:
  [1] error(s::String)
    @ Base ./error.jl:35
  [2] finalize_mlir_fn(result::Nothing, traced_args::Vector{…}, linear_args::Vector{…}, skipped_args::Vector{…}, seen_args::Reactant.OrderedIdDict{…}, fnbody::Reactant.MLIR.IR.Block, func::Reactant.MLIR.IR.Operation, mod::Reactant.MLIR.IR.Module, name::String, in_tys::Vector{…}, do_transpose::Bool, optimize_then_pad::Bool, inv_map::IdDict{…}, args_in_result::Symbol, resprefix::Symbol, argprefix::Symbol, resargprefix::Symbol, verify_arg_names::NTuple{…}, return_dialect::Symbol, traced_args_to_shardings::Reactant.OrderedIdDict{…}, output_shardings::Nothing, sym_visibility::Reactant.MLIR.IR.Attribute, num_replicas::Int64, runtime::Nothing, construct_function_without_args::Bool, args::Vector{…}, N::Int64, concretein::Bool, toscalar::Bool)
    @ Reactant.TracedUtils ~/.julia/packages/Reactant/Tk0YL/src/TracedUtils.jl:818
  [3] make_mlir_fn(f::var"#66#68", args::Vector{…}, kwargs::Tuple{}, name::String, concretein::Bool; toscalar::Bool, return_dialect::Symbol, args_in_result::Symbol, construct_function_without_args::Bool, do_transpose::Bool, within_autodiff::Bool, input_shardings::Nothing, output_shardings::Nothing, runtime::Nothing, verify_arg_names::NTuple{…}, argprefix::Symbol, resprefix::Symbol, resargprefix::Symbol, num_replicas::Int64, optimize_then_pad::Bool)
    @ Reactant.TracedUtils ~/.julia/packages/Reactant/Tk0YL/src/TracedUtils.jl:390
  [4] make_mlir_fn
    @ ~/.julia/packages/Reactant/Tk0YL/src/TracedUtils.jl:299 [inlined]
  [5] while_loop(::var"#65#67", ::var"#66#68", ::Base.RefValue{…}, ::Vararg{…}; track_numbers::Type, verify_arg_names::NTuple{…}, checkpointing::Bool, mincut::Bool, location::Reactant.MLIR.IR.Location)
    @ Reactant.Ops ~/.julia/packages/Reactant/Tk0YL/src/Ops.jl:2362
  [6] #traced_while#162
    @ ~/.julia/packages/Reactant/Tk0YL/src/ControlFlow.jl:20 [inlined]
  [7] traced_while
    @ ~/.julia/packages/Reactant/Tk0YL/src/ControlFlow.jl:11 [inlined]
  [8] macro expansion
    @ ~/.julia/packages/ReactantCore/KHVpc/src/ReactantCore.jl:390 [inlined]
  [9] temp
    @ ./REPL[62]:8
 [10] call_with_reactant(::typeof(temp), ::Reactant.TracedRNumber{Float64})
    @ Reactant ~/.julia/packages/Reactant/Tk0YL/src/utils.jl:1195
 [11] make_mlir_fn(f::typeof(temp), args::Tuple{…}, kwargs::Tuple{}, name::String, concretein::Bool; toscalar::Bool, return_dialect::Symbol, args_in_result::Symbol, construct_function_without_args::Bool, do_transpose::Bool, within_autodiff::Bool, input_shardings::Nothing, output_shardings::Nothing, runtime::Nothing, verify_arg_names::Nothing, argprefix::Symbol, resprefix::Symbol, resargprefix::Symbol, num_replicas::Int64, optimize_then_pad::Bool)
    @ Reactant.TracedUtils ~/.julia/packages/Reactant/Tk0YL/src/TracedUtils.jl:370
 [12] elem_apply(f::Function, args::Reactant.TracedRArray{Float64, 1})
    @ Reactant.TracedUtils ~/.julia/packages/Reactant/Tk0YL/src/TracedUtils.jl:1211
 [13] _copyto!
    @ ~/.julia/packages/Reactant/Tk0YL/src/TracedRArray.jl:397
 [14] call_with_reactant(::Reactant.EnsureReturnType{…}, ::typeof(Reactant.TracedRArrayOverrides._copyto!), ::Reactant.TracedRArray{…}, ::Base.Broadcast.Broadcasted{…})
    @ Reactant ~/.julia/packages/Reactant/Tk0YL/src/utils.jl:1195
 [15] copyto!
    @ ~/.julia/packages/Reactant/Tk0YL/src/TracedRArray.jl:336
 [16] call_with_reactant(::Reactant.EnsureReturnType{…}, ::typeof(copyto!), ::Reactant.TracedRArray{…}, ::Base.Broadcast.Broadcasted{…})
    @ Reactant ~/.julia/packages/Reactant/Tk0YL/src/utils.jl:1195
 [17] copyto!
    @ ./broadcast.jl:956 [inlined]
 [18] _copy
    @ ~/.julia/packages/Reactant/Tk0YL/src/TracedRArray.jl:322
 [19] call_with_reactant(::Reactant.EnsureReturnType{…}, ::typeof(Reactant.TracedRArrayOverrides._copy), ::Base.Broadcast.Broadcasted{…})
    @ Reactant ~/.julia/packages/Reactant/Tk0YL/src/utils.jl:1195
 [20] copy
    @ ~/.julia/packages/Reactant/Tk0YL/src/TracedRArray.jl:326
 [21] call_with_reactant(::Reactant.EnsureReturnType{…}, ::typeof(copy), ::Base.Broadcast.Broadcasted{…})
    @ Reactant ~/.julia/packages/Reactant/Tk0YL/src/utils.jl:1195
 [22] materialize
    @ ./broadcast.jl:903 [inlined]
 [23] BroadcastFunction
    @ ./broadcast.jl:1381 [inlined]
 [24] #apply#102
    @ ~/.julia/packages/Reactant/Tk0YL/src/utils.jl:119
 [25] call_with_reactant(::Reactant.EnsureReturnType{…}, ::Reactant.var"##apply#102", ::@Kwargs{}, ::typeof(Reactant.apply), ::Base.Broadcast.BroadcastFunction{…}, ::Reactant.TracedRArray{…})
    @ Reactant ~/.julia/packages/Reactant/Tk0YL/src/utils.jl:1195
 [26] apply
    @ ~/.julia/packages/Reactant/Tk0YL/src/utils.jl:118
 [27] call_with_reactant(::typeof(Reactant.apply), ::Base.Broadcast.BroadcastFunction{…}, ::Reactant.TracedRArray{…})
    @ Reactant ~/.julia/packages/Reactant/Tk0YL/src/utils.jl:1195
 [28] make_mlir_fn(f::typeof(Reactant.apply), args::Tuple{…}, kwargs::@NamedTuple{}, name::String, concretein::Bool; toscalar::Bool, return_dialect::Symbol, args_in_result::Symbol, construct_function_without_args::Bool, do_transpose::Bool, within_autodiff::Bool, input_shardings::Nothing, output_shardings::Nothing, runtime::Val{…}, verify_arg_names::Nothing, argprefix::Symbol, resprefix::Symbol, resargprefix::Symbol, num_replicas::Int64, optimize_then_pad::Bool)
    @ Reactant.TracedUtils ~/.julia/packages/Reactant/Tk0YL/src/TracedUtils.jl:370
 [29] make_mlir_fn
    @ ~/.julia/packages/Reactant/Tk0YL/src/TracedUtils.jl:299 [inlined]
 [30] #make_mlir_fn#7
    @ ~/.julia/packages/Reactant/Tk0YL/src/TracedUtils.jl:322 [inlined]
 [31] make_mlir_fn
    @ ~/.julia/packages/Reactant/Tk0YL/src/TracedUtils.jl:299 [inlined]
 [32] compile_mlir!(mod::Reactant.MLIR.IR.Module, f::Base.Broadcast.BroadcastFunction{…}, args::Tuple{…}, compile_options::CompileOptions, debugcache::Vector{…}, callcache::Dict{…}, sdycache::Dict{…}, sdygroupidcache::Tuple{…}; fn_kwargs::@NamedTuple{}, backend::String, runtime::Val{…}, legalize_stablehlo_to_mhlo::Bool, client::Reactant.XLA.PJRT.Client, kwargs::@Kwargs{})
    @ Reactant.Compiler ~/.julia/packages/Reactant/Tk0YL/src/Compiler.jl:1289
 [33] compile_mlir!
    @ ~/.julia/packages/Reactant/Tk0YL/src/Compiler.jl:1248 [inlined]
 [34] compile_xla(ctx::Reactant.MLIR.IR.Context, f::Function, args::Tuple{…}; before_xla_optimizations::Bool, client::Nothing, serializable::Bool, kwargs::@Kwargs{…})
    @ Reactant.Compiler ~/.julia/packages/Reactant/Tk0YL/src/Compiler.jl:3503
 [35] compile_xla
    @ ~/.julia/packages/Reactant/Tk0YL/src/Compiler.jl:3478 [inlined]
 [36] compile(ctx::Reactant.MLIR.IR.Context, f::Function, args::Tuple{…}; kwargs::@Kwargs{…})
    @ Reactant.Compiler ~/.julia/packages/Reactant/Tk0YL/src/Compiler.jl:3599
 [37] macro expansion
    @ ~/.julia/packages/Reactant/Tk0YL/src/Compiler.jl:2556 [inlined]
 [38] macro expansion
    @ ~/.julia/packages/LLVM/Bd7kA/src/base.jl:113 [inlined]
 [39] top-level scope
    @ ~/.julia/packages/Reactant/Tk0YL/src/Compiler.jl:2555
Some type information was truncated. Use `show(err)` to see complete types.

[jumerckx]

I was able to reduce this further:

using Reactant

function temp(m::T) where {T}
    a = one(T)
    pow2 = a

    Reactant.@trace for _ = 1:8
        pow2 = pow2 + a
    end

    return a
end

ms = Float64[0.01, 0.1, 0.5, 0.9]
ms_r = Reactant.to_rarray(ms)

Reactant.@compile temp.(ms_r)

Error

ERROR: Types do not match between function arguments and results.
The following arguments should be traced but were not: pow2 (path=(6, 1), type=Reactant.TracedRNumber{Float64})
The following arguments should be returned but were not: 
argprefix = ##loop_bodyarg#296
resprefix = ##loop_bodyres#297
verify_arg_names = (Symbol("##i#277"), Symbol("##step#282"), Symbol("##limit#283"), Symbol("##start#281"), :a, :pow2)
argtys = DataType[Base.RefValue{Reactant.TracedRNumber{Int64}}, Base.RefValue{Reactant.TracedRNumber{Int64}}, Base.RefValue{Reactant.TracedRNumber{Int64}}, Base.RefValue{Reactant.TracedRNumber{Int64}}, Base.RefValue{Reactant.TracedRNumber{Float64}}, Base.RefValue{Reactant.TracedRNumber{Float64}}]
Traced Arg Paths: 
(Ptr{Nothing}(0x000077e2ecab9a10), ((Symbol("##loop_bodyarg#296"), 1, 1),))
(Ptr{Nothing}(0x000077e2ecab9bf0), ((Symbol("##loop_bodyarg#296"), 2, 1), (Symbol("##loop_bodyresarg#298"), 2, 1)))
(Ptr{Nothing}(0x000077e2ecab9dd0), ((Symbol("##loop_bodyarg#296"), 3, 1), (Symbol("##loop_bodyresarg#298"), 3, 1)))
(Ptr{Nothing}(0x000077e2ecab9fd0), ((Symbol("##loop_bodyarg#296"), 4, 1), (Symbol("##loop_bodyresarg#298"), 4, 1)))
(Ptr{Nothing}(0x000077e2ecaba1d0), ((Symbol("##loop_bodyarg#296"), 5, 1), (Symbol("##loop_bodyresarg#298"), 5, 1)))

Traced Res Paths: 
(Ptr{Nothing}(0x000077e2eb975410), ((Symbol("##loop_bodyresarg#298"), 1, 1),))
(Ptr{Nothing}(0x000077e2eb975f10), ((Symbol("##loop_bodyresarg#298"), 6, 1),))
(Ptr{Nothing}(0x000077e2ecab9bf0), ((Symbol("##loop_bodyarg#296"), 2, 1), (Symbol("##loop_bodyresarg#298"), 2, 1)))
(Ptr{Nothing}(0x000077e2ecab9dd0), ((Symbol("##loop_bodyarg#296"), 3, 1), (Symbol("##loop_bodyresarg#298"), 3, 1)))
(Ptr{Nothing}(0x000077e2ecab9fd0), ((Symbol("##loop_bodyarg#296"), 4, 1), (Symbol("##loop_bodyresarg#298"), 4, 1)))
(Ptr{Nothing}(0x000077e2ecaba1d0), ((Symbol("##loop_bodyarg#296"), 5, 1), (Symbol("##loop_bodyresarg#298"), 5, 1)))

Traced Arg NI Paths: 
(Ptr{Nothing}(0x000077e2ecab9a10), ((Symbol("##loop_bodyarg#296"), 1, 1),))

Traced Res NI Paths: 
(Ptr{Nothing}(0x000077e2eb975410), ((Symbol("##loop_bodyresarg#298"), 1, 1),))
(Ptr{Nothing}(0x000077e2eb975f10), ((Symbol("##loop_bodyresarg#298"), 6, 1),))

traced_result : Nothing

Stacktrace:
  [1] error(s::String)
    @ Base ./error.jl:44
  [2] finalize_mlir_fn(result::Nothing, traced_args::Vector{…}, linear_args::Vector{…}, skipped_args::Vector{…}, seen_args::Reactant.OrderedIdDict{…}, fnbody::Reactant.MLIR.IR.Block, func::Reactant.MLIR.IR.Operation, mod::Reactant.MLIR.IR.Module, name::String, in_tys::Vector{…}, do_transpose::Bool, optimize_then_pad::Bool, inv_map::IdDict{…}, args_in_result::Symbol, resprefix::Symbol, argprefix::Symbol, resargprefix::Symbol, verify_arg_names::NTuple{…}, return_dialect::Symbol, traced_args_to_shardings::Reactant.OrderedIdDict{…}, output_shardings::Nothing, sym_visibility::Reactant.MLIR.IR.Attribute, num_replicas::Int64, runtime::Nothing, construct_function_without_args::Bool, args::Vector{…}, N::Int64, concretein::Bool, toscalar::Bool)
    @ Reactant.TracedUtils /mnt/jumerckx/Reactant2/src/TracedUtils.jl:818
  [3] make_mlir_fn(f::var"#temp##2#temp##3", args::Vector{…}, kwargs::Tuple{}, name::String, concretein::Bool; toscalar::Bool, return_dialect::Symbol, args_in_result::Symbol, construct_function_without_args::Bool, do_transpose::Bool, within_autodiff::Bool, input_shardings::Nothing, output_shardings::Nothing, runtime::Nothing, verify_arg_names::NTuple{…}, argprefix::Symbol, resprefix::Symbol, resargprefix::Symbol, num_replicas::Int64, optimize_then_pad::Bool)
    @ Reactant.TracedUtils /mnt/jumerckx/Reactant2/src/TracedUtils.jl:390
  [4] make_mlir_fn
    @ /mnt/jumerckx/Reactant2/src/TracedUtils.jl:299 [inlined]
  [5] while_loop(::var"#temp##0#temp##1", ::var"#temp##2#temp##3", ::Base.RefValue{…}, ::Vararg{…}; track_numbers::Type, verify_arg_names::NTuple{…}, checkpointing::Bool, mincut::Bool, location::Reactant.MLIR.IR.Location)
    @ Reactant.Ops /mnt/jumerckx/Reactant2/src/Ops.jl:2362
  [6] #traced_while#186
    @ /mnt/jumerckx/Reactant2/src/ControlFlow.jl:20 [inlined]
  [7] traced_while
    @ /mnt/jumerckx/Reactant2/src/ControlFlow.jl:11 [inlined]
  [8] macro expansion
    @ /mnt/jumerckx/Reactant2/lib/ReactantCore/src/ReactantCore.jl:390 [inlined]
  [9] temp
    @ ~/Reactant2/scratch.jl:7
 [10] call_with_reactant
    @ ./none:-1 [inlined]
 [11] call_with_reactant(::var"#temp", ::Reactant.TracedRNumber{Float64})
    @ Reactant /mnt/jumerckx/Reactant2/src/utils.jl:0
 [12] make_mlir_fn(f::var"#temp", args::Tuple{…}, kwargs::Tuple{}, name::String, concretein::Bool; toscalar::Bool, return_dialect::Symbol, args_in_result::Symbol, construct_function_without_args::Bool, do_transpose::Bool, within_autodiff::Bool, input_shardings::Nothing, output_shardings::Nothing, runtime::Nothing, verify_arg_names::Nothing, argprefix::Symbol, resprefix::Symbol, resargprefix::Symbol, num_replicas::Int64, optimize_then_pad::Bool)
    @ Reactant.TracedUtils /mnt/jumerckx/Reactant2/src/TracedUtils.jl:370
 [13] elem_apply(f::Function, args::Reactant.TracedRArray{Float64, 1})
    @ Reactant.TracedUtils /mnt/jumerckx/Reactant2/src/TracedUtils.jl:1211
 [14] _copyto!
    @ /mnt/jumerckx/Reactant2/src/TracedRArray.jl:401
 [15] call_with_reactant
    @ ./none:-1 [inlined]
 [16] call_with_reactant(::Reactant.EnsureReturnType{…}, ::typeof(Reactant.TracedRArrayOverrides._copyto!), ::Reactant.TracedRArray{…}, ::Base.Broadcast.Broadcasted{…})
    @ Reactant /mnt/jumerckx/Reactant2/src/utils.jl:0
 [17] call_with_reactant
    @ ./none:-1 [inlined]
 [18] call_with_reactant(::Reactant.EnsureReturnType{…}, ::typeof(copyto!), ::Reactant.TracedRArray{…}, ::Base.Broadcast.Broadcasted{…})
    @ Reactant /mnt/jumerckx/Reactant2/src/utils.jl:0
 [19] copyto!
    @ ./broadcast.jl:947 [inlined]
 [20] _copy
    @ /mnt/jumerckx/Reactant2/src/TracedRArray.jl:326
 [21] call_with_reactant
    @ ./none:-1 [inlined]
 [22] call_with_reactant(::Reactant.EnsureReturnType{…}, ::typeof(Reactant.TracedRArrayOverrides._copy), ::Base.Broadcast.Broadcasted{…})
    @ Reactant /mnt/jumerckx/Reactant2/src/utils.jl:0
 [23] call_with_reactant
    @ ./none:-1 [inlined]
 [24] call_with_reactant(::Reactant.EnsureReturnType{…}, ::typeof(copy), ::Base.Broadcast.Broadcasted{…})
    @ Reactant /mnt/jumerckx/Reactant2/src/utils.jl:0
 [25] materialize
    @ ./broadcast.jl:894 [inlined]
 [26] BroadcastFunction
    @ ./broadcast.jl:1387 [inlined]
 [27] #apply#128
    @ /mnt/jumerckx/Reactant2/src/utils.jl:119
 [28] call_with_reactant
    @ ./none:-1 [inlined]
 [29] call_with_reactant(::Reactant.EnsureReturnType{…}, ::Reactant.var"##apply#128", ::@Kwargs{}, ::typeof(Reactant.apply), ::Base.Broadcast.BroadcastFunction{…}, ::Reactant.TracedRArray{…})
    @ Reactant /mnt/jumerckx/Reactant2/src/utils.jl:0
 [30] call_with_reactant
    @ ./none:-1 [inlined]
 [31] call_with_reactant(::typeof(Reactant.apply), ::Base.Broadcast.BroadcastFunction{…}, ::Reactant.TracedRArray{…})
    @ Reactant /mnt/jumerckx/Reactant2/src/utils.jl:0
 [32] make_mlir_fn(f::typeof(Reactant.apply), args::Tuple{…}, kwargs::@NamedTuple{}, name::String, concretein::Bool; toscalar::Bool, return_dialect::Symbol, args_in_result::Symbol, construct_function_without_args::Bool, do_transpose::Bool, within_autodiff::Bool, input_shardings::Nothing, output_shardings::Nothing, runtime::Val{…}, verify_arg_names::Nothing, argprefix::Symbol, resprefix::Symbol, resargprefix::Symbol, num_replicas::Int64, optimize_then_pad::Bool)
    @ Reactant.TracedUtils /mnt/jumerckx/Reactant2/src/TracedUtils.jl:370
 [33] make_mlir_fn
    @ /mnt/jumerckx/Reactant2/src/TracedUtils.jl:299 [inlined]
 [34] #make_mlir_fn#4
    @ /mnt/jumerckx/Reactant2/src/TracedUtils.jl:322 [inlined]
 [35] make_mlir_fn
    @ /mnt/jumerckx/Reactant2/src/TracedUtils.jl:299 [inlined]
 [36] compile_mlir!(mod::Reactant.MLIR.IR.Module, f::Base.Broadcast.BroadcastFunction{…}, args::Tuple{…}, compile_options::Reactant.CompileOptions, debugcache::Vector{…}, callcache::Dict{…}, sdycache::Dict{…}, sdygroupidcache::Tuple{…}; fn_kwargs::@NamedTuple{}, backend::String, runtime::Val{…}, legalize_stablehlo_to_mhlo::Bool, client::Reactant.XLA.PJRT.Client, kwargs::@Kwargs{})
    @ Reactant.Compiler /mnt/jumerckx/Reactant2/src/compiler/Compiler.jl:278
 [37] compile_xla(ctx::Reactant.MLIR.IR.Context, f::Function, args::Tuple{…}; before_xla_optimizations::Bool, client::Nothing, serializable::Bool, kwargs::@Kwargs{…})
    @ Reactant.Compiler /mnt/jumerckx/Reactant2/src/compiler/Compiler.jl:1330
 [38] compile_xla
    @ /mnt/jumerckx/Reactant2/src/compiler/Compiler.jl:1305 [inlined]
 [39] compile(ctx::Reactant.MLIR.IR.Context, f::Function, args::Tuple{…}; kwargs::@Kwargs{…})
    @ Reactant.Compiler /mnt/jumerckx/Reactant2/src/compiler/Compiler.jl:1422
 [40] top-level scope
    @ ~/Reactant2/scratch.jl:17
 [41] macro expansion
    @ /mnt/jumerckx/Reactant2/src/compiler/Macros.jl:255 [inlined]
 [42] macro expansion
    @ ~/.julia/packages/LLVM/upRII/src/base.jl:113 [inlined]
 [43] macro expansion
    @ /mnt/jumerckx/Reactant2/src/compiler/Macros.jl:256 [inlined]
Some type information was truncated. Use `show(err)` to see complete types.

The issue is with the aliasing of pow2 and a. When the loop body function is being generated, Refs are generated that point to the same traced value.
The linearized argument list for this function will contain this traced value only once.
However, the list of result values for the loop body function will contain an additional traced value, explaining the mismatch.
I had a go at implementing a fix, with the help of some LLM, but I'm not confident this is always correct: #2900

[jumerckx]

For context, since this is difficult to fix automatically, we decided that this is now illegal to do in traced loops. So now your code throws an error which is hopefully more helpful:

ERROR: Reactant.@trace: the aliasing pattern of loop-carried variables changed during the loop body.
`a` and `pow2` referred to the same object before the loop body but not after.
Fix: break the aliasing before the `@trace` loop, e.g. `pow2 = copy(pow2)`.