`remake_buffer` skips eltype promotion for mixed-portion `ParameterIndex` collections → `MethodError: Float64(::Dual)`

**Describe the bug 🐞**

`setp_oop` / `setsym_oop` setters whose symbols span *different* `MTKParameters` portions
(e.g. `Initial(v)` in `Initials` + a plain parameter in `Tunable`) fail with

```
MethodError: no method matching Float64(::ForwardDiff.Dual{...})
```

as soon as the values are non-`Float64` (ForwardDiff Duals being the common case).
Homogeneous single-portion setters promote correctly. This breaks
`ForwardDiff.gradient` over any solve-based loss that tunes an initial condition together
with a parameter — e.g. parameter estimation with mixed u0/p search spaces


**Expected behavior**

no error.

**Minimal Reproducible Example 👇**

```julia
using ModelingToolkit
using ModelingToolkit: D_nounits as D, t_nounits as t
using SymbolicIndexingInterface
using ForwardDiff: Dual

@parameters m=1.5 d=9.0
@variables s(t)=1.0 v(t)=1.0
eqs = [D(s) ~ v, m * D(v) ~ 1 - d * v]
sys = mtkcompile(System(eqs, t; name = :model))
prob = ODEProblem(sys, [], (0.0, 1.0))

xd = [Dual(1.1, 1.0), Dual(4.3, 1.0)]

setp_oop(prob, [Initial(sys.v)])(prob, xd[1:1])   # works, initials promoted to Dual
setp_oop(prob, [sys.d])(prob, xd[2:2])            # works, tunable promoted to Dual
setp_oop(prob, [Initial(sys.v), sys.d])(prob, xd) # MethodError
```

**Error & Stacktrace ⚠️**

```julia
ERROR: MethodError: no method matching Float64(::ForwardDiff.Dual{Nothing, Float64, 1})
The type `Float64` exists, but no method is defined for this combination of argument types when trying to construct it.

Closest candidates are:
  (::Type{T})(::Real, ::RoundingMode) where T<:AbstractFloat
   @ Base rounding.jl:265
  (::Type{T})(::T) where T<:Number
   @ Core boot.jl:965
  Float64(::Int16)
   @ Base float.jl:245
  ...

Stacktrace:
 [1] convert(::Type{Float64}, x::ForwardDiff.Dual{Nothing, Float64, 1})
   @ Base ./number.jl:7
 [2] setindex!
   @ ./array.jl:985 [inlined]
 [3] set_parameter!(p::ModelingToolkitBase.MTKParameters{…}, val::ForwardDiff.Dual{…}, pidx::ModelingToolkitBase.ParameterIndex{…})
   @ ModelingToolkitBase ~/.julia/packages/ModelingToolkitBase/tQDig/src/systems/parameter_buffer.jl:505
 [4] setindex!(p::ModelingToolkitBase.MTKParameters{…}, val::ForwardDiff.Dual{…}, pind::ModelingToolkitBase.ParameterIndex{…})
   @ ModelingToolkitBase ~/.julia/packages/ModelingToolkitBase/tQDig/src/systems/parameter_buffer.jl:1135
 [5] macro expansion
   @ ~/.julia/packages/ModelingToolkitBase/tQDig/src/systems/parameter_buffer.jl:919 [inlined]
 [6] _remake_buffer
   @ ~/.julia/packages/ModelingToolkitBase/tQDig/src/systems/parameter_buffer.jl:784 [inlined]
 [7] remake_buffer
   @ ~/.julia/packages/ModelingToolkitBase/tQDig/src/systems/parameter_buffer.jl:659 [inlined]
 [8] (::SymbolicIndexingInterface.OOPSetter{…})(valp::SciMLBase.ODEProblem{…}, val::Vector{…})
   @ SymbolicIndexingInterface ~/.julia/packages/SymbolicIndexingInterface/v22YL/src/value_provider_interface.jl:272
 [9] top-level scope
   @ ~/work/dev/upstream_bugs/remake_buffer_mixed_promotion/issue_draft.md:41
Some type information was truncated. Use `show(err)` to see complete types.
```

**Environment (please complete the following information):**

  - Output of `using Pkg; Pkg.status()`

```julia
Status `~/work/dev/upstream_bugs/remake_buffer_mixed_promotion/Project.toml`
  [f6369f11] ForwardDiff v1.4.3
  [961ee093] ModelingToolkit v11.38.0
  [2efcf032] SymbolicIndexingInterface v0.3.51
```

  - Output of `using Pkg; Pkg.status(; mode = PKGMODE_MANIFEST)`

```julia
Status `~/work/dev/upstream_bugs/remake_buffer_mixed_promotion/Manifest.toml`
  [47edcb42] ADTypes v1.22.3
  [14f7f29c] AMD v0.5.3
  [6e696c72] AbstractPlutoDingetjes v1.4.0
  [1520ce14] AbstractTrees v0.4.5
  [7d9f7c33] Accessors v0.1.45
  [79e6a3ab] Adapt v4.7.0
  [ec485272] ArnoldiMethod v0.4.0
  [4fba245c] ArrayInterface v7.28.1
  [4c555306] ArrayLayouts v1.12.2
  [aae01518] BandedMatrices v1.11.0
  [e2ed5e7c] Bijections v0.2.2
  [b2a6c25c] BinaryHeaps v1.0.4
  [caf10ac8] BipartiteGraphs v0.1.10
  [8e7c35d0] BlockArrays v1.10.0
  [70df07ce] BracketingNonlinearSolve v1.12.4
⌅ [861a8166] Combinatorics v1.0.2
  [38540f10] CommonSolve v0.2.12
  [bbf7d656] CommonSubexpressions v0.3.1
  [f70d9fcc] CommonWorldInvalidations v1.1.2
  [34da2185] Compat v4.18.1
  [b152e2b5] CompositeTypes v0.1.4
  [a33af91c] CompositionsBase v0.1.2
  [2569d6c7] ConcreteStructs v0.2.7
  [187b0558] ConstructionBase v1.6.0
  [a8cc5b0e] Crayons v4.2.0
  [9a962f9c] DataAPI v1.16.0
  [864edb3b] DataStructures v0.19.6
  [e2d170a0] DataValueInterfaces v1.0.0
  [2b5f629d] DiffEqBase v7.10.0
  [459566f4] DiffEqCallbacks v4.19.0
  [163ba53b] DiffResults v1.1.0
  [b552c78f] DiffRules v1.16.0
  [a0c0ee7d] DifferentiationInterface v0.7.20
  [ffbed154] DocStringExtensions v0.9.5
  [5b8099bc] DomainSets v0.8.1
  [7c1d4256] DynamicPolynomials v0.6.6
  [4e289a0a] EnumX v1.0.7
  [f151be2c] EnzymeCore v0.8.21
  [e2ba6199] ExprTools v0.1.11
  [55351af7] ExproniconLite v0.10.14
  [7034ab61] FastBroadcast v1.3.5
  [9aa1b823] FastClosures v0.3.2
  [a4df4552] FastPower v1.4.0
  [1a297f60] FillArrays v1.17.0
  [64ca27bc] FindFirstFunctions v3.2.1
  [6a86dc24] FiniteDiff v2.32.0
  [f6369f11] ForwardDiff v1.4.3
  [a85aefff] FunctionMaps v0.1.2
  [069b7b12] FunctionWrappers v1.1.3
  [77dc65aa] FunctionWrappersWrappers v1.12.0
  [46192b85] GPUArraysCore v0.2.0
  [86223c79] Graphs v1.14.0
  [3263718b] ImplicitDiscreteSolve v2.1.5
  [d25df0c9] Inflate v0.1.5
  [18e54dd8] IntegerMathUtils v0.1.4
  [8197267c] IntervalSets v0.7.14
  [3587e190] InverseFunctions v0.1.17
  [92d709cd] IrrationalConstants v0.2.6
  [82899510] IteratorInterfaceExtensions v1.0.0
  [692b3bcd] JLLWrappers v1.8.0
  [ae98c720] Jieko v0.2.1
  [ccbc3e58] JumpProcesses v9.29.2
  [ba0b0d4f] Krylov v0.10.8
  [b964fa9f] LaTeXStrings v1.4.0
  [87fe0de2] LineSearch v0.1.12
  [7ed4a6bd] LinearSolve v5.4.0
  [2ab3a3ac] LogExpFunctions v1.0.1
  [e6f89c97] LoggingExtras v1.2.0
  [1914dd2f] MacroTools v0.5.16
  [bb5d69b7] MaybeInplace v0.1.7
  [961ee093] ModelingToolkit v11.38.0
  [7771a370] ModelingToolkitBase v1.59.0
  [6bb917b9] ModelingToolkitTearing v1.20.4
  [2e0e35c7] Moshi v0.3.12
  [46d2c3a1] MuladdMacro v0.2.7
  [102ac46a] MultivariatePolynomials v0.5.19
  [d8a4904e] MutableArithmetics v1.8.0
  [77ba4419] NaNMath v1.1.4
  [be0214bd] NonlinearSolveBase v2.39.1
  [5959db7a] NonlinearSolveFirstOrder v2.2.3
  [6fe1bfb0] OffsetArrays v1.17.0
⌅ [bac558e1] OrderedCollections v1.8.2
  [bbf590c4] OrdinaryDiffEqCore v4.12.0
  [e409e4f3] PoissonRandom v0.4.13
  [d236fae5] PreallocationTools v1.4.0
  [aea7be01] PrecompileTools v1.3.4
  [21216c6a] Preferences v1.5.2
  [08abe8d2] PrettyTables v3.4.4
  [27ebfcd6] Primes v0.5.7
  [0c0d3e7f] PureKLU v1.2.0
  [988b38a3] ReadOnlyArrays v0.2.0
  [795d4caa] ReadOnlyDicts v1.0.1
  [3cdcf5f2] RecipesBase v1.3.4
  [731186ca] RecursiveArrayTools v4.3.5
  [189a3867] Reexport v1.2.2
  [9fe22ead] RespecializeParams v1.2.0
  [7e49a35a] RuntimeGeneratedFunctions v0.5.24
  [9dfe8606] SCCNonlinearSolve v1.14.0
  [0bca4576] SciMLBase v3.41.0
  [19f34311] SciMLJacobianOperators v0.1.16
  [a6db7da4] SciMLLogging v2.0.3
  [c0aeaf25] SciMLOperators v1.25.2
  [431bcebd] SciMLPublic v1.2.4
  [53ae85a6] SciMLStructures v1.10.4
  [efcf1570] Setfield v1.1.2
  [727e6d20] SimpleNonlinearSolve v2.13.1
  [699a6c99] SimpleTraits v0.9.6
  [a57abbd0] SparseColumnPivotedQR v2.1.5
  [276daf66] SpecialFunctions v2.8.1
  [0c0c59c1] StarAlgebras v0.3.0
  [64909d44] StateSelection v1.11.0
  [90137ffa] StaticArrays v1.9.18
  [1e83bf80] StaticArraysCore v1.4.4
  [10745b16] Statistics v1.11.1
  [892a3eda] StringManipulation v0.4.7
  [2efcf032] SymbolicIndexingInterface v0.3.51
  [19f23fe9] SymbolicLimits v1.1.4
  [d1185830] SymbolicUtils v4.44.0
  [0c5d862f] Symbolics v7.35.0
  [3783bdb8] TableTraits v1.0.1
  [bd369af6] Tables v1.13.0
  [ed4db957] TaskLocalValues v0.1.3
  [8ea1fca8] TermInterface v2.0.0
  [a759f4b9] TimerOutputs v1.1.0
  [781d530d] TruncatedStacktraces v1.4.0
  [3a884ed6] UnPack v1.0.2
  [d30d5f5c] WeakCacheSets v0.1.0
  [1d5cc7b8] IntelOpenMP_jll v2025.2.0+0
  [856f044c] MKL_jll v2025.2.0+0
  [efe28fd5] OpenSpecFun_jll v0.5.6+0
  [1317d2d5] oneTBB_jll v2022.3.0+0
  [0dad84c5] ArgTools v1.1.2
  [56f22d72] Artifacts v1.11.0
  [2a0f44e3] Base64 v1.11.0
  [ade2ca70] Dates v1.11.0
  [8ba89e20] Distributed v1.11.0
  [f43a241f] Downloads v1.7.0
  [7b1f6079] FileWatching v1.11.0
  [9fa8497b] Future v1.11.0
  [b77e0a4c] InteractiveUtils v1.11.0
  [ac6e5ff7] JuliaSyntaxHighlighting v1.12.0
  [4af54fe1] LazyArtifacts v1.11.0
  [b27032c2] LibCURL v0.6.4
  [76f85450] LibGit2 v1.11.0
  [8f399da3] Libdl v1.11.0
  [37e2e46d] LinearAlgebra v1.12.0
  [56ddb016] Logging v1.11.0
  [d6f4376e] Markdown v1.11.0
  [ca575930] NetworkOptions v1.3.0
  [44cfe95a] Pkg v1.12.1
  [de0858da] Printf v1.11.0
  [3fa0cd96] REPL v1.11.0
  [9a3f8284] Random v1.11.0
  [ea8e919c] SHA v0.7.0
  [9e88b42a] Serialization v1.11.0
  [6462fe0b] Sockets v1.11.0
  [2f01184e] SparseArrays v1.12.0
  [f489334b] StyledStrings v1.11.0
  [fa267f1f] TOML v1.0.3
  [a4e569a6] Tar v1.10.0
  [8dfed614] Test v1.11.0
  [cf7118a7] UUIDs v1.11.0
  [4ec0a83e] Unicode v1.11.0
  [e66e0078] CompilerSupportLibraries_jll v1.3.0+1
  [deac9b47] LibCURL_jll v8.15.0+0
  [e37daf67] LibGit2_jll v1.9.0+0
  [29816b5a] LibSSH2_jll v1.11.3+1
  [14a3606d] MozillaCACerts_jll v2025.11.4
  [4536629a] OpenBLAS_jll v0.3.29+0
  [05823500] OpenLibm_jll v0.8.7+0
  [458c3c95] OpenSSL_jll v3.5.4+0
  [bea87d4a] SuiteSparse_jll v7.8.3+2
  [83775a58] Zlib_jll v1.3.1+2
  [8e850b90] libblastrampoline_jll v5.15.0+0
  [8e850ede] nghttp2_jll v1.64.0+1
  [3f19e933] p7zip_jll v17.7.0+0
Info Packages marked with ⌅ have new versions available but compatibility constraints restrict them from upgrading. To see why use `status --outdated -m`
```

  - Output of `versioninfo()`

```julia
Julia Version 1.12.6
Commit 15346901f00 (2026-04-09 19:20 UTC)
Build Info:
  Official https://julialang.org release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 32 × Intel(R) Core(TM) i9-14900K
  WORD_SIZE: 64
  LLVM: libLLVM-18.1.7 (ORCJIT, alderlake)
  GC: Built with stock GC
Threads: 32 default, 1 interactive, 32 GC (on 32 virtual cores)
Environment:
  JULIA_EDITOR = code
  JULIA_VSCODE_REPL = 1
```

**Additional context**

### Root cause

The `@generated` fast path of `_remake_buffer`
(ModelingToolkitBase v1.59.0, `src/systems/parameter_buffer.jl:784`, comment
"For type-inference when using `SII.setp_oop`") decides per-portion buffer promotion
**from the type parameters alone**:

```julia
# parameter_buffer.jl:826-834
idxT <: ParameterIndex{SciMLStructures.Tunable}  || continue
idxT <: ParameterIndex{SciMLStructures.Initials} || continue
```

with `idxT = eltype(idxs)`. A mixed index vector (built by SII as
`parameter_index.((indp,), sym)`, `parameter_indexing.jl:834-837`; the
`BatchedInterface` route in `batched_interface.jl:351` splices an equivalent vector)
has the abstract typejoin eltype `ParameterIndex{P, Int64} where P`, which is a subtype
of **none** of the per-portion checks — so *no* buffer is promoted, and the generated
`setindex!` loop (`parameter_buffer.jl:919`) hits `convert(Float64, ::Dual)`.

The existing fallback to the dynamic path only inspects the values:

```julia
# parameter_buffer.jl:791
if vals <: AbstractArray && !isconcretetype(eltype(vals))
```

`eltype(vals) = Dual{...}` is concrete, so the fallback doesn't trigger. The dynamic
`__remake_buffer` (Any buffers + narrowing) would have handled this case correctly.


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`remake_buffer` skips eltype promotion for mixed-portion `ParameterIndex` collections → `MethodError: Float64(::Dual)` #4864

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remake_buffer skips eltype promotion for mixed-portion ParameterIndex collections → MethodError: Float64(::Dual) #4864

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`remake_buffer` skips eltype promotion for mixed-portion `ParameterIndex` collections → `MethodError: Float64(::Dual)` #4864