check length on unflatten!!

[penelopeysm]

Closes #987

Uncovered a bug in the test suite ....

[github-actions]

Benchmark Report

• this PR's head: 1d499c480193ecea2a020ac2846f871bd3f84a72
• base branch: 23bd87cb89359b5ca9e45eb956165b12ae3c10d4

Computer Information

Julia Version 1.11.9
Commit 53a02c0720c (2026-02-06 00:27 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 4 × AMD EPYC 7763 64-Core Processor
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, znver3)
Threads: 1 default, 0 interactive, 1 GC (on 4 virtual cores)

Benchmark Results

┌───────────────────────┬───────┬─────────────┬────────┬───────────────────────────────┬────────────────────────────┬─────────────────────────────────┐
│                       │       │             │        │       t(eval) / t(ref)        │     t(grad) / t(eval)      │        t(grad) / t(ref)         │
│                       │       │             │        │ ─────────┬──────────┬──────── │ ───────┬─────────┬──────── │ ──────────┬───────────┬──────── │
│                 Model │   Dim │  AD Backend │ Linked │     base │  this PR │ speedup │   base │ this PR │ speedup │      base │   this PR │ speedup │
├───────────────────────┼───────┼─────────────┼────────┼──────────┼──────────┼─────────┼────────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│               Dynamic │    10 │    mooncake │   true │   257.67 │   275.74 │    0.93 │  11.10 │    7.58 │    1.46 │   2859.12 │   2089.97 │    1.37 │
│                   LDA │    12 │ reversediff │   true │  2350.67 │  2595.02 │    0.91 │   2.18 │    1.97 │    1.11 │   5135.38 │   5112.12 │    1.00 │
│   Loop univariate 10k │ 10000 │    mooncake │   true │ 28586.78 │ 30429.96 │    0.94 │   6.32 │    6.52 │    0.97 │ 180695.25 │ 198294.30 │    0.91 │
├───────────────────────┼───────┼─────────────┼────────┼──────────┼──────────┼─────────┼────────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│    Loop univariate 1k │  1000 │    mooncake │   true │  2905.88 │  3087.52 │    0.94 │   6.17 │    6.37 │    0.97 │  17938.63 │  19678.82 │    0.91 │
│      Multivariate 10k │ 10000 │    mooncake │   true │ 29250.91 │ 31652.56 │    0.92 │   9.86 │    9.82 │    1.00 │ 288472.60 │ 310681.00 │    0.93 │
│       Multivariate 1k │  1000 │    mooncake │   true │  3188.37 │  3350.65 │    0.95 │   9.22 │    9.36 │    0.99 │  29395.59 │  31348.93 │    0.94 │
├───────────────────────┼───────┼─────────────┼────────┼──────────┼──────────┼─────────┼────────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│ Simple assume observe │     1 │ forwarddiff │  false │     1.02 │     0.86 │    1.18 │   8.32 │   10.39 │    0.80 │      8.48 │      8.95 │    0.95 │
│           Smorgasbord │   201 │ forwarddiff │  false │   896.66 │   929.93 │    0.96 │  74.16 │   73.98 │    1.00 │  66494.12 │  68796.14 │    0.97 │
│           Smorgasbord │   201 │      enzyme │   true │  1197.34 │  1269.31 │    0.94 │   4.90 │    4.90 │    1.00 │   5862.92 │   6217.75 │    0.94 │
├───────────────────────┼───────┼─────────────┼────────┼──────────┼──────────┼─────────┼────────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│           Smorgasbord │   201 │ forwarddiff │   true │  1599.68 │  1694.48 │    0.94 │  58.97 │   52.79 │    1.12 │  94338.24 │  89456.37 │    1.05 │
│           Smorgasbord │   201 │    mooncake │   true │  1213.89 │  1290.88 │    0.94 │   4.75 │    4.63 │    1.03 │   5771.87 │   5970.76 │    0.97 │
│           Smorgasbord │   201 │ reversediff │   true │  1252.72 │  1641.82 │    0.76 │ 123.14 │   98.52 │    1.25 │ 154261.30 │ 161748.88 │    0.95 │
├───────────────────────┼───────┼─────────────┼────────┼──────────┼──────────┼─────────┼────────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│              Submodel │     1 │    mooncake │   true │     0.93 │     0.86 │    1.08 │  23.26 │   26.98 │    0.86 │     21.71 │     23.22 │    0.93 │
└───────────────────────┴───────┴─────────────┴────────┴──────────┴──────────┴─────────┴────────┴─────────┴─────────┴───────────┴───────────┴─────────┘

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.43%. Comparing base (61eda05) to head (1d499c4).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1341      +/-   ##
==========================================
+ Coverage   78.13%   78.43%   +0.29%     
==========================================
  Files          50       50              
  Lines        3590     3593       +3     
==========================================
+ Hits         2805     2818      +13     
+ Misses        785      775      -10     

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[github-actions]

DynamicPPL.jl documentation for PR #1341 is available at:
https://TuringLang.github.io/DynamicPPL.jl/previews/PR1341/