fix(reduce_sum_static): wrap Args with ref_type_t to materialize Eigen expressions

[tmchow]

Summary

Closes #3304. reduce_sum wraps its shared arguments with ref_type_t<Args&&>... before handing them to TBB so that Eigen expression-template temporaries (e.g. M.row(0), matrix.col(i)) materialize instead of being held as references to a destroyed temporary. reduce_sum_static's STAN_THREADS branch was not updated in the same pass and still forwarded plain Args....

A caller that passes a temporary Eigen expression as a shared argument to reduce_sum_static can therefore end up with a dangling reference once the temporary falls out of scope at the call site.

Fix

stan/math/prim/functor/reduce_sum_static.hpp:

return internal::reduce_sum_impl<ReduceFunction, void, return_type, Vec,
-                                 Args...>()(std::forward<Vec>(vmapped), false,
-                                            grainsize, msgs,
-                                            std::forward<Args>(args)...);
+                                 ref_type_t<Args&&>...>()(
+    std::forward<Vec>(vmapped), false, grainsize, msgs,
+    std::forward<Args>(args)...);

This exactly mirrors the wrapping already in place in reduce_sum.hpp (see line 208).

Notes

• ref_type_t is already in scope via stan/math/prim/meta.hpp (already #included).
• No API change; the behavior only changes for callers that were relying on temporary Eigen expressions staying alive longer than the standard allows, which is exactly the UB being fixed.
• The non-threaded branch is unchanged: it either forwards directly to ReduceFunction() (no reduce_sum_impl instantiation) or returns zero.

Closes #3304

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_regr/gp_regr.stan	0.09	0.09	0.98	-2.08% slower
gp_regr/gen_gp_data.stan	0.02	0.02	0.99	-0.7% slower
arK/arK.stan	1.72	1.74	0.99	-1.11% slower
eight_schools/eight_schools.stan	0.05	0.05	0.95	-5.54% slower
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	8.32	8.39	0.99	-0.87% slower
pkpd/one_comp_mm_elim_abs.stan	18.44	18.42	1.0	0.13% faster
pkpd/sim_one_comp_mm_elim_abs.stan	0.24	0.24	1.0	0.11% faster
sir/sir.stan	68.32	67.4	1.01	1.34% faster
gp_pois_regr/gp_pois_regr.stan	2.68	2.69	1.0	-0.39% slower
low_dim_gauss_mix/low_dim_gauss_mix.stan	2.54	2.57	0.99	-1.25% slower
irt_2pl/irt_2pl.stan	3.92	3.91	1.0	0.37% faster
arma/arma.stan	0.27	0.27	1.0	0.39% faster
garch/garch.stan	0.4	0.4	1.0	0.13% faster
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.01	0.01	0.97	-3.36% slower
performance.compilation	214.91	217.48	0.99	-1.2% slower
Mean result: 0.9910021999617704

---

Jenkins Console Log
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Commit hash: 240e2dc95a112f175bb961a03b53357d10955996

---

Machine information

No LSB modules are available. Distributor ID: Ubuntu Description: Ubuntu 20.04.3 LTS Release: 20.04 Codename: focal

CPU:
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Little Endian
Address sizes: 46 bits physical, 48 bits virtual
CPU(s): 80
On-line CPU(s) list: 0-79
Thread(s) per core: 2
Core(s) per socket: 20
Socket(s): 2
NUMA node(s): 2
Vendor ID: GenuineIntel
CPU family: 6
Model: 85
Model name: Intel(R) Xeon(R) Gold 6148 CPU @ 2.40GHz
Stepping: 4
CPU MHz: 2400.000
CPU max MHz: 3700.0000
CPU min MHz: 1000.0000
BogoMIPS: 4800.00
Virtualization: VT-x
L1d cache: 1.3 MiB
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L2 cache: 40 MiB
L3 cache: 55 MiB
NUMA node0 CPU(s): 0,2,4,6,8,10,12,14,16,18,20,22,24,26,28,30,32,34,36,38,40,42,44,46,48,50,52,54,56,58,60,62,64,66,68,70,72,74,76,78
NUMA node1 CPU(s): 1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31,33,35,37,39,41,43,45,47,49,51,53,55,57,59,61,63,65,67,69,71,73,75,77,79
Vulnerability Gather data sampling: Mitigation; Microcode
Vulnerability Itlb multihit: KVM: Mitigation: VMX disabled
Vulnerability L1tf: Mitigation; PTE Inversion; VMX conditional cache flushes, SMT vulnerable
Vulnerability Mds: Mitigation; Clear CPU buffers; SMT vulnerable
Vulnerability Meltdown: Mitigation; PTI
Vulnerability Mmio stale data: Mitigation; Clear CPU buffers; SMT vulnerable
Vulnerability Reg file data sampling: Not affected
Vulnerability Retbleed: Mitigation; IBRS
Vulnerability Spec rstack overflow: Not affected
Vulnerability Spec store bypass: Mitigation; Speculative Store Bypass disabled via prctl
Vulnerability Spectre v1: Mitigation; usercopy/swapgs barriers and __user pointer sanitization
Vulnerability Spectre v2: Mitigation; IBRS; IBPB conditional; STIBP conditional; RSB filling; PBRSB-eIBRS Not affected; BHI Not affected
Vulnerability Srbds: Not affected
Vulnerability Tsx async abort: Mitigation; Clear CPU buffers; SMT vulnerable
Vulnerability Vmscape: Mitigation; IBPB before exit to userspace
Flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb cat_l3 cdp_l3 invpcid_single pti intel_ppin ssbd mba ibrs ibpb stibp tpr_shadow vnmi flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 erms invpcid rtm cqm mpx rdt_a avx512f avx512dq rdseed adx smap clflushopt clwb intel_pt avx512cd avx512bw avx512vl xsaveopt xsavec xgetbv1 xsaves cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local dtherm ida arat pln pts hwp hwp_act_window hwp_epp hwp_pkg_req pku ospke md_clear flush_l1d arch_capabilities

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