[SteveBronder]

Summary

Strict Clang test builds reject unused declarations and copies of named rvalue references. Remove the unused declarations and forward evaluated unit-vector and empty softmax results with their value categories preserved. The OpenCL reduction cleanup removes discarded copies that consume temporary inputs before constructing the returned expression. Add ownership and temporary-input regressions.

Part 1 of the CMake migration split from #3463.

This is the bottom of the native six-PR stack and targets develop.

Tests

• Reproduced the Clang 10 baseline diagnostics; cleaned probes compile with -Wall -Werror.
• Independent Make unit-vector, softmax, and log-softmax tests pass.
• The combined stack passes 18 focused expression tests for those functions.
• OpenCL temporary-input runtime regressions require GPU CI.

Side Effects

Returned numerical formulas are unchanged. Returning evaluated objects preserves their ownership; discarded reduction copies no longer consume temporary inputs.

Release notes

Fix strict-warning test compilation and ownership of temporary OpenCL reduction inputs.

Checklist

• Copyright holder: Simons Foundation
• The complete basic-test and CI matrix is passing
• C++ idioms and Doxygen documentation have completed review
• Focused validation is passing as described above; remaining coverage is listed

Related PRs

• 1/7: [CMake 1/7] Fix test compilation with strict compiler warnings (this PR)
• 2/7: [CMake 2/7] Share OpenCL kernel-cache registration across translation units
• 3/7: [CMake 3/7] Isolate TBB construction tests from process-wide initialization
• 4/7: [CMake 4/7] Make probability test fixtures safe for grouped linking
• 5/7: [CMake 5/7] Make unit tests safe for grouped execution
• 6/7: [CMake 6/7] Initialize CVODES adjoint quadrature derivative buffers
• 7/7: [CMake 7/7] Add CMake builds and migrate the test runner and CI

Native stack: https://github.com/stan-dev/math/stack/3509

[stan-buildbot]

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
stat_comp_benchmarks/benchmarks/gp_regr/gp_regr.stan	0.25	0.24	1.02	1.99% faster
stat_comp_benchmarks/benchmarks/gp_regr/gen_gp_data.stan	0.06	0.06	1.0	0.19% faster
stat_comp_benchmarks/benchmarks/low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	1.0	-0.28% slower
stat_comp_benchmarks/benchmarks/arma/arma.stan	0.7	0.69	1.01	0.89% faster
stat_comp_benchmarks/benchmarks/arK/arK.stan	3.4	3.4	1.0	-0.11% slower
stat_comp_benchmarks/benchmarks/pkpd/one_comp_mm_elim_abs.stan	42.62	42.75	1.0	-0.32% slower
stat_comp_benchmarks/benchmarks/pkpd/sim_one_comp_mm_elim_abs.stan	0.58	0.59	0.99	-0.52% slower
stat_comp_benchmarks/benchmarks/gp_pois_regr/gp_pois_regr.stan	4.66	4.67	1.0	-0.23% slower
stat_comp_benchmarks/benchmarks/low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	21.74	21.74	1.0	0.01% faster
stat_comp_benchmarks/benchmarks/garch/garch.stan	1.0	1.0	1.0	0.05% faster
stat_comp_benchmarks/benchmarks/eight_schools/eight_schools.stan	0.11	0.11	1.0	-0.14% slower
stat_comp_benchmarks/benchmarks/irt_2pl/irt_2pl.stan	9.3	9.27	1.0	0.4% faster
stat_comp_benchmarks/benchmarks/low_dim_gauss_mix/low_dim_gauss_mix.stan	6.57	6.58	1.0	-0.17% slower
stat_comp_benchmarks/benchmarks/sir/sir.stan	223.36	223.87	1.0	-0.23% slower
performance.compilation	356.88	366.19	0.97	-2.61% slower
Mean result: 0.9993778080035196

Jenkins Console Log
Jenkins Build Stages
Commit hash: ae9f8bf236716062b00a3acbd8b688a2766a7317

Machine information

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:                           43 bits physical, 48 bits virtual
CPU(s):                                  256
On-line CPU(s) list:                     0-255
Thread(s) per core:                      2
Core(s) per socket:                      64
Socket(s):                               2
NUMA node(s):                            2
Vendor ID:                               AuthenticAMD
CPU family:                              23
Model:                                   49
Model name:                              AMD EPYC 7742 64-Core Processor
Stepping:                                0
Frequency boost:                         enabled
CPU MHz:                                 1496.811
CPU max MHz:                             3416.0681
CPU min MHz:                             1500.0000
BogoMIPS:                                4491.56
Virtualization:                          AMD-V
L1d cache:                               4 MiB
L1i cache:                               4 MiB
L2 cache:                                64 MiB
L3 cache:                                512 MiB
NUMA node0 CPU(s):                       0-63,128-191
NUMA node1 CPU(s):                       64-127,192-255
Vulnerability Gather data sampling:      Not affected
Vulnerability Indirect target selection: Not affected
Vulnerability Itlb multihit:             Not affected
Vulnerability L1tf:                      Not affected
Vulnerability Mds:                       Not affected
Vulnerability Meltdown:                  Not affected
Vulnerability Mmio stale data:           Not affected
Vulnerability Old microcode:             Not affected
Vulnerability Reg file data sampling:    Not affected
Vulnerability Retbleed:                  Mitigation; untrained return thunk; SMT enabled with STIBP protection
Vulnerability Spec rstack overflow:      Mitigation; Safe RET
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; Retpolines; IBPB conditional; STIBP always-on; RSB filling; PBRSB-eIBRS Not affected; BHI Not affected
Vulnerability Srbds:                     Not affected
Vulnerability Tsa:                       Not affected
Vulnerability Tsx async abort:           Not affected
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 mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm constant_tsc rep_good nopl xtopology nonstop_tsc cpuid extd_apicid aperfmperf rapl pni pclmulqdq monitor ssse3 fma cx16 sse4_1 sse4_2 x2apic movbe popcnt aes xsave avx f16c rdrand lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ibs skinit wdt tce topoext perfctr_core perfctr_nb bpext perfctr_llc mwaitx cpb cat_l3 cdp_l3 hw_pstate ssbd mba ibrs ibpb stibp vmmcall fsgsbase bmi1 avx2 smep bmi2 cqm rdt_a rdseed adx smap clflushopt clwb sha_ni xsaveopt xsavec xgetbv1 xsaves cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local clzero irperf xsaveerptr rdpru wbnoinvd amd_ppin arat npt lbrv svm_lock nrip_save tsc_scale vmcb_clean flushbyasid decodeassists pausefilter pfthreshold avic v_vmsave_vmload vgif v_spec_ctrl umip rdpid overflow_recov succor smca sev sev_es

G++:

g++ (Ubuntu 9.4.0-1ubuntu1~20.04) 9.4.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Clang:

clang version 10.0.0-4ubuntu1 
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin