Allow as<void> #2432

Job	Run time
arm (clang, aarch64, -Wno-psabi)	34m 59s
externals (clang, x86, -msse2)	45m 31s
arm (clang, arm, -Wno-psabi)	36m 20s
macosx (clang, x86_osx, -mavx)	1h 48m 15s
externals (gcc, x86, -msse2)	54m 14s
no_pch (clang, x86, -msse2)	59m 25s
nosimd (clang, x86, -DEVE_NO_SIMD)	38m 59s
no_pch (gcc, x86, -msse2)	1h 2m 12s
x86-clang (clang, x86, -msse4.2)	16m 2s
nosimd (gcc, x86, -DEVE_NO_SIMD)	47m 41s
riscv (clang, riscv, -mrvv-vector-bits=128)	35m 23s
x86-clang (clang, x86, -mavx)	21m 43s
x86-clang (clang, x86, -mavx2)	20m 43s
other-arch (gcc, sve256, -Wno-psabi)	1h 10m 25s
other-arch (gcc, sve512, -Wno-psabi)	2h 26m 6s
other-arch (gcc, ppc64, -Wno-psabi)	47m 55s
avx512 (clang, x86, -march=skylake-avx512)	37m 31s
avx512 (gcc, x86, -march=skylake-avx512)	55m 41s
other-cases (clang_libcpp, x86, -msse2)	26m 1s
other-cases (clang, x86, -mavx2 -mbmi2 -DEVE_USE_BMI_ON_AVX2)	28m 36s
other-cases (gcc, x86, -mavx2 -mbmi2 -DEVE_USE_BMI_ON_AVX2)	1h 8m 50s
x86-gcc (gcc, x86, -msse4.2)	16m 23s
x86-gcc (gcc, x86, -mavx)	22m 3s
x86-gcc (gcc, x86, -mavx2)	21m 12s
	18h 42m 10s

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