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The wrapper rewrites a package only when it was built with the same Go release as the toolchain running the build, and compared releases as go1.27. But the simd package can change in a patch release without go/types seeing it: go1.27.0 declares ShiftAllRight(uint64) in its stubs and compiles it as taking a uint8, go1.27.1 takes a uint64. A wrapper built with go1.27.1 or later rewrote a go1.27.0 build into code that type-checked here and then failed to compile, so the build failed: cannot use uint64(31) (constant 31 of type uint64) as uint8 value in argument to _v1.ShiftAllRight The wrapper now compares the whole version word (go1.27.3, go1.28rc1, a development build's go1.28-6ae0b1f8a9, or a vendor's go1.27.3-acme), as toolexec.md already asks: build loopvec-toolexec with the same Go toolchain that runs the build. Experiments are left out of it: the compiler reports go1.27.3 X:simd, while a wrapper built with GOEXPERIMENT=simd reports go1.27.3-X:simd. A version it cannot find never matches.
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I tried the wrapper on goav1, which was still on go1.27.0, with a wrapper built by 1.27.1, and the build failed instead of falling back:
go1.27.0's simd stubs declare
ShiftAllRight(uint64), but the compiler implements it with auint8(1.27.1 fixed that). go/types only sees the stubs, so the rewritten package type-checks in the wrapper and then fails in the compiler. The release check comparedgo1.27togo1.27, so nothing stopped it.This compares the whole version word instead:
go1.27.3,go1.28rc1, tip'sgo1.28-<hash>, or a vendor build such asgo1.27.3-acme. Experiments are left out, and the two sides spell them differently: the compiler saysgo1.27.3 X:simd, while a wrapper built withGOEXPERIMENT=simdreportsgo1.27.3-X:simd. A version it can't find never matches. toolexec.md already says to build the wrapper with the toolchain that runs the build; now a mismatch leaves packages unchanged with the existing debug message.toolexec_toolchain_mismatch.txtgets a same-release, different-patch case, and a unit test covers the version strings (releases, prereleases, tip, vendor builds, with and without experiments). The scripts build the wrapper withoutGOEXPERIMENT, so I also checked by hand that a wrapper built withGOEXPERIMENT=simdrewrites under go1.27.3 and skips under go1.27.1.