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reflect: Index(X).Type() returning slice element type instead of actual type #65306

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notyourusualaccountname2 opened this issue Jan 26, 2024 · 1 comment
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reflect: Index(X).Type() returning slice element type instead of actual type #65306

notyourusualaccountname2 opened this issue Jan 26, 2024 · 1 comment
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compiler/runtime Issues related to the Go compiler and/or runtime. FrozenDueToAge

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@notyourusualaccountname2
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Go version

go1.22-20240109-RC01 linux/amd64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/usr/local/google/home/gopher/.cache/go-build'
GOENV='/usr/local/google/home/gopher/.config/go/env'
GOEXE=''
GOEXPERIMENT='fieldtrack,boringcrypto'
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/usr/local/google/home/gopher/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/usr/local/google/home/raffaelemeyer/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/lib/google-golang'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/lib/google-golang/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.22-20240109-RC01 cl/597041403 +dcbe772469 X:fieldtrack,boringcrypto'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='0'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -fno-caret-diagnostics -Qunused-arguments -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1517257978=/tmp/go-build -gno-record-gcc-switches'

What did you do?

When accessing an element from a slice via reflection (ie. via the Index method of reflect.Value), the returned reflect.Value's type is the slice's Elem type, not the actual type of the value.

ie.

var x = []any{string("")}

reflect.ValueOf(x).Index(0).Type() // returns "interface {}" instead of "string"

This seems strange behavior, especially since

reflect.ValueOf(x[0]).Type() // returns "string"

See https://go.dev/play/p/YNfd_PClNWe

What did you see happen?

var x = []any{string("")}

reflect.ValueOf(x).Index(0).Type() // returns "interface {}" instead of "string"

What did you expect to see?

I'd expect the Index method to return the actual underlying value of the slice, so its Type method should point to the actual value type.
@gopherbot gopherbot added the compiler/runtime Issues related to the Go compiler and/or runtime. label Jan 26, 2024
@seankhliao
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This is working as intended. For the second example, passing interface types to ValueOf results in passing the underlying type as interfaces don't nest.

Unlike many projects, the Go project does not use GitHub Issues for general discussion or asking questions. GitHub Issues are used for tracking bugs and proposals only.

For questions please refer to https://github.com/golang/go/wiki/Questions

@seankhliao seankhliao closed this as not planned Won't fix, can't repro, duplicate, stale Jan 26, 2024
@golang golang locked and limited conversation to collaborators Jan 25, 2025
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@gopherbot @seankhliao @notyourusualaccountname2