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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"reflect: Select panics if array length greater than 1\u003c\u003c16","articleBody":"### What version of Go are you using (`go version`)?\r\n\r\n\u003cpre\u003e\r\n$ go version\r\n1.13.7\r\n\u003c/pre\u003e\r\n\r\n### Does this issue reproduce with the latest release?\r\n\r\n[Yes, here is a go playground example](https://play.golang.org/p/88C_ON8hcrZ)\r\n\r\n### What operating system and processor architecture are you using (`go env`)?\r\n\r\n\u003cdetails\u003e\u003csummary\u003e\u003ccode\u003ego env\u003c/code\u003e Output\u003c/summary\u003e\u003cbr\u003e\u003cpre\u003e\r\n$ go env\r\nGO111MODULE=\"\"\r\nGOARCH=\"amd64\"\r\nGOBIN=\"\"\r\nGOCACHE=\"/Users/milanpatel/Library/Caches/go-build\"\r\nGOENV=\"/Users/milanpatel/Library/Application Support/go/env\"\r\nGOEXE=\"\"\r\nGOFLAGS=\"\"\r\nGOHOSTARCH=\"amd64\"\r\nGOHOSTOS=\"darwin\"\r\nGONOPROXY=\"\"\r\nGONOSUMDB=\"\"\r\nGOOS=\"darwin\"\r\nGOPATH=\"/Users/milanpatel/go\"\r\nGOPRIVATE=\"\"\r\nGOPROXY=\"https://proxy.golang.org,direct\"\r\nGOROOT=\"/usr/local/Cellar/go/1.13.7/libexec\"\r\nGOSUMDB=\"sum.golang.org\"\r\nGOTMPDIR=\"\"\r\nGOTOOLDIR=\"/usr/local/Cellar/go/1.13.7/libexec/pkg/tool/darwin_amd64\"\r\nGCCGO=\"gccgo\"\r\nAR=\"ar\"\r\nCC=\"clang\"\r\nCXX=\"clang++\"\r\nCGO_ENABLED=\"1\"\r\nGOMOD=\"\"\r\nCGO_CFLAGS=\"-g -O2\"\r\nCGO_CPPFLAGS=\"\"\r\nCGO_CXXFLAGS=\"-g -O2\"\r\nCGO_FFLAGS=\"-g -O2\"\r\nCGO_LDFLAGS=\"-g -O2\"\r\nPKG_CONFIG=\"pkg-config\"\r\nGOGCCFLAGS=\"-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/gx/lcfzy_f56tlb_czrp7z2ptyc0000gn/T/go-build327901560=/tmp/go-build -gno-record-gcc-switches -fno-common\"\r\n\u003c/pre\u003e\u003c/details\u003e\r\n\r\n### What did you do?\r\n\r\nWe ran into an issue today where if the number of reflect cases in an array exceeds 1\u003c\u003c16, the call to `reflect.Select` panics. The function doc for the select implementation suggests that it's the length of the array that is used to recreate the pointer to the array, determined by a passed in variable `ncases`.\r\n\r\n```\r\n// selectgo implements the select statement.\r\n//\r\n// cas0 points to an array of type [ncases]scase, and order0 points to\r\n// an array of type [2*ncases]uint16. Both reside on the goroutine's\r\n// stack (regardless of any escaping in selectgo).\r\n//\r\n// selectgo returns the index of the chosen scase, which matches the\r\n// ordinal position of its respective select{recv,send,default} call.\r\n// Also, if the chosen scase was a receive operation, it reports whether\r\n// a value was received.\r\nfunc selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {\r\n```\r\nIn fact, that doesn't appear to be true:\r\n```\r\n\tcas1 := (*[1 \u003c\u003c 16]scase)(unsafe.Pointer(cas0))\r\n\t...\r\n\tscases := cas1[:ncases:ncases] // \u003c- panics here \r\n```","author":{"url":"https://github.com/thempatel","@type":"Person","name":"thempatel"},"datePublished":"2020-02-21T02:53:36.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":7},"url":"https://github.com/37350/go/issues/37350"}