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runtime: strange panic message #12046

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bronze1man opened this issue Aug 6, 2015 · 5 comments
Closed

runtime: strange panic message #12046

bronze1man opened this issue Aug 6, 2015 · 5 comments
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@bronze1man
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I have following program.

package main

type st string
func main() {
    s:=st("1")
    panic("a="+s)
}

http://play.golang.org/p/0DCktht8U2

I think it should not compile, but it compile and got the panic message:

panic: (main.st) (0x63660,0x1020a040)

goroutine 1 [running]:
main.main()
    /tmp/sandbox341206984/main.go:6 +0x100

goroutine 2 [runnable]:
runtime.forcegchelper()
    /usr/src/go/src/runtime/proc.go:90
runtime.goexit()
    /usr/src/go/src/runtime/asm_amd64p32.s:1086 +0x1

goroutine 3 [runnable]:
runtime.bgsweep()
    /usr/src/go/src/runtime/mgc0.go:82
runtime.goexit()
    /usr/src/go/src/runtime/asm_amd64p32.s:1086 +0x1
@cznic
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cznic commented Aug 6, 2015

I think it should not compile...

Please discuss why it should not compile, the program seems legal to me.

... but it compile and got the panic message:

Somewhere in the runtime the stack trace printing routine attempts to output the value panic was called with. It handles predeclared types, like eg. string, and probably some others. The argument to panic is converted to an interface{}, I think the two numbers printed are the two items in the interface{} runtime struct {*typeInfo, *value}.

FTR: converting the argument to string makes the trace look more usual: http://play.golang.org/p/einqqW9zTp

@ianlancetaylor
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This is working as expected. The panic messages is telling you the type followed by the value. panic handles the predeclared type "string" specially, and it handles types that implement the fmt.Stringer method specially.

@bronze1man
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"why it should not compile?"
Type st should not add with string type. It looks like different types.

@ianlancetaylor
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The literal "a=" is an untyped constant. This is not string + st, it's untyped + st.

http://golang.org/ref/spec#Constants

@bronze1man
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Thanks. I just forgot that.

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@cznic @bronze1man @ianlancetaylor @gopherbot