cmd/compile: panic line number of deferred functions is ambiguous

[dsnet]

Using go1.6

Consider the following:

package main

func Foo(b bool) {
    if b {
        panic("ha")
    }
}

func main() {         // Line  9
    defer Foo(false)  // Line 10
    defer Foo(true)   // Line 11
    defer Foo(false)  // Line 12
    defer Foo(false)  // Line 13
}                     // Line 14

Currently, this prints:

panic: ha

goroutine 1 [running]:
panic(0x94e60, 0x1030a040)
    /usr/local/go/src/runtime/panic.go:481 +0x700
main.Foo(0x10327f01, 0xb4a00)
    /tmp/sandbox118729948/main.go:5 +0x80
main.main()
    /tmp/sandbox118729948/main.go:14 +0x13b

However, the stack trace reports that the panic occurs at Line 14 of main, which is technically correct since it is at the end of the function and now executing deferred functions. However, it is ambiguous exactly which of the calls to Foo was part of the real stack trace.

Any thoughts on having the printed line number be the line number where the defer occurred? In this example, it would be Line 11.

[ianlancetaylor]

See also #14646 and https://tip.golang.org/test/fixedbugs/issue14646.go .

That is, we could change it, but the current behavior is intentional.

[rsc]

Working as intended. The line where the defer was executed is gone.