On Sat, Dec 28, 2024 at 5:38 PM Kaiming Yang <[email protected]> wrote:
>
> I'm trying to understand execution order of assignment statement, as spec
> mentioned:
>
> > The assignment proceeds in two phases. First, the operands of index
> > expressions and pointer indirections (including implicit pointer
> > indirections in selectors) on the left and the expressions on the right are
> > all evaluated in the usual order. Second, the assignments are carried out
> > in left-to-right order.
>
> I crafted a example about it: https://go.dev/play/p/6RulNQwIk4n, the core of
> that example is this line:
>
> phase_2, foo.bar.value = "??????", compute_value()
>
> where `foo.bar.value` causes panic due to nil pointer.
>
> The execution result shows `compute_value()` is invoked, but `phase_2`
> variable is not updated. Seems like the panic happened after phase 1 is
> complete but before phase 2 started.
>
> I assume the panic should happen in one of two phases, but not in-between. If
> panic happened in phase 1, `compute_value()` should not be invoked due to it
> happen later in same phase; If panic happened in phase 2, `phase_2` variable
> should have been changed already.
>
> Is the behavior working-as-indented? if so, what am I missing to understand
> it?
Thanks. This looks like a bug to me. I think the panic should occur
before compute_value is called. When I run your program with gccgo I
get
original
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x4037fb]
goroutine 1 [running]:
main.main
foo.go:22
Want to open a bug report at https://go.dev/issue? Thanks.
Ian
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