go1.2 reports multiple-value f() in single-value context, and doesn't ICE.
go1.4 compiles it successfully (!)
go1.[56] ICEs with cgen_wb of type interface {}
go1.[789] to current ICE with not lowered: ITab PTR64 PTR64

I think go1.2 is correct. I think if we reported that error correctly, we'd never pass the bad code to the backend, and then we wouldn't ICE.

go1.4 generated code is "correct", in that it does convert the *int to an interface{} and appends it to a []interface{}.

Hmmm, looks like append of the result of a multi-return function does generally work.
Maybe go1.4 is correct?

Oh, yes, the code should compile correctly, like it did with go1.4.

go/types and gccgo accept the code. (Sorry, I should have mentioned that in the original report.)

Looking at cmd/compile's -W output, I think maybe this is an issue in walk instead. Currently we're rewriting append(f()) into something like:

t0, t1 := f()
append([]interface{}(t0), t1)

I think what we actually mean to do is rewrite it to:

t0, t1 := f()
append(t0, interface{}(t1))

Still happening with go version devel +9761a162f0 Thu Mar 29 16:37:26 2018 +0000 linux/amd64

This has been broken for a while, so punting to 1.12, but marking as a release-blocker for that release.

Looks like there is a missing CONVIFACE here.
Before order:

.   AS l(7) tc(1)
.   .   NAME-_ a(true) l(7) x(0) tc(1) SLICE-[]interface {}
.   .   APPEND l(7) tc(1) SLICE-[]interface {}
.   .   APPEND-list
.   .   .   CALLFUNC l(7) tc(1) STRUCT-([]interface {}, *int)
.   .   .   .   NAME-p.f a(true) l(3) x(0) class(PFUNC) tc(1) used FUNC-func() ([]interface {}, *int)

After order:

.   AS2FUNC l(7) tc(1)
.   AS2FUNC-list
.   .   NAME-p..autotmp_2 a(true) l(7) x(0) class(PAUTO) esc(N) tc(1) assigned used SLICE-[]interface {}

.   .   NAME-p..autotmp_3 a(true) l(7) x(0) class(PAUTO) esc(N) tc(1) assigned used PTR-*int
.   AS2FUNC-rlist
.   .   CALLFUNC l(7) tc(1) STRUCT-([]interface {}, *int)
.   .   .   NAME-p.f a(true) l(3) x(0) class(PFUNC) tc(1) used FUNC-func() ([]interface {}, *int)

.   AS2 l(7) tc(1)
.   AS2-list
.   .   NAME-p..autotmp_0 a(true) l(7) x(0) class(PAUTO) esc(N) tc(1) assigned used SLICE-[]interface {}

.   .   NAME-p..autotmp_1 a(true) l(7) x(0) class(PAUTO) esc(N) tc(1) assigned used PTR-*int
.   AS2-rlist
.   .   NAME-p..autotmp_2 a(true) l(7) x(0) class(PAUTO) esc(N) tc(1) assigned used SLICE-[]interface {}

.   .   NAME-p..autotmp_3 a(true) l(7) x(0) class(PAUTO) esc(N) tc(1) assigned used PTR-*int
.   AS l(7) tc(1)
.   .   NAME-_ a(true) l(7) x(0) tc(1) SLICE-[]interface {}
.   .   APPEND l(7) tc(1) SLICE-[]interface {}
.   .   APPEND-list
.   .   .   NAME-p..autotmp_0 a(true) l(7) x(0) class(PAUTO) esc(N) tc(1) assigned used SLICE-[]interface {}
.   .   .   NAME-p..autotmp_1 a(true) l(7) x(0) class(PAUTO) esc(N) tc(1) assigned used PTR-*int

Nowhere is there a CONVIFACE to convert the second append arugment from *int to interface{}.
Normally the typecheck pass, even before order, inserts CONVIFACE and the like ops to convert function arguments to the right types. We just don't do that here.
I think copyRet in order.go needs to use temporaries corresponding to the outer function's required argument types instead of the inner function's result types. Then the typecheck call it does should insert the conversions required.

Change https://golang.org/cl/142718 mentions this issue: cmd/compile: In append(f()), type convert appended items