cmd/compile: "cannot use _ as value" errors initializing blank fields in struct literal

[mdempsky]

Following up on #38690, this code is valid and accepted by go/types, but cmd/compile fails to accept it:

package p

type t struct{ _ u }
type u [10]int

func f(x int) t   { return t{u{1 / x, 1 % x}} }
func g(p *int) t  { return t{u{*p}} }
func h(s []int) t { return t{u{s[0]}} }

These are expressions where candiscard(value) returns false.

Note that only one error message is printed, but that's because we hit a Fatalf after the error (use go tool compile -d panic to see it). You can also comment out some of the functions or shuffle their order to see that any of them alone triggers the failure.

Fails to compile in Go 1.14 too, so not a regression.

/cc @cuonglm

[mdempsky]

It looks like one possible fix is to have splitnode return nblank whenever var_ == nblank.

[mdempsky]

Slightly better test case that provides better coverage of splitnode:

package p

type t struct{ _ u }
type u [10]struct{ f int }

func f(x int) t   { return t{u{{1 / x}, {1 % x}}} }
func g(p *int) t  { return t{u{{*p}}} }
func h(s []int) t { return t{u{{s[0]}}} }

[gopherbot]

Change https://golang.org/cl/232617 mentions this issue: cmd/compile: another fix initializing blank fields in struct literal