x/tools/gopls: Incorrect warnings that yield maybe called again

[renthraysk]

Go version

go version go1.23.3 linux/amd64 & gopls version golang.org/x/tools/gopls v0.17.0-pre.3

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/ren/.cache/go-build'
GOENV='/home/ren/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/ren/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/ren/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.23.3'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/ren/.config/go/telemetry'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/home/ren/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build3925782217=/tmp/go-build -gno-record-gcc-switches'

What did you do?

package main

import "iter"

func D() iter.Seq[uint8] {
    return func(yield func(uint8) bool) {
        ok := yield(0x01)
        ok = ok && yield(0x01)
        ok = ok && yield(0x01)
        ok = ok && yield(0x01)
    }
}

What did you see happen?

Get 3 warnings that yield maybe called again.

What did you expect to see?

No warnings.

[gabyhelp]

Related Issues

• x/tools/gopls: range over func without variable shows 'simplify range expression' warning #65799 (closed)
• x/tools/gopls: False alarm with unused cancel() of derived context #69790 (closed)
• x/tools/gopls: No warning "loop variable xx captured by func literal". #60016 (closed)
• x/tools/gopls: #52425 (closed)
• iter: `iter.Seq` returns incorrect values for subsequent next calls #64646 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[adonovan]

Smaller repro:

package main

func f(yield func(int) bool) {
	ok := yield(1)      // yield may be called again (on L5) after returning false
	ok = ok && yield(1) // yield may be called again (on L6) after returning false
	ok = ok && yield(1)
}

The first call to yield is unconditional, and the second depends on the result of the first, so there's no problem there. But the third depends on the phi(false, t1) node representing the two alternatives of ok && yield(1):

// func f(yield func(int) bool):
// 0:                                                                entry P:0 S:2
// 	t0 = yield(1:int)                                                  bool
// 	if t0 goto 1 else 2
// 1:                                                            binop.rhs P:1 S:1
// 	t1 = yield(1:int)                                                  bool
// 	jump 2
// 2:                                                           binop.done P:2 S:2
// 	t2 = phi [0: false:bool, 1: t1] #&&                                bool
// 	if t2 goto 3 else 4
// 3:                                                            binop.rhs P:1 S:1
// 	t3 = yield(1:int)                                                  bool
// 	jump 4
// 4:                                                           binop.done P:2 S:0
// 	t4 = phi [2: false:bool, 3: t3] #&&                                bool
// 	return

The analyzer should work harder to recursively analyze phi nodes used as conditions, especially when the phi operands are constants.

[gopherbot]

Change https://go.dev/cl/632117 mentions this issue: gopls/internal/analysis/yield: peephole-optimize phi(false, x)

[gopherbot]

Change https://go.dev/cl/633196 mentions this issue: gopls/internal/analysis/yield: add comment about dataflow

[gopherbot]

Change https://go.dev/cl/633709 mentions this issue: gopls/internal/analysis/yield: add comment about dataflow