go/printer: printing replaced const decls outputs broken code

[haruyama480]

Go version

go1.25.5 darwin/amd64

Output of go env in your module/workspace:

AR='ar'
CC='clang'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='clang++'
GCCGO='gccgo'
GO111MODULE=''
GOARCH='arm64'
GOARM64='v8.0'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/Users/xx/Library/Caches/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/Users/xx/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/vs/2qqlhqss7s34tmqvys6fl0rh0000gn/T/go-build4097758846=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD='/dev/null'
GOMODCACHE='/Users/xx/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/xx/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/xx/Library/Application Support/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.25.5'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

I just replaced names in a const decl.

For example, replaced following a const variable name 'B' to 'CCCCCCC'.

const (
	A = 1 // comment1

	B = 2 // comment2
)

https://go.dev/play/p/mGFEL11ib29

What did you see happen?

Printing replaced const decl generates broken code.

const (
	A	= 1	// comment1

	CCCCCCC// comment2
	= 2
)

What did you expect to see?

generates valid code.

const (
	A = 1 // comment1

	CCCCCCC = 2 // comment2
)

[gabyhelp]

Related Issues

• go/printer: printing const decl containing comment not attached to a const spec messes up indentation when using SourcePos mode #77305
• go/printer: output differs to "go fmt" for consts and structs #69843 (closed)
• x/tools/cmd/bundle: bundle discard some comments #20627 (closed)
• go/printer: unhappy formatting of const declaration #4808
• go/printer: comments were unexpected inserted in `import` decl #40546 (closed)
• go/printer: Comment-LineBreak-LineBreak-SelectorExpr-Comment AST modification issue #70978

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

[mateusz834]

Duplicate of #20744

[mateusz834]

Oh, sorry this is a bad code being produced, this should not happen even with free floating comments behaving like that.

[mateusz834]

I wonder whether this is a dup of #70978, could you try the fix in CL 638637 and report back whether it fixes the issue?

[haruyama480]

I've added a test and confirmed your CL fixes!

diff.patch

diff --git a/src/go/printer/printer_test.go b/src/go/printer/printer_test.go
index bde0089a24..e2c97b9e9d 100644
--- a/src/go/printer/printer_test.go
+++ b/src/go/printer/printer_test.go
@@ -273,6 +273,44 @@ func TestBadNodes(t *testing.T) {
 	}
 }
 
+func TestReplaceConstDecl(t *testing.T) {
+	src := `package p
+
+const (
+	A = 1 // comment1
+
+	B = 2 // comment2
+)
+`
+	dist := `package p
+
+const (
+	A	= 1	// comment1
+
+	CCCCCCC	= 2	// comment2
+)
+`
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "input.go", src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// replace B with CCCCCCC
+	genDecl := f.Decls[0].(*ast.GenDecl)
+	spec := genDecl.Specs[1].(*ast.ValueSpec)
+	spec.Names[0].Name = "CCCCCCC"
+
+	var buf bytes.Buffer
+	if err := Fprint(&buf, fset, f); err != nil {
+		t.Fatal(err)
+	}
+	got := buf.String()
+	if got != dist {
+		t.Errorf("got:\n%s\nwant:\n%s\n", got, dist)
+	}
+}
+
 // testComment verifies that f can be parsed again after printing it
 // with its first comment set to comment at any possible source offset.
 func testComment(t *testing.T, f *ast.File, srclen int, comment *ast.Comment) {

% git fetch https://go.googlesource.com/go refs/changes/37/638637/3 && git checkout FETCH_HEAD
% patch < diff.patch                                                                          
patching file 'src/go/printer/printer_test.go'
% (cd src; ../bin/go tool dist test -run go/printer)

##### Test execution environment.
# GOARCH: arm64
# CPU: Apple M1 Ultra
# GOOS: darwin
# OS Version: Darwin 25.2.0 Darwin Kernel Version 25.2.0: Tue Nov 18 21:09:40 PST 2025; root:xnu-12377.61.12~1/RELEASE_ARM64_T6000 arm64

##### Testing packages.
ok      go/printer      0.499s

ALL TESTS PASSED (some were excluded)

[mateusz834]

Thanks, i will update that CL soon to include that testcase.

[mateusz834]

CC @griesemer - since that CL is waiting on a review from you.

[haruyama480]

Oh, I see this CL has been active for a year.
Given the lack of workarounds, it might get prioritized higher.

(I hope backports, though)