cmd/compile: initialize posBaseMap correctly

The posBaseMap is used to identify a file's syntax tree node
given a source position. The position is mapped to the file
base which is then used to look up the file node in posBaseMap.

When posBaseMap is initialized, the file position base
is not the file base if there's a line directive before
the package clause. This can happen in cgo-generated files,
for instance due to an import "C" declaration.

If the wrong file position base is used during initialization,
looking up a file given a position will not find the file.

If a version error occurs and the corresponding file is
not found, the old code panicked with a null pointer exception.

Make sure to consistently initialize the posBaseMap by factoring
out the code computing the file base from a given position.

While at it, check for a nil file pointer. This should not happen
anymore, but don't crash if it happens (at the cost of a slightly
less informative error message).

Fixes #67141.

Change-Id: I4a6af88699c32ad01fffce124b06bb7f9e06f43d
Reviewed-on: https://go-review.googlesource.com/c/go/+/586238
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>

src/cmd/compile/internal/noder/irgen.go

@@ -34,7 +34,13 @@ func checkFiles(m posMap, noders []*noder) (*types2.Package, *types2.Info) {
 	posBaseMap := make(map[*syntax.PosBase]*syntax.File)
 	for i, p := range noders {
 		files[i] = p.file
-		posBaseMap[p.file.Pos().Base()] = p.file
+		// The file.Pos() is the position of the package clause.
+		// If there's a //line directive before that, file.Pos().Base()
+		// refers to that directive, not the file itself.
+		// Make sure to consistently map back to file base, here and
+		// when we look for a file in the conf.Error handler below,
+		// otherwise the file may not be found (was go.dev/issue/67141).
+		posBaseMap[fileBase(p.file.Pos())] = p.file
 	}
 
 	// typechecking
@@ -69,13 +75,12 @@ func checkFiles(m posMap, noders []*noder) (*types2.Package, *types2.Info) {
 		terr := err.(types2.Error)
 		msg := terr.Msg
 		if versionErrorRx.MatchString(msg) {
-			posBase := terr.Pos.Base()
-			for !posBase.IsFileBase() { // line directive base
-				posBase = posBase.Pos().Base()
-			}
+			posBase := fileBase(terr.Pos)
 			fileVersion := info.FileVersions[posBase]
 			file := posBaseMap[posBase]
-			if file.GoVersion == fileVersion {
+			if file == nil {
+				// This should never happen, but be careful and don't crash.
+			} else if file.GoVersion == fileVersion {
 				// If we have a version error caused by //go:build, report it.
 				msg = fmt.Sprintf("%s (file declares //go:build %s)", msg, fileVersion)
 			} else {
@@ -150,6 +155,15 @@ func checkFiles(m posMap, noders []*noder) (*types2.Package, *types2.Info) {
 	return pkg, info
 }
 
+// fileBase returns a file's position base given a position in the file.
+func fileBase(pos syntax.Pos) *syntax.PosBase {
+	base := pos.Base()
+	for !base.IsFileBase() { // line directive base
+		base = base.Pos().Base()
+	}
+	return base
+}
+
 // A cycleFinder detects anonymous interface cycles (go.dev/issue/56103).
 type cycleFinder struct {
 	cyclic map[*types2.Interface]bool

test/fixedbugs/issue67141.go

@@ -0,0 +1,15 @@
+// errorcheck -lang=go1.22
+
+//go:build go1.21
+
+// We need a line directive before the package clause,
+// but don't change file name or position so that the
+// error message appears at the right place.
+
+//line issue67141.go:10
+package p
+
+func _() {
+	for range 10 { // ERROR "cannot range over 10"
+	}
+}