go/parser: goto; is incorrectly valid

[mateusz834]

While poking around the go/types internals, i noticed that nothing really "type-checks" gotos without labels.

First pass:

go/src/go/types/stmt.go

Lines 578 to 608 in 500675a

	case *ast.BranchStmt:
	if s.Label != nil {
	check.hasLabel = true
	return // checked in 2nd pass (check.labels)
	}
	switch s.Tok {
	case token.BREAK:
	if ctxt&breakOk == 0 {
	check.error(s, MisplacedBreak, "break not in for, switch, or select statement")
	}
	case token.CONTINUE:
	if ctxt&continueOk == 0 {
	check.error(s, MisplacedContinue, "continue not in for statement")
	}
	case token.FALLTHROUGH:
	if ctxt&fallthroughOk == 0 {
	var msg string
	switch {
	case ctxt&finalSwitchCase != 0:
	msg = "cannot fallthrough final case in switch"
	case ctxt&inTypeSwitch != 0:
	msg = "cannot fallthrough in type switch"
	default:
	msg = "fallthrough statement out of place"
	}
	check.error(s, MisplacedFallthrough, msg)
	}
	default:
	check.errorf(s, InvalidSyntaxTree, "branch statement: %s", s.Tok)
	}

Second pass:

go/src/go/types/labels.go

Lines 173 to 176 in 500675a

	case *ast.BranchStmt:
	if s.Label == nil {
	return // checked in 1st pass (check.stmt)
	}

Also it also turns out that:

func test() {
    goto;
}

Is accepted by the go/parser. The spec disallows gotos without labels, so we should either return an appropriate error in go/types or go/parser. Currently go/types returns an InvalidSyntaxTree error.

go/src/go/types/stmt.go

Lines 605 to 607 in 500675a

	default:
	check.errorf(s, InvalidSyntaxTree, "branch statement: %s", s.Tok)
	}

CC @griesemer @adonovan

[gabyhelp]

Related Issues

• go/parser, go/types: accepts invalid non-if/block statements after "else" #13475 (closed)
• spec: goto to outer function #8043 (closed)
• gccgo: goto over type/const declarations is erroneously rejected #19089 (closed)
• cmd/compile: goto over type/const declarations is erroneously rejected #19088 (closed)

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

[mateusz834]

Actually, this looks like a bug in go/parser:

go/src/go/parser/parser.go

Lines 2051 to 2056 in 500675a

	pos := p.expect(tok)
	var label *ast.Ident
	if tok != token.FALLTHROUGH && p.tok == token.IDENT {
	label = p.parseIdent()
	}
	p.expectSemi()

Will send a CL.

[mateusz834]

cmd/compile/internal/syntax currently reports this as an error:

go/src/cmd/compile/internal/syntax/parser.go

Lines 2660 to 2667 in 500675a

	case _Goto:
	s := new(BranchStmt)
	s.pos = p.pos()
	s.Tok = _Goto
	p.next()
	s.Label = p.name()
	return s

[gopherbot]

Change https://go.dev/cl/638395 mentions this issue: go/parser: require label after goto