func (check *Checker) binary(x *operand, e syntax.Expr, lhs, rhs syntax.Expr, op syntax.Operator) {
	var y operand

	check.expr(nil, x, lhs)
	check.expr(nil, &y, rhs)

	if x.mode == invalid {
		return
	}
	if y.mode == invalid {
		x.mode = invalid
		x.expr = y.expr
		return
	}

	if isShift(op) {
		check.shift(x, &y, e, op)
		return
	}

func (check *Checker) binary(x *operand, e ast.Expr, lhs, rhs ast.Expr, op token.Token, opPos token.Pos) {
	var y operand

	check.expr(nil, x, lhs)
	check.expr(nil, &y, rhs)

	if x.mode == invalid {
		return
	}
	if y.mode == invalid {
		x.mode = invalid
		x.expr = y.expr
		return
	}

	if isShift(op) {
		check.shift(x, &y, e, op)
		return
	}

	var inst *syntax.IndexExpr // function instantiation, if any
	if iexpr, _ := call.Fun.(*syntax.IndexExpr); iexpr != nil {
		if check.indexExpr(x, iexpr) {
			// Delay function instantiation to argument checking,
			// where we combine type and value arguments for type
			// inference.
			assert(x.mode == value)
			inst = iexpr
		}
		x.expr = iexpr
		check.record(x)
	} else {
		check.exprOrType(x, call.Fun, true)

func (p *parser) pexpr(x Expr, keep_parens bool) Expr {
	if trace {
		defer p.trace("pexpr")()
	}

	if x == nil {
		x = p.operand(keep_parens)
	}

loop:
	for {
		pos := p.pos()
		switch p.tok {
		case _Dot:
			p.next()
			switch p.tok {
			case _Name:
				// pexpr '.' sym
				t := new(SelectorExpr)
				t.pos = pos

	case *types.Struct:
		texpr := TypeExpr(f, pkg, typ) // typ because we want the name here.
		if texpr == nil {
			return nil
		}
		return &ast.CompositeLit{
			Type: texpr,
		}
	}
	return nil
}

// IsZeroValue checks whether the given expression is a 'zero value' (as determined by output of
// analysisinternal.ZeroValue)
func IsZeroValue(expr ast.Expr) bool {
	switch e := expr.(type) {

		// var decl w/o init expr
		if s.Type == nil {
			check.error(s, code, "missing type or init expr")
		}
	case l < r:
		if l < len(s.Values) {
			// init exprs from s
			n := s.Values[l]
			check.errorf(n, code, "extra init expr %s", n)
			// TODO(gri) avoid declared and not used error here
		} else {
			// init exprs "inherited"
			check.errorf(s, code, "extra init expr at %s", check.fset.Position(init.Pos()))

		}
	}

	var buf bytes.Buffer

	var expr string
	if x.expr != nil {
		expr = ExprString(x.expr)
	} else {
		switch x.mode {
		case builtin:
			expr = predeclaredFuncs[x.id].name
		case typexpr:
			expr = TypeString(x.typ, qf)
		case constant_:
			expr = x.val.String()
		}
	}

	// <expr> (
	if expr != "" {
		buf.WriteString(expr)
		buf.WriteString(" (")
	}

        data class Failed(val errors: Collection<DocumentError>) : ExprConversion
    }


    private
    fun exprToValue(expr: Expr): ExprConversion = when (expr) {
        is Literal<*> -> ExprConversion.Converted(literalNode(expr))
        is FunctionCall -> run {
            val errors = mutableListOf<DocumentError>()

		}
	}

	var buf bytes.Buffer

	var expr string
	if x.expr != nil {
		expr = ExprString(x.expr)
	} else {
		switch x.mode {
		case builtin:
			expr = predeclaredFuncs[x.id].name
		case typexpr:
			expr = TypeString(x.typ, qf)
		case constant_:
			expr = x.val.String()
		}
	}

	// <expr> (
	if expr != "" {
		buf.WriteString(expr)
		buf.WriteString(" (")
	}

}


sealed interface FunctionArgument : LanguageTreeElement {
    sealed interface ValueArgument : FunctionArgument {
        val expr: Expr
    }

    data class Positional(override val expr: Expr, override val sourceData: SourceData) : ValueArgument
    data class Named(val name: String, override val expr: Expr, override val sourceData: SourceData) : ValueArgument