go/ast: reflect communication operator changes accurately in ast

- go/ast: introduce SendStmt; adjust SelectStmt
- go/parser: accept new communication syntax, minor
  unrelated cleanups
- go/printer: adjustments for new ast, fewer binary
  expression precedences
- go/token: remove one binary precedence

Adjusted dependent code. gofmt -w src -misc. Ran all tests.

R=rsc, gri
CC=golang-dev
https://golang.org/cl/3989056

9 files changed, 130 insertions, 92 deletions

diff --git a/misc/cgo/stdio/chain.go b/misc/cgo/stdio/chain.go
index c2b105072e..c188b2dd92 100644
--- a/misc/cgo/stdio/chain.go
+++ b/misc/cgo/stdio/chain.go
@@ -23,7 +23,7 @@ func link(left chan<- int, right <-chan int) {
 	for {
 		v := <-right
 		stdio.Stdout.WriteString(strconv.Itoa(v) + "\n")
-		left <- 1+v
+		left <- 1 + v
 	}
 }
 
diff --git a/src/cmd/cgo/ast.go b/src/cmd/cgo/ast.go
index 9bb8a55fd4..2eae22aed9 100644
--- a/src/cmd/cgo/ast.go
+++ b/src/cmd/cgo/ast.go
@@ -305,6 +305,9 @@ func (f *File) walk(x interface{}, context string, visit func(*File, interface{}
 		f.walk(n.Stmt, "stmt", visit)
 	case *ast.ExprStmt:
 		f.walk(&n.X, "expr", visit)
+	case *ast.SendStmt:
+		f.walk(&n.Chan, "expr", visit)
+		f.walk(&n.Value, "expr", visit)
 	case *ast.IncDecStmt:
 		f.walk(&n.X, "expr", visit)
 	case *ast.AssignStmt:
@@ -343,8 +346,7 @@ func (f *File) walk(x interface{}, context string, visit func(*File, interface{}
 		f.walk(n.Assign, "stmt", visit)
 		f.walk(n.Body, "stmt", visit)
 	case *ast.CommClause:
-		f.walk(n.Lhs, "expr", visit)
-		f.walk(n.Rhs, "expr", visit)
+		f.walk(n.Comm, "stmt", visit)
 		f.walk(n.Body, "stmt", visit)
 	case *ast.SelectStmt:
 		f.walk(n.Body, "stmt", visit)
diff --git a/src/pkg/go/ast/ast.go b/src/pkg/go/ast/ast.go
index cf2ce36df8..2e8f0973fa 100644
--- a/src/pkg/go/ast/ast.go
+++ b/src/pkg/go/ast/ast.go
@@ -535,6 +535,13 @@ type (
 		X Expr // expression
 	}
 
+	// A SendStmt node represents a send statement.
+	SendStmt struct {
+		Chan  Expr
+		Arrow token.Pos // position of "<-"
+		Value Expr
+	}
+
 	// An IncDecStmt node represents an increment or decrement statement.
 	IncDecStmt struct {
 		X      Expr
@@ -629,11 +636,10 @@ type (
 
 	// A CommClause node represents a case of a select statement.
 	CommClause struct {
-		Case     token.Pos   // position of "case" or "default" keyword
-		Tok      token.Token // ASSIGN or DEFINE (valid only if Lhs != nil)
-		Lhs, Rhs Expr        // Rhs == nil means default case
-		Colon    token.Pos   // position of ":"
-		Body     []Stmt      // statement list; or nil
+		Case  token.Pos // position of "case" or "default" keyword
+		Comm  Stmt      // send or receive statement; nil means default case
+		Colon token.Pos // position of ":"
+		Body  []Stmt    // statement list; or nil
 	}
 
 	// An SelectStmt node represents a select statement.
@@ -670,6 +676,7 @@ func (s *DeclStmt) Pos() token.Pos       { return s.Decl.Pos() }
 func (s *EmptyStmt) Pos() token.Pos      { return s.Semicolon }
 func (s *LabeledStmt) Pos() token.Pos    { return s.Label.Pos() }
 func (s *ExprStmt) Pos() token.Pos       { return s.X.Pos() }
+func (s *SendStmt) Pos() token.Pos       { return s.Chan.Pos() }
 func (s *IncDecStmt) Pos() token.Pos     { return s.X.Pos() }
 func (s *AssignStmt) Pos() token.Pos     { return s.Lhs[0].Pos() }
 func (s *GoStmt) Pos() token.Pos         { return s.Go }
@@ -695,6 +702,7 @@ func (s *EmptyStmt) End() token.Pos {
 }
 func (s *LabeledStmt) End() token.Pos { return s.Stmt.End() }
 func (s *ExprStmt) End() token.Pos    { return s.X.End() }
+func (s *SendStmt) End() token.Pos    { return s.Value.End() }
 func (s *IncDecStmt) End() token.Pos {
 	return s.TokPos + 2 /* len("++") */
 }
@@ -753,6 +761,7 @@ func (s *DeclStmt) stmtNode()       {}
 func (s *EmptyStmt) stmtNode()      {}
 func (s *LabeledStmt) stmtNode()    {}
 func (s *ExprStmt) stmtNode()       {}
+func (s *SendStmt) stmtNode()       {}
 func (s *IncDecStmt) stmtNode()     {}
 func (s *AssignStmt) stmtNode()     {}
 func (s *GoStmt) stmtNode()         {}
diff --git a/src/pkg/go/ast/walk.go b/src/pkg/go/ast/walk.go
index 875a92f3f4..d90c789363 100644
--- a/src/pkg/go/ast/walk.go
+++ b/src/pkg/go/ast/walk.go
@@ -258,11 +258,8 @@ func Walk(v Visitor, node Node) {
 		Walk(v, n.Body)
 
 	case *CommClause:
-		if n.Lhs != nil {
-			Walk(v, n.Lhs)
-		}
-		if n.Rhs != nil {
-			Walk(v, n.Rhs)
+		if n.Comm != nil {
+			Walk(v, n.Comm)
 		}
 		walkStmtList(v, n.Body)
 
diff --git a/src/pkg/go/parser/parser.go b/src/pkg/go/parser/parser.go
index f1746e0405..2395b81587 100644
--- a/src/pkg/go/parser/parser.go
+++ b/src/pkg/go/parser/parser.go
@@ -1193,18 +1193,6 @@ func (p *parser) parseSimpleStmt(labelOk bool) ast.Stmt {
 	x := p.parseExprList()
 
 	switch p.tok {
-	case token.COLON:
-		// labeled statement
-		colon := p.pos
-		p.next()
-		if labelOk && len(x) == 1 {
-			if label, isIdent := x[0].(*ast.Ident); isIdent {
-				return &ast.LabeledStmt{label, colon, p.parseStmt()}
-			}
-		}
-		p.error(x[0].Pos(), "illegal label declaration")
-		return &ast.BadStmt{x[0].Pos(), colon + 1}
-
 	case
 		token.DEFINE, token.ASSIGN, token.ADD_ASSIGN,
 		token.SUB_ASSIGN, token.MUL_ASSIGN, token.QUO_ASSIGN,
@@ -1218,11 +1206,29 @@ func (p *parser) parseSimpleStmt(labelOk bool) ast.Stmt {
 	}
 
 	if len(x) > 1 {
-		p.error(x[0].Pos(), "only one expression allowed")
+		p.errorExpected(x[0].Pos(), "1 expression")
 		// continue with first expression
 	}
 
-	if p.tok == token.INC || p.tok == token.DEC {
+	switch p.tok {
+	case token.COLON:
+		// labeled statement
+		colon := p.pos
+		p.next()
+		if label, isIdent := x[0].(*ast.Ident); labelOk && isIdent {
+			return &ast.LabeledStmt{label, colon, p.parseStmt()}
+		}
+		p.error(x[0].Pos(), "illegal label declaration")
+		return &ast.BadStmt{x[0].Pos(), colon + 1}
+
+	case token.ARROW:
+		// send statement
+		arrow := p.pos
+		p.next() // consume "<-"
+		y := p.parseExpr()
+		return &ast.SendStmt{x[0], arrow, y}
+
+	case token.INC, token.DEC:
 		// increment or decrement
 		s := &ast.IncDecStmt{x[0], p.pos, p.tok}
 		p.next() // consume "++" or "--"
@@ -1486,28 +1492,52 @@ func (p *parser) parseCommClause() *ast.CommClause {
 
 	// CommCase
 	pos := p.pos
-	var tok token.Token
-	var lhs, rhs ast.Expr
+	var comm ast.Stmt
 	if p.tok == token.CASE {
 		p.next()
+		lhs := p.parseExprList()
 		if p.tok == token.ARROW {
-			// RecvExpr without assignment
-			rhs = p.parseExpr()
+			// SendStmt
+			if len(lhs) > 1 {
+				p.errorExpected(lhs[0].Pos(), "1 expression")
+				// continue with first expression
+			}
+			arrow := p.pos
+			p.next()
+			rhs := p.parseExpr()
+			comm = &ast.SendStmt{lhs[0], arrow, rhs}
 		} else {
-			// SendExpr or RecvExpr
-			rhs = p.parseExpr()
+			// RecvStmt
+			pos := p.pos
+			tok := p.tok
+			var rhs ast.Expr
 			if p.tok == token.ASSIGN || p.tok == token.DEFINE {
-				// RecvExpr with assignment
-				tok = p.tok
+				// RecvStmt with assignment
+				if len(lhs) > 2 {
+					p.errorExpected(lhs[0].Pos(), "1 or 2 expressions")
+					// continue with first two expressions
+					lhs = lhs[0:2]
+				}
 				p.next()
-				lhs = rhs
-				if p.tok == token.ARROW {
-					rhs = p.parseExpr()
-				} else {
-					p.expect(token.ARROW) // use expect() error handling
+				rhs = p.parseExpr()
+			} else {
+				// rhs must be single receive operation
+				if len(lhs) > 1 {
+					p.errorExpected(lhs[0].Pos(), "1 expression")
+					// continue with first expression
 				}
+				rhs = lhs[0]
+				lhs = nil // there is no lhs
+			}
+			if x, isUnary := rhs.(*ast.UnaryExpr); !isUnary || x.Op != token.ARROW {
+				p.errorExpected(rhs.Pos(), "send or receive operation")
+				rhs = &ast.BadExpr{rhs.Pos(), rhs.End()}
+			}
+			if lhs != nil {
+				comm = &ast.AssignStmt{lhs, pos, tok, []ast.Expr{rhs}}
+			} else {
+				comm = &ast.ExprStmt{rhs}
 			}
-			// else SendExpr
 		}
 	} else {
 		p.expect(token.DEFAULT)
@@ -1516,7 +1546,7 @@ func (p *parser) parseCommClause() *ast.CommClause {
 	colon := p.expect(token.COLON)
 	body := p.parseStmtList()
 
-	return &ast.CommClause{pos, tok, lhs, rhs, colon, body}
+	return &ast.CommClause{pos, comm, colon, body}
 }
 
 
@@ -1568,7 +1598,7 @@ func (p *parser) parseForStmt() ast.Stmt {
 		}
 		// check rhs
 		if len(as.Rhs) != 1 {
-			p.errorExpected(as.Rhs[0].Pos(), "1 expressions")
+			p.errorExpected(as.Rhs[0].Pos(), "1 expression")
 			return &ast.BadStmt{pos, body.End()}
 		}
 		if rhs, isUnary := as.Rhs[0].(*ast.UnaryExpr); isUnary && rhs.Op == token.RANGE {
diff --git a/src/pkg/go/parser/parser_test.go b/src/pkg/go/parser/parser_test.go
index 56bd80ef1f..5a7f05ca83 100644
--- a/src/pkg/go/parser/parser_test.go
+++ b/src/pkg/go/parser/parser_test.go
@@ -46,6 +46,7 @@ var validPrograms = []interface{}{
 	`package main; type T []int; func g(int) bool { return true }; func f() { if g(T{42}[0]) {} };`,
 	`package main; type T []int; func f() { for _ = range []int{T{42}[0]} {} };`,
 	`package main; var a = T{{1, 2}, {3, 4}}`,
+	`package main; func f() { select { case <- c: case c <- d: case c <- <- d: case <-c <- d: } };`,
 }
 
 
diff --git a/src/pkg/go/printer/nodes.go b/src/pkg/go/printer/nodes.go
index 8207996dcd..7933c2f182 100644
--- a/src/pkg/go/printer/nodes.go
+++ b/src/pkg/go/printer/nodes.go
@@ -506,12 +506,12 @@ const (
 )
 
 
-func walkBinary(e *ast.BinaryExpr) (has5, has6 bool, maxProblem int) {
+func walkBinary(e *ast.BinaryExpr) (has4, has5 bool, maxProblem int) {
 	switch e.Op.Precedence() {
+	case 4:
+		has4 = true
 	case 5:
 		has5 = true
-	case 6:
-		has6 = true
 	}
 
 	switch l := e.X.(type) {
@@ -521,9 +521,9 @@ func walkBinary(e *ast.BinaryExpr) (has5, has6 bool, maxProblem int) {
 			// pretend this is an *ast.ParenExpr and do nothing.
 			break
 		}
-		h5, h6, mp := walkBinary(l)
+		h4, h5, mp := walkBinary(l)
+		has4 = has4 || h4
 		has5 = has5 || h5
-		has6 = has6 || h6
 		if maxProblem < mp {
 			maxProblem = mp
 		}
@@ -536,25 +536,25 @@ func walkBinary(e *ast.BinaryExpr) (has5, has6 bool, maxProblem int) {
 			// pretend this is an *ast.ParenExpr and do nothing.
 			break
 		}
-		h5, h6, mp := walkBinary(r)
+		h4, h5, mp := walkBinary(r)
+		has4 = has4 || h4
 		has5 = has5 || h5
-		has6 = has6 || h6
 		if maxProblem < mp {
 			maxProblem = mp
 		}
 
 	case *ast.StarExpr:
 		if e.Op.String() == "/" {
-			maxProblem = 6
+			maxProblem = 5
 		}
 
 	case *ast.UnaryExpr:
 		switch e.Op.String() + r.Op.String() {
 		case "/*", "&&", "&^":
-			maxProblem = 6
+			maxProblem = 5
 		case "++", "--":
-			if maxProblem < 5 {
-				maxProblem = 5
+			if maxProblem < 4 {
+				maxProblem = 4
 			}
 		}
 	}
@@ -563,20 +563,20 @@ func walkBinary(e *ast.BinaryExpr) (has5, has6 bool, maxProblem int) {
 
 
 func cutoff(e *ast.BinaryExpr, depth int) int {
-	has5, has6, maxProblem := walkBinary(e)
+	has4, has5, maxProblem := walkBinary(e)
 	if maxProblem > 0 {
 		return maxProblem + 1
 	}
-	if has5 && has6 {
+	if has4 && has5 {
 		if depth == 1 {
-			return 6
+			return 5
 		}
-		return 5
+		return 4
 	}
 	if depth == 1 {
-		return 7
+		return 6
 	}
-	return 5
+	return 4
 }
 
 
@@ -603,15 +603,14 @@ func reduceDepth(depth int) int {
 // (Algorithm suggestion by Russ Cox.)
 //
 // The precedences are:
-//	6             *  /  %  <<  >>  &  &^
-//	5             +  -  |  ^
-//	4             ==  !=  <  <=  >  >=
-//	3             <-
+//	5             *  /  %  <<  >>  &  &^
+//	4             +  -  |  ^
+//	3             ==  !=  <  <=  >  >=
 //	2             &&
 //	1             ||
 //
-// The only decision is whether there will be spaces around levels 5 and 6.
-// There are never spaces at level 7 (unary), and always spaces at levels 4 and below.
+// The only decision is whether there will be spaces around levels 4 and 5.
+// There are never spaces at level 6 (unary), and always spaces at levels 3 and below.
 //
 // To choose the cutoff, look at the whole expression but excluding primary
 // expressions (function calls, parenthesized exprs), and apply these rules:
@@ -619,21 +618,21 @@ func reduceDepth(depth int) int {
 //	1) If there is a binary operator with a right side unary operand
 //	   that would clash without a space, the cutoff must be (in order):
 //
-//		/*	7
-//		&&	7
-//		&^	7
-//		++	6
-//		--	6
+//		/*	6
+//		&&	6
+//		&^	6
+//		++	5
+//		--	5
 //
 //         (Comparison operators always have spaces around them.)
 //
-//	2) If there is a mix of level 6 and level 5 operators, then the cutoff
-//	   is 6 (use spaces to distinguish precedence) in Normal mode
-//	   and 5 (never use spaces) in Compact mode.
+//	2) If there is a mix of level 5 and level 4 operators, then the cutoff
+//	   is 5 (use spaces to distinguish precedence) in Normal mode
+//	   and 4 (never use spaces) in Compact mode.
 //
-//	3) If there are no level 5 operators or no level 6 operators, then the
-//	   cutoff is 7 (always use spaces) in Normal mode
-//	   and 5 (never use spaces) in Compact mode.
+//	3) If there are no level 4 operators or no level 5 operators, then the
+//	   cutoff is 6 (always use spaces) in Normal mode
+//	   and 4 (never use spaces) in Compact mode.
 //
 // Sets multiLine to true if the binary expression spans multiple lines.
 func (p *printer) binaryExpr(x *ast.BinaryExpr, prec1, cutoff, depth int, multiLine *bool) {
@@ -1083,6 +1082,12 @@ func (p *printer) stmt(stmt ast.Stmt, nextIsRBrace bool, multiLine *bool) {
 		const depth = 1
 		p.expr0(s.X, depth, multiLine)
 
+	case *ast.SendStmt:
+		const depth = 1
+		p.expr0(s.Chan, depth, multiLine)
+		p.print(blank, s.Arrow, token.ARROW, blank)
+		p.expr0(s.Value, depth, multiLine)
+
 	case *ast.IncDecStmt:
 		const depth = 1
 		p.expr0(s.X, depth+1, multiLine)
@@ -1179,13 +1184,9 @@ func (p *printer) stmt(stmt ast.Stmt, nextIsRBrace bool, multiLine *bool) {
 		*multiLine = true
 
 	case *ast.CommClause:
-		if s.Rhs != nil {
+		if s.Comm != nil {
 			p.print(token.CASE, blank)
-			if s.Lhs != nil {
-				p.expr(s.Lhs, multiLine)
-				p.print(blank, s.Tok, blank)
-			}
-			p.expr(s.Rhs, multiLine)
+			p.stmt(s.Comm, false, ignoreMultiLine)
 		} else {
 			p.print(token.DEFAULT)
 		}
diff --git a/src/pkg/go/token/token.go b/src/pkg/go/token/token.go
index 1bd81c1b14..2a2d3ecc4f 100644
--- a/src/pkg/go/token/token.go
+++ b/src/pkg/go/token/token.go
@@ -252,8 +252,8 @@ func (tok Token) String() string {
 //
 const (
 	LowestPrec  = 0 // non-operators
-	UnaryPrec   = 7
-	HighestPrec = 8
+	UnaryPrec   = 6
+	HighestPrec = 7
 )
 
 
@@ -267,14 +267,12 @@ func (op Token) Precedence() int {
 		return 1
 	case LAND:
 		return 2
-	case ARROW:
-		return 3
 	case EQL, NEQ, LSS, LEQ, GTR, GEQ:
-		return 4
+		return 3
 	case ADD, SUB, OR, XOR:
-		return 5
+		return 4
 	case MUL, QUO, REM, SHL, SHR, AND, AND_NOT:
-		return 6
+		return 5
 	}
 	return LowestPrec
 }
diff --git a/src/pkg/netchan/netchan_test.go b/src/pkg/netchan/netchan_test.go
index 6d7d63f988..4076aefebf 100644
--- a/src/pkg/netchan/netchan_test.go
+++ b/src/pkg/netchan/netchan_test.go
@@ -23,7 +23,7 @@ func exportSend(exp *Exporter, n int, t *testing.T, done chan bool) {
 	}
 	go func() {
 		for i := 0; i < n; i++ {
-			ch <- base+i
+			ch <- base + i
 		}
 		close(ch)
 		if done != nil {
@@ -61,7 +61,7 @@ func importSend(imp *Importer, n int, t *testing.T, done chan bool) {
 	}
 	go func() {
 		for i := 0; i < n; i++ {
-			ch <- base+i
+			ch <- base + i
 		}
 		close(ch)
 		if done != nil {