go/parser: better error message if = is seen instead of ==

Fixes #4519.

R=rsc
CC=golang-dev
https://golang.org/cl/7733044

diff --git a/src/pkg/go/parser/parser.go b/src/pkg/go/parser/parser.go
index a021a5abe5b37fb3a5c1bde118a0e2b704c64509..46e0e0d87c8dd5498e6ada21ed9f73faa19c2e50 100644 (file)
--- a/src/pkg/go/parser/parser.go
+++ b/src/pkg/go/parser/parser.go
@@ -48,7 +48,8 @@ type parser struct {
        syncCnt int       // number of calls to syncXXX without progress
 
        // Non-syntactic parser control
-       exprLev int // < 0: in control clause, >= 0: in expression
+       exprLev int  // < 0: in control clause, >= 0: in expression
+       inRhs   bool // if set, the parser is parsing a rhs expression
 
        // Ordinary identifier scopes
        pkgScope   *ast.Scope        // pkgScope.Outer == nil
@@ -539,6 +540,8 @@ func (p *parser) parseExprList(lhs bool) (list []ast.Expr) {
 }
 
 func (p *parser) parseLhsList() []ast.Expr {
+       old := p.inRhs
+       p.inRhs = false
        list := p.parseExprList(true)
        switch p.tok {
        case token.DEFINE:
@@ -560,11 +563,16 @@ func (p *parser) parseLhsList() []ast.Expr {
                        p.resolve(x)
                }
        }
+       p.inRhs = old
        return list
 }
 
 func (p *parser) parseRhsList() []ast.Expr {
-       return p.parseExprList(false)
+       old := p.inRhs
+       p.inRhs = true
+       list := p.parseExprList(false)
+       p.inRhs = old
+       return list
 }
 
 // ----------------------------------------------------------------------------
@@ -1505,6 +1513,14 @@ func (p *parser) parseUnaryExpr(lhs bool) ast.Expr {
        return p.parsePrimaryExpr(lhs)
 }
 
+func (p *parser) tokPrec() (token.Token, int) {
+       tok := p.tok
+       if p.inRhs && tok == token.ASSIGN {
+               tok = token.EQL
+       }
+       return tok, tok.Precedence()
+}
+
 // If lhs is set and the result is an identifier, it is not resolved.
 func (p *parser) parseBinaryExpr(lhs bool, prec1 int) ast.Expr {
        if p.trace {
@@ -1512,10 +1528,13 @@ func (p *parser) parseBinaryExpr(lhs bool, prec1 int) ast.Expr {
        }
 
        x := p.parseUnaryExpr(lhs)
-       for prec := p.tok.Precedence(); prec >= prec1; prec-- {
-               for p.tok.Precedence() == prec {
-                       pos, op := p.pos, p.tok
-                       p.next()
+       for _, prec := p.tokPrec(); prec >= prec1; prec-- {
+               for {
+                       op, oprec := p.tokPrec()
+                       if oprec != prec {
+                               break
+                       }
+                       pos := p.expect(op)
                        if lhs {
                                p.resolve(x)
                                lhs = false
@@ -1541,11 +1560,19 @@ func (p *parser) parseExpr(lhs bool) ast.Expr {
 }
 
 func (p *parser) parseRhs() ast.Expr {
-       return p.checkExpr(p.parseExpr(false))
+       old := p.inRhs
+       p.inRhs = true
+       x := p.checkExpr(p.parseExpr(false))
+       p.inRhs = old
+       return x
 }
 
 func (p *parser) parseRhsOrType() ast.Expr {
-       return p.checkExprOrType(p.parseExpr(false))
+       old := p.inRhs
+       p.inRhs = true
+       x := p.checkExprOrType(p.parseExpr(false))
+       p.inRhs = old
+       return x
 }
 
 // ----------------------------------------------------------------------------

diff --git a/src/pkg/go/parser/short_test.go b/src/pkg/go/parser/short_test.go
index c62f7e050649303f19f77b35f44f0dedc66d721f..62277c0d26b45ca7959eae701abe134d0493c707 100644 (file)
--- a/src/pkg/go/parser/short_test.go
+++ b/src/pkg/go/parser/short_test.go
@@ -71,6 +71,9 @@ var invalids = []string{
        `package p; func f() { _ = (<-<- /* ERROR "expected 'chan'" */ chan int)(nil) };`,
        `package p; func f() { _ = (<-chan<-chan<-chan<-chan<-chan<- /* ERROR "expected channel type" */ int)(nil) };`,
        `package p; func f() { var t []int; t /* ERROR "expected identifier on left side of :=" */ [0] := 0 };`,
+       `package p; func f() { if x := g(); x = /* ERROR "expected '=='" */ 0 {}};`,
+       `package p; func f() { _ = x = /* ERROR "expected '=='" */ 0 {}};`,
+       `package p; func f() { _ = 1 == func()int { var x bool; x = x = /* ERROR "expected '=='" */ true; return x }() };`,
 }
 
 func TestInvalid(t *testing.T) {