go/types: better error message for certain unary/binary expression errors

Port of https://go-review.googlesource.com/11363.

Fixes #11367.

Change-Id: Ie0a82bcfab782c514d1947e7a6b190e286afd159
Reviewed-on: https://go-review.googlesource.com/11367Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/expr.go

@@ -78,7 +78,8 @@ func (check *Checker) op(m opPredicates, x *operand, op token.Token) bool {
 	return true
 }
 
-func (check *Checker) unary(x *operand, op token.Token) {
+// The unary expression e may be nil. It's passed in for better error messages only.
+func (check *Checker) unary(x *operand, e *ast.UnaryExpr, op token.Token) {
 	switch op {
 	case token.AND:
 		// spec: "As an exception to the addressability
@@ -125,6 +126,9 @@ func (check *Checker) unary(x *operand, op token.Token) {
 		// Typed constants must be representable in
 		// their type after each constant operation.
 		if isTyped(typ) {
+			if e != nil {
+				x.expr = e // for better error message
+			}
 			check.representable(x, typ)
 		}
 		return
@@ -721,7 +725,8 @@ var binaryOpPredicates = opPredicates{
 	token.LOR:  isBoolean,
 }
 
-func (check *Checker) binary(x *operand, lhs, rhs ast.Expr, op token.Token) {
+// The binary expression e may be nil. It's passed in for better error messages only.
+func (check *Checker) binary(x *operand, e *ast.BinaryExpr, lhs, rhs ast.Expr, op token.Token) {
 	var y operand
 
 	check.expr(x, lhs)
@@ -787,6 +792,9 @@ func (check *Checker) binary(x *operand, lhs, rhs ast.Expr, op token.Token) {
 		// Typed constants must be representable in
 		// their type after each constant operation.
 		if isTyped(typ) {
+			if e != nil {
+				x.expr = e // for better error message
+			}
 			check.representable(x, typ)
 		}
 		return
@@ -1374,7 +1382,7 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 		if x.mode == invalid {
 			goto Error
 		}
-		check.unary(x, e.Op)
+		check.unary(x, e, e.Op)
 		if x.mode == invalid {
 			goto Error
 		}
@@ -1384,7 +1392,7 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 		}
 
 	case *ast.BinaryExpr:
-		check.binary(x, e.X, e.Y, e.Op)
+		check.binary(x, e, e.X, e.Y, e.Op)
 		if x.mode == invalid {
 			goto Error
 		}

src/go/types/stmt.go

@@ -277,7 +277,7 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 		}
 		var x operand
 		Y := &ast.BasicLit{ValuePos: s.X.Pos(), Kind: token.INT, Value: "1"} // use x's position
-		check.binary(&x, s.X, Y, op)
+		check.binary(&x, nil, s.X, Y, op)
 		if x.mode == invalid {
 			return
 		}
@@ -309,7 +309,7 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 				return
 			}
 			var x operand
-			check.binary(&x, s.Lhs[0], s.Rhs[0], op)
+			check.binary(&x, nil, s.Lhs[0], s.Rhs[0], op)
 			if x.mode == invalid {
 				return
 			}

src/go/types/testdata/expr0.src

@@ -25,6 +25,12 @@ var (
 	b12 = <-b0 /* ERROR "cannot receive" */
 	b13 = & & /* ERROR "cannot take address" */ b0
 
+	// byte
+	_ = byte(0)
+	_ = byte(- /* ERROR "cannot convert" */ 1)
+	_ = - /* ERROR "-byte\(1\) \(constant -1 of type byte\) overflows byte" */ byte(1) // test for issue 11367
+	_ = byte /* ERROR "overflows byte" */ (0) - byte(1)
+
 	// int
 	i0 = 1
 	i1 int = i0