cmd/gc: print x++ (not x += 1) in errors about x++

Fixes #8311.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews, r
https://golang.org/cl/146270043

src/cmd/gc/fmt.c

@@ -810,6 +810,13 @@ stmtfmt(Fmt *f, Node *n)
 		break;
 
 	case OASOP:
+		if(n->implicit) {
+			if(n->etype == OADD)
+				fmtprint(f, "%N++", n->left);
+			else
+				fmtprint(f, "%N--", n->left);
+			break;
+		}
 		fmtprint(f, "%N %#O= %N", n->left, n->etype, n->right);
 		break;
 

src/cmd/gc/go.y

@@ -460,11 +460,13 @@ simple_stmt:
 |	expr LINC
 	{
 		$$ = nod(OASOP, $1, nodintconst(1));
+		$$->implicit = 1;
 		$$->etype = OADD;
 	}
 |	expr LDEC
 	{
 		$$ = nod(OASOP, $1, nodintconst(1));
+		$$->implicit = 1;
 		$$->etype = OSUB;
 	}
 

src/cmd/gc/typecheck.c

@@ -600,6 +600,10 @@ reswitch:
 		}
 		if(t->etype != TIDEAL && !eqtype(l->type, r->type)) {
 			defaultlit2(&l, &r, 1);
+			if(n->op == OASOP && n->implicit) {
+				yyerror("invalid operation: %N (non-numeric type %T)", n, l->type);
+				goto error;
+			}
 			yyerror("invalid operation: %N (mismatched types %T and %T)", n, l->type, r->type);
 			goto error;
 		}

test/fixedbugs/issue8311.go

+// errorcheck
+
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// issue 8311.
+// error for x++ should say x++ not x += 1
+
+package p
+
+func f() {
+	var x []byte
+	x++ // ERROR "invalid operation: x[+][+]"
+
+}