cmd/compile: don't emit conversion error in non-numeric increment/decrement

In increment and decrement statements, explicit check that the type
of operand is numeric earlier. This avoids a related but less clear
error about converting "1" to be emitted.

So, when compiling

	package main

	func main() {
		var x bool
		x++
	}

instead of emitting two errors

	prog.go:5: cannot convert 1 to type bool
	prog.go:5: invalid operation: x++ (non-numeric type bool)

just emits the second error.

Fixes #12525.

Change-Id: I6e81330703765bef0d6eb6c57098c1336af7c799
Reviewed-on: https://go-review.googlesource.com/20245
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/typecheck.go

@@ -521,6 +521,11 @@ OpSwitch:
 				n.Type = nil
 				return
 			}
+			if n.Implicit && !okforarith[l.Type.Etype] {
+				Yyerror("invalid operation: %v (non-numeric type %v)", n, l.Type)
+				n.Type = nil
+				return
+			}
 			// TODO(marvin): Fix Node.EType type union.
 			op = Op(n.Etype)
 		} else {
@@ -632,12 +637,6 @@ OpSwitch:
 
 		if t.Etype != TIDEAL && !Eqtype(l.Type, r.Type) {
 			defaultlit2(&l, &r, true)
-			if n.Op == OASOP && n.Implicit {
-				Yyerror("invalid operation: %v (non-numeric type %v)", n, l.Type)
-				n.Type = nil
-				return
-			}
-
 			if Isinter(r.Type) == Isinter(l.Type) || aop == 0 {
 				Yyerror("invalid operation: %v (mismatched types %v and %v)", n, l.Type, r.Type)
 				n.Type = nil

test/fixedbugs/issue12525.go

@@ -0,0 +1,26 @@
+// errorcheck
+
+// Copyright 2016 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 12525: confusing error trying to increment boolean value
+
+package main
+
+func main() {
+	var i int
+	i++
+
+	var f float64
+	f++
+
+	var c complex128
+	c++
+
+	var b bool
+	b++ // ERROR "invalid operation: b\+\+ \(non-numeric type bool\)"
+
+	var s string
+	s-- // ERROR "invalid operation: s-- \(non-numeric type string\)"
+}