go/types: don't emit conversion error in non-numeric increment/decrement

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

So, when checking

	package main

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

instead of emitting the error

	prog.go:5:2: cannot convert 1 (untyped int constant) to bool

emits

	prog.go:5:2: invalid operation: x++ (non-numeric type bool).

Updates #12525.

Change-Id: I00aa6bd0bb23267a2fe10ea3f5a0b20bbf3552bc
Reviewed-on: https://go-review.googlesource.com/20244
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/go/types/stmt.go

@@ -346,7 +346,17 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 			check.invalidAST(s.TokPos, "unknown inc/dec operation %s", s.Tok)
 			return
 		}
+
 		var x operand
+		check.expr(&x, s.X)
+		if x.mode == invalid {
+			return
+		}
+		if !isNumeric(x.typ) {
+			check.invalidOp(s.X.Pos(), "%s%s (non-numeric type %s)", s.X, s.Tok, x.typ)
+			return
+		}
+
 		Y := &ast.BasicLit{ValuePos: s.X.Pos(), Kind: token.INT, Value: "1"} // use x's position
 		check.binary(&x, nil, s.X, Y, op)
 		if x.mode == invalid {

src/go/types/testdata/stmt0.src

@@ -164,7 +164,7 @@ func incdecs() {
 	const c = 3.14
 	c /* ERROR "cannot assign" */ ++
 	s := "foo"
-	s /* ERROR "cannot convert" */ --
+	s /* ERROR "invalid operation" */ --
 	3.14 /* ERROR "cannot assign" */ ++
 	var (
 		x int