Ensure "exit" without status code doesn't set exit code (#193)

Also update stringer version, as I was getting this error with the
current version for some reason:
golang/go#52710

Author: benhoyt

internal/compiler/compiler.go

@@ -462,10 +462,10 @@ func (c *compiler) stmt(stmt ast.Stmt) {
 	case *ast.ExitStmt:
 		if s.Status != nil {
 			c.expr(s.Status)
+			c.add(ExitStatus)
 		} else {
-			c.expr(&ast.NumExpr{0})
+			c.add(Exit)
 		}
-		c.add(Exit)
 
 	case *ast.DeleteStmt:
 		scope, index := c.arrayInfo(s.Array)

internal/compiler/opcode_string.go

@@ -1,6 +1,6 @@
 package compiler
 
-//go:generate go run golang.org/x/tools/cmd/stringer@v0.1.8 -type=Opcode,AugOp,BuiltinOp
+//go:generate go run golang.org/x/tools/cmd/stringer@v0.10.0 -type=Opcode,AugOp,BuiltinOp
 
 // Opcode represents a single virtual machine instruction (or argument). The
 // comments beside each opcode show any arguments that instruction consumes.
@@ -109,6 +109,7 @@ const (
 	Next
 	Nextfile
 	Exit
+	ExitStatus
 	ForIn // varScope varIndex arrayScope arrayIndex offset
 	BreakForIn
 

internal/compiler/opcodes.go

@@ -1607,6 +1607,9 @@ func TestExit(t *testing.T) {
 		{`{ print "x"; exit 1+2; print "y" }  END { print "z" }`, "x\nz\n", 3},
 		{`END { print "x"; exit; print "y" }`, "x\n", 0},
 		{`END { print "x"; exit 1+2; print "y" }`, "x\n", 3},
+		{`BEGIN { print "x"; exit 42 }  END { print "z"; exit }`, "x\nz\n", 42},
+		{`BEGIN { print "x"; exit }  END { print "z"; exit 43 }`, "x\nz\n", 43},
+		{`BEGIN { print "x"; exit 44 }  { print "a"; exit }`, "x\n", 44},
 	}
 	for _, test := range tests {
 		t.Run(test.src, func(t *testing.T) {

interp/interp_test.go

@@ -591,10 +591,13 @@ func (p *interp) execute(code []compiler.Opcode) error {
 			return errNextfile
 
 		case compiler.Exit:
-			p.exitStatus = int(p.pop().num())
 			// Return special errExit value "caught" by top-level executor
 			return errExit
 
+		case compiler.ExitStatus:
+			p.exitStatus = int(p.pop().num())
+			return errExit
+
 		case compiler.ForIn:
 			varScope := code[ip]
 			varIndex := code[ip+1]