compiler: emit error for non-byte subslices

They are not supported for now, as VM has only
`SUBSTR` opcode for Buffers (`[]byte` in Go).

pkg/compiler/codegen.go

@@ -517,6 +517,10 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 		return nil
 
 	case *ast.SliceExpr:
+		if isCompoundSlice(c.typeOf(n.X.(*ast.Ident)).Underlying()) {
+			c.prog.Err = errors.New("subslices are supported only for []byte")
+			return nil
+		}
 		name := n.X.(*ast.Ident).Name
 		c.emitLoadVar("", name)
 

pkg/compiler/slice_test.go

@@ -316,6 +316,17 @@ func TestSliceOperations(t *testing.T) {
 	runTestCases(t, sliceTestCases)
 }
 
+func TestSubsliceCompound(t *testing.T) {
+	src := `package foo
+	func Main() []int {
+		a := []int{0, 1, 2, 3}
+		b := a[1:3]
+		return b
+	}`
+	_, err := compiler.Compile("", strings.NewReader(src))
+	require.Error(t, err)
+}
+
 func TestJumps(t *testing.T) {
 	src := `
 	package foo