compiler: prohibit to compile type assertion with two return values

Close #2692.

docs/compiler.md

@@ -28,6 +28,9 @@ a dialect of Go rather than a complete port of the language:
  * converting value to interface type doesn't change the underlying type,
    original value will always be used, therefore it never panics and always "succeeds";
    it's up to the programmer whether it's a correct use of a value
+ * type assertion with two return values is not supported; single return value (of the desired type)
+   is supported; type assertion panics if value can't be asserted to the desired type, therefore
+   it's up to the programmer whether assert can be performed successfully.
 
 ## VM API (interop layer)
 Compiler translates interop function calls into NEO VM syscalls or (for custom

pkg/compiler/codegen.go

@@ -161,6 +161,9 @@ const (
 	varArgument
 )
 
+// ErrUnsupportedTypeAssertion is returned when type assertion statement is not supported by the compiler.
+var ErrUnsupportedTypeAssertion = errors.New("type assertion with two return values is not supported")
+
 // newLabel creates a new label to jump to.
 func (c *codegen) newLabel() (l uint16) {
 	li := len(c.l)
@@ -584,6 +587,14 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 		for _, spec := range n.Specs {
 			switch t := spec.(type) {
 			case *ast.ValueSpec:
+				// Filter out type assertion with two return values: var i, ok = v.(int)
+				if len(t.Names) == 2 && len(t.Values) == 1 && n.Tok == token.VAR {
+					err := checkTypeAssertWithOK(t.Values[0])
+					if err != nil {
+						c.prog.Err = err
+						return nil
+					}
+				}
 				multiRet := n.Tok == token.VAR && len(t.Values) != 0 && len(t.Names) != len(t.Values)
 				for _, id := range t.Names {
 					if id.Name != "_" {
@@ -630,6 +641,14 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 		return nil
 
 	case *ast.AssignStmt:
+		// Filter out type assertion with two return values: i, ok = v.(int)
+		if len(n.Lhs) == 2 && len(n.Rhs) == 1 && (n.Tok == token.DEFINE || n.Tok == token.ASSIGN) {
+			err := checkTypeAssertWithOK(n.Rhs[0])
+			if err != nil {
+				c.prog.Err = err
+				return nil
+			}
+		}
 		multiRet := len(n.Rhs) != len(n.Lhs)
 		c.saveSequencePoint(n)
 		// Assign operations are grouped https://github.com/golang/go/blob/master/src/go/types/stmt.go#L160
@@ -1346,6 +1365,14 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 	return c
 }
 
+func checkTypeAssertWithOK(n ast.Node) error {
+	if t, ok := n.(*ast.TypeAssertExpr); ok &&
+		t.Type != nil { // not a type switch
+		return ErrUnsupportedTypeAssertion
+	}
+	return nil
+}
+
 // packVarArgs packs variadic arguments into an array
 // and returns the amount of arguments packed.
 func (c *codegen) packVarArgs(n *ast.CallExpr, typ *types.Signature) int {

pkg/compiler/convert_test.go

@@ -81,6 +81,7 @@ func TestConvert(t *testing.T) {
 }
 
 func TestTypeAssertion(t *testing.T) {
+	t.Run("inside return statement", func(t *testing.T) {
 		src := `package foo
 				func Main() int {
 					a := []byte{1}
@@ -89,6 +90,84 @@ func TestTypeAssertion(t *testing.T) {
 					return u.(int)
 				}`
 		eval(t, src, big.NewInt(1))
+	})
+	t.Run("inside general declaration", func(t *testing.T) {
+		src := `package foo
+				func Main() int {
+					a := []byte{1}
+					var u interface{}
+					u = a
+					var ret = u.(int)
+					return ret
+				}`
+		eval(t, src, big.NewInt(1))
+	})
+	t.Run("inside assignment statement", func(t *testing.T) {
+		src := `package foo
+				func Main() int {
+					a := []byte{1}
+					var u interface{}
+					u = a
+					var ret int
+					ret = u.(int)
+					return ret
+				}`
+		eval(t, src, big.NewInt(1))
+	})
+	t.Run("inside definition statement", func(t *testing.T) {
+		src := `package foo
+				func Main() int {
+					a := []byte{1}
+					var u interface{}
+					u = a
+					ret := u.(int)
+					return ret
+				}`
+		eval(t, src, big.NewInt(1))
+	})
+}
+
+func TestTypeAssertionWithOK(t *testing.T) {
+	t.Run("inside general declaration", func(t *testing.T) {
+		src := `package foo
+				func Main() bool {
+					a := 1
+					var u interface{}
+					u = a
+					var _, ok = u.(int)	//	*ast.GenDecl
+					return ok
+				}`
+		_, _, err := compiler.CompileWithOptions("foo.go", strings.NewReader(src), nil)
+		require.Error(t, err)
+		require.ErrorIs(t, err, compiler.ErrUnsupportedTypeAssertion)
+	})
+	t.Run("inside assignment statement", func(t *testing.T) {
+		src := `package foo
+				func Main() bool {
+					a := 1
+					var u interface{}
+					u = a
+					var ok bool
+					_, ok = u.(int)	// *ast.AssignStmt
+					return ok
+				}`
+		_, _, err := compiler.CompileWithOptions("foo.go", strings.NewReader(src), nil)
+		require.Error(t, err)
+		require.ErrorIs(t, err, compiler.ErrUnsupportedTypeAssertion)
+	})
+	t.Run("inside definition statement", func(t *testing.T) {
+		src := `package foo
+				func Main() bool {
+					a := 1
+					var u interface{}
+					u = a
+					_, ok := u.(int)	// *ast.AssignStmt
+					return ok
+				}`
+		_, _, err := compiler.CompileWithOptions("foo.go", strings.NewReader(src), nil)
+		require.Error(t, err)
+		require.ErrorIs(t, err, compiler.ErrUnsupportedTypeAssertion)
+	})
 }
 
 func TestTypeConversion(t *testing.T) {