Handle interfaces that implement interfaces

codegen/config/binder.go

@@ -206,7 +206,7 @@ func (t *TypeReference) IsPtr() bool {
 }
 
 func (t *TypeReference) IsNilable() bool {
-	return isNilable(t.GO)
+	return IsNilable(t.GO)
 }
 
 func (t *TypeReference) IsSlice() bool {
@@ -403,14 +403,14 @@ func (b *Binder) CopyModifiersFromAst(t *ast.Type, base types.Type) types.Type {
 		_, isInterface = named.Underlying().(*types.Interface)
 	}
 
-	if !isInterface && !isNilable(base) && !t.NonNull {
+	if !isInterface && !IsNilable(base) && !t.NonNull {
 		return types.NewPointer(base)
 	}
 
 	return base
 }
 
-func isNilable(t types.Type) bool {
+func IsNilable(t types.Type) bool {
 	if namedType, isNamed := t.(*types.Named); isNamed {
 		t = namedType.Underlying()
 	}

codegen/data.go

@@ -80,7 +80,10 @@ func BuildData(cfg *config.Config) (*Data, error) {
 			s.Inputs = append(s.Inputs, input)
 
 		case ast.Union, ast.Interface:
-			s.Interfaces[schemaType.Name] = b.buildInterface(schemaType)
+			s.Interfaces[schemaType.Name], err = b.buildInterface(schemaType)
+			if err != nil {
+				return nil, errors.Wrap(err, "unable to bind to interface")
+			}
 		}
 	}
 

codegen/interface.go

@@ -1,9 +1,13 @@
 package codegen
 
 import (
+	"fmt"
 	"go/types"
 
+	"github.com/pkg/errors"
 	"github.com/vektah/gqlparser/ast"
+
+	"github.com/99designs/gqlgen/codegen/config"
 )
 
 type Interface struct {
@@ -16,11 +20,11 @@ type Interface struct {
 type InterfaceImplementor struct {
 	*ast.Definition
 
-	Interface *Interface
-	Type      types.Type
+	Type    types.Type
+	TakeRef bool
 }
 
-func (b *builder) buildInterface(typ *ast.Definition) *Interface {
+func (b *builder) buildInterface(typ *ast.Definition) (*Interface, error) {
 	obj, err := b.Binder.DefaultUserObject(typ.Name)
 	if err != nil {
 		panic(err)
@@ -32,32 +36,53 @@ func (b *builder) buildInterface(typ *ast.Definition) *Interface {
 		InTypemap:  b.Config.Models.UserDefined(typ.Name),
 	}
 
+	interfaceType, err := findGoInterface(i.Type)
+	if interfaceType == nil || err != nil {
+		return nil, fmt.Errorf("%s is not an interface", i.Type)
+	}
+
 	for _, implementor := range b.Schema.GetPossibleTypes(typ) {
 		obj, err := b.Binder.DefaultUserObject(implementor.Name)
 		if err != nil {
-			panic(err)
+			return nil, fmt.Errorf("%s has no backing go type", implementor.Name)
 		}
 
-		i.Implementors = append(i.Implementors, InterfaceImplementor{
-			Definition: implementor,
-			Type:       obj,
-			Interface:  i,
-		})
-	}
+		implementorType, err := findGoNamedType(obj)
+		if err != nil {
+			return nil, errors.Wrapf(err, "can not find backing go type %s", obj.String())
+		} else if implementorType == nil {
+			return nil, fmt.Errorf("can not find backing go type %s", obj.String())
+		}
 
-	return i
-}
+		anyValid := false
 
-func (i *InterfaceImplementor) ValueReceiver() bool {
-	interfaceType, err := findGoInterface(i.Interface.Type)
-	if interfaceType == nil || err != nil {
-		return true
-	}
+		// first check if the value receiver can be nil, eg can we type switch on case Thing:
+		if types.Implements(implementorType, interfaceType) {
+			i.Implementors = append(i.Implementors, InterfaceImplementor{
+				Definition: implementor,
+				Type:       obj,
+				TakeRef:    !types.IsInterface(obj),
+			})
+			anyValid = true
+		}
+
+		// then check if the pointer receiver can be nil, eg can we type switch on case *Thing:
+		if types.Implements(types.NewPointer(implementorType), interfaceType) {
+			i.Implementors = append(i.Implementors, InterfaceImplementor{
+				Definition: implementor,
+				Type:       types.NewPointer(obj),
+			})
+			anyValid = true
+		}
 
-	implementorType, err := findGoNamedType(i.Type)
-	if implementorType == nil || err != nil {
-		return true
+		if !anyValid {
+			return nil, fmt.Errorf("%s does not satisfy the interface %s", implementorType.String(), i.Type.String())
+		}
 	}
 
-	return types.Implements(implementorType, interfaceType)
+	return i, nil
+}
+
+func (i *InterfaceImplementor) CanBeNil() bool {
+	return config.IsNilable(i.Type)
 }

codegen/interface.gotpl

@@ -5,15 +5,13 @@ func (ec *executionContext) _{{$interface.Name}}(ctx context.Context, sel ast.Se
 	case nil:
 		return graphql.Null
 	{{- range $implementor := $interface.Implementors }}
-		{{- if $implementor.ValueReceiver }}
-			case {{$implementor.Type | ref}}:
-				return ec._{{$implementor.Name}}(ctx, sel, &obj)
-		{{- end}}
-		case *{{$implementor.Type | ref}}:
-			if obj == nil {
-				return graphql.Null
-			}
-			return ec._{{$implementor.Name}}(ctx, sel, obj)
+		case {{$implementor.Type | ref}}:
+			{{- if $implementor.CanBeNil }}
+				if obj == nil {
+					return graphql.Null
+				}
+			{{- end }}
+			return ec._{{$implementor.Name}}(ctx, sel, {{ if $implementor.TakeRef }}&{{ end }}obj)
 	{{- end }}
 	default:
 		panic(fmt.Errorf("unexpected type %T", obj))

codegen/testserver/interfaces.go

@@ -47,6 +47,25 @@ func (n *ConcreteNodeA) Child() (Node, error) {
 	return n.child, nil
 }
 
+//  Implements the Node interface with another interface
+type ConcreteNodeInterface interface {
+	Node
+	ID() string
+}
+
+type ConcreteNodeInterfaceImplementor struct{}
+
+func (c ConcreteNodeInterfaceImplementor) ID() string {
+	return "CNII"
+}
+
+func (c ConcreteNodeInterfaceImplementor) Child() (Node, error) {
+	return &ConcreteNodeA{
+		ID:   "Child",
+		Name: "child",
+	}, nil
+}
+
 type BackedByInterface interface {
 	ThisShouldBind() string
 	ThisShouldBindWithError() (string, error)

codegen/testserver/interfaces.graphql

@@ -54,3 +54,9 @@ type ConcreteNodeA implements Node {
     child: Node!
     name: String!
 }
+
+""" Implements the Node interface with another interface """
+type ConcreteNodeInterface implements Node {
+    id: ID!
+    child: Node!
+}

codegen/testserver/interfaces_test.go

@@ -176,4 +176,25 @@ func TestInterfaces(t *testing.T) {
 		err := c.Post(`{ notAnInterface { id, thisShouldBind, thisShouldBindWithError } }`, &resp)
 		require.EqualError(t, err, `[{"message":"boom","path":["notAnInterface","thisShouldBindWithError"]}]`)
 	})
+
+	t.Run("interfaces can implement other interfaces", func(t *testing.T) {
+		resolvers := &Stub{}
+		resolvers.QueryResolver.Node = func(ctx context.Context) (node Node, err error) {
+			return ConcreteNodeInterfaceImplementor{}, nil
+		}
+
+		c := client.New(handler.NewDefaultServer(NewExecutableSchema(Config{Resolvers: resolvers})))
+
+		var resp struct {
+			Node struct {
+				ID    string
+				Child struct {
+					ID string
+				}
+			}
+		}
+		c.MustPost(`{ node { id, child { id } } }`, &resp)
+		require.Equal(t, "CNII", resp.Node.ID)
+		require.Equal(t, "Child", resp.Node.Child.ID)
+	})
 }