param: Pointers are not parameter objects (#81)

This narrows down the definition of parameter objects to only structs
embedding `dig.Param`. Pointers to structs embedding `dig.Param` are
treated like any other struct pointer.

dig.go

@@ -116,7 +116,7 @@ func (c *Container) provideInstance(val interface{}) error {
 	if vtype == _errType {
 		return errors.New("can't provide errors")
 	}
-	if vtype.Implements(_parameterObjectType) {
+	if isParameterObject(vtype) {
 		return errors.New("can't provide parameter objects")
 	}
 	if _, ok := c.nodes[vtype]; ok {
@@ -135,7 +135,7 @@ func (c *Container) provideConstructor(ctor interface{}, ctype reflect.Type) err
 			// Don't register errors into the container.
 			continue
 		}
-		if rt.Implements(_parameterObjectType) {
+		if isParameterObject(rt) {
 			return errors.New("can't provide parameter objects")
 		}
 		if _, ok := returnTypes[rt]; ok {
@@ -180,7 +180,7 @@ func (c *Container) get(t reflect.Type) (reflect.Value, error) {
 		return v, nil
 	}
 
-	if t.Implements(_parameterObjectType) {
+	if isParameterObject(t) {
 		// We do not want parameter objects to be cached.
 		return c.createParamObject(t)
 	}
@@ -270,7 +270,7 @@ func newNode(provides reflect.Type, ctor interface{}, ctype reflect.Type) (node,
 
 // Retrives the dependencies for the parameter of a constructor.
 func getCtorParamDependencies(t reflect.Type) (deps []reflect.Type) {
-	if !t.Implements(_parameterObjectType) {
+	if !isParameterObject(t) {
 		deps = append(deps, t)
 		return
 	}
@@ -334,17 +334,14 @@ type parameterObject interface {
 	parameterObject()
 }
 
+func isParameterObject(t reflect.Type) bool {
+	return t.Implements(_parameterObjectType) && t.Kind() == reflect.Struct
+}
+
 // Returns a new Param parent object with all the dependency fields
 // populated from the dig container.
 func (c *Container) createParamObject(t reflect.Type) (reflect.Value, error) {
 	dest := reflect.New(t).Elem()
-	result := dest
-	for t.Kind() == reflect.Ptr {
-		t = t.Elem()
-		dest.Set(reflect.New(t))
-		dest = dest.Elem()
-	}
-
 	for i := 0; i < t.NumField(); i++ {
 		f := t.Field(i)
 		if f.PkgPath != "" {
@@ -357,12 +354,12 @@ func (c *Container) createParamObject(t reflect.Type) (reflect.Value, error) {
 			case "true", "yes":
 				v = reflect.Zero(f.Type)
 			default:
-				return result, fmt.Errorf(
+				return dest, fmt.Errorf(
 					"could not get field %v (type %v) of %v: %v", f.Name, f.Type, t, err)
 			}
 		}
 
 		dest.Field(i).Set(v)
 	}
-	return result, nil
+	return dest, nil
 }

dig_test.go

@@ -247,25 +247,6 @@ func TestEndToEndSuccess(t *testing.T) {
 		}))
 	})
 
-	t.Run("param pointer", func(t *testing.T) {
-		c := New()
-
-		require.NoError(t, c.Provide(func() io.Writer {
-			return new(bytes.Buffer)
-		}), "provide failed")
-
-		type Args struct {
-			Param
-
-			Writer io.Writer
-		}
-
-		require.NoError(t, c.Invoke(func(args *Args) {
-			require.NotNil(t, args, "args must not be nil")
-			require.NotNil(t, args.Writer, "writer must not be nil")
-		}), "invoke failed")
-	})
-
 	t.Run("invoke param", func(t *testing.T) {
 		c := New()
 		require.NoError(t, c.Provide(func() *bytes.Buffer {
@@ -316,15 +297,15 @@ func TestEndToEndSuccess(t *testing.T) {
 			Param
 
 			Buffer  *bytes.Buffer
-			Another *anotherParam
+			Another anotherParam
 		}
 
 		c := New()
 		require.NoError(t, c.Provide(func() *bytes.Buffer {
 			return new(bytes.Buffer)
 		}), "provide must not fail")
 
-		require.NoError(t, c.Invoke(func(p *someParam) {
+		require.NoError(t, c.Invoke(func(p someParam) {
 			require.NotNil(t, p, "someParam must not be nil")
 			require.NotNil(t, p.Buffer, "someParam must not be nil")
 			require.NotNil(t, p.Another, "someParam must not be nil")
@@ -502,10 +483,9 @@ func TestCanProvideErrorLikeType(t *testing.T) {
 			c := New()
 			require.NoError(t, c.Provide(tt), "provide must not fail")
 
-			require.NoError(t, c.Invoke(
-				func(err *someError) {
-					assert.NotNil(t, err, "invoke received nil")
-				}), "invoke must not fail")
+			require.NoError(t, c.Invoke(func(err *someError) {
+				assert.NotNil(t, err, "invoke received nil")
+			}), "invoke must not fail")
 		})
 	}
 }
@@ -525,10 +505,13 @@ func TestCantProvideParameterObjects(t *testing.T) {
 	t.Run("pointer", func(t *testing.T) {
 		type Args struct{ Param }
 
+		args := &Args{}
+
 		c := New()
-		err := c.Provide(&Args{})
-		require.Error(t, err, "provide should fail")
-		require.Contains(t, err.Error(), "can't provide parameter objects")
+		require.NoError(t, c.Provide(args), "provide failed")
+		require.NoError(t, c.Invoke(func(a *Args) {
+			require.True(t, args == a, "args must match")
+		}), "invoke failed")
 	})
 
 	t.Run("constructor", func(t *testing.T) {
@@ -542,15 +525,18 @@ func TestCantProvideParameterObjects(t *testing.T) {
 		require.Contains(t, err.Error(), "can't provide parameter objects")
 	})
 
-	t.Run("constructor pointer", func(t *testing.T) {
+	t.Run("pointer from constructor", func(t *testing.T) {
 		type Args struct{ Param }
 
+		args := &Args{}
+
 		c := New()
-		err := c.Provide(func() (*Args, error) {
-			panic("great sadness")
-		})
-		require.Error(t, err, "provide should fail")
-		require.Contains(t, err.Error(), "can't provide parameter objects")
+		require.NoError(t, c.Provide(func() (*Args, error) {
+			return args, nil
+		}), "provide failed")
+		require.NoError(t, c.Invoke(func(a *Args) {
+			require.True(t, args == a, "args must match")
+		}), "invoke failed")
 	})
 }
 
@@ -684,7 +670,7 @@ func TestInvokeFailures(t *testing.T) {
 		}
 
 		c := New()
-		err := c.Invoke(func(a *args) {
+		err := c.Invoke(func(a args) {
 			t.Fatal("function must not be called")
 		})