fix(generator): support nil returns when using interface typed params

As referenced on #513
Types using generic typed interface params, should allow nil values on return

Author: jfragosoperez

.mockery.yaml

@@ -70,3 +70,11 @@ packages:
       filename: "mock_{{.InterfaceName}}_test.go"
       inpackage: True
       keeptree: False
+  github.com/vektra/mockery/v2/pkg/fixtures/iface_typed_param:
+    config:
+      all: True
+      dir: "{{.InterfaceDir}}"
+      mockname: "{{.InterfaceName}}"
+      outpkg: "{{.PackageName}}_test"
+      filename: "{{.InterfaceNameSnake}}_mock_test.go"
+      keeptree: True    

mocks/github.com/vektra/mockery/v2/pkg/fixtures/EmbeddedGet.go

@@ -0,0 +1,7 @@
+package iface_typed_param
+
+import "io"
+
+type GetterIfaceTypedParam[T io.Reader] interface {
+	Get() T
+}

mocks/github.com/vektra/mockery/v2/pkg/fixtures/GetGeneric.go

@@ -0,0 +1,29 @@
+package iface_typed_param_test
+
+import (
+	"bufio"
+	"net/http"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestIfaceWithIfaceTypedParamReturnValues(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name      string
+		returnVal *bufio.Reader
+	}{
+		{"nil return val", nil},
+		{"returning val", bufio.NewReader(http.NoBody)},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(st *testing.T) {
+			m := NewGetterIfaceTypedParam[*bufio.Reader](st)
+			m.EXPECT().Get().Return(test.returnVal)
+
+			assert.Equal(st, test.returnVal, m.Get())
+		})
+	}
+}

mocks/github.com/vektra/mockery/v2/pkg/fixtures/ReplaceGeneric.go

@@ -665,7 +665,7 @@ func isNillable(typ types.Type) bool {
 	switch t := typ.(type) {
 	case *types.Pointer, *types.Array, *types.Map, *types.Interface, *types.Signature, *types.Chan, *types.Slice:
 		return true
-	case *types.Named, *types.Alias:
+	case *types.Named, *types.Alias, *types.TypeParam:
 		return isNillable(t.Underlying())
 	}
 	return false