fix(generator): support for nil returns when using interface typed pa…

…rams

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

pkg/fixtures/requester_iface.go

@@ -5,3 +5,7 @@ import "io"
 type RequesterIface interface {
 	Get() io.Reader
 }
+
+type RequesterIfaceWithInterfaceTypedParam[T io.Reader] interface {
+	Get() T
+}

pkg/generator.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

pkg/generator_test.go

@@ -337,6 +337,22 @@ func (s *GeneratorSuite) TestGeneratorChecksInterfacesForNilable() {
 	s.checkGeneration("requester_iface.go", "RequesterIface", false, "", "")
 }
 
+func (s *GeneratorSuite) TestGeneratorChecksInterfacesWithGenericInterfaceTypedParamForNilable() {
+	expectedBytes, err := os.ReadFile(getMocksPath("RequesterIfaceWithInterfaceTypedParam.go"))
+	s.Require().NoError(err)
+
+	expected := string(expectedBytes)
+	expected = expected[strings.Index(expected, "// RequesterIfaceWithInterfaceTypedParam is"):]
+
+	cfg := GeneratorConfig{
+		StructName:     "RequesterIfaceWithInterfaceTypedParam",
+		WithExpecter:   true,
+		UnrollVariadic: false,
+	}
+
+	s.checkGenerationWithConfig("requester_iface.go", "RequesterIfaceWithInterfaceTypedParam", cfg, expected)
+}
+
 func (s *GeneratorSuite) TestGeneratorTreatsAnyAsNilable() {
 	cfg := GeneratorConfig{
 		WithExpecter: true,