avoid name collisions between receiver and params/results

Fixes #46

impl.go

@@ -349,13 +349,30 @@ var tmpl = template.Must(template.New("test").Parse(stub))
 // genStubs won't generate stubs for
 // already implemented methods of receiver.
 func genStubs(recv string, fns []Func, implemented map[string]bool) []byte {
-	var buf bytes.Buffer
+	var recvName string
+	if recvs := strings.Fields(recv); len(recvs) > 1 {
+		recvName = recvs[0]
+	}
+
+	// (r *recv) F(r string) {} => (r *recv) F(_ string)
+	fixParams := func(params []Param) {
+		for i, p := range params {
+			if p.Name == recvName {
+				params[i].Name = "_"
+			}
+		}
+	}
+
+	buf := new(bytes.Buffer)
 	for _, fn := range fns {
 		if implemented[fn.Name] {
 			continue
 		}
+
+		fixParams(fn.Params)
+		fixParams(fn.Res)
 		meth := Method{Recv: recv, Func: fn}
-		tmpl.Execute(&buf, meth)
+		tmpl.Execute(buf, meth)
 	}
 
 	pretty, err := format.Source(buf.Bytes())

impl_test.go

@@ -647,3 +647,45 @@ func TestStubGenerationForImplemented(t *testing.T) {
 		})
 	}
 }
+
+func TestStubGenerationForRepeatedName(t *testing.T) {
+	cases := []struct {
+		desc    string
+		iface   string
+		recv    string
+		recvPkg string
+		want    string
+	}{
+		{
+			desc:    "receiver and in.Params with the same name",
+			iface:   "github.com/josharian/impl/testdata.Interface6",
+			recv:    "arg1 *Implemented",
+			recvPkg: "testdata",
+			want:    testdata.Interface7Output,
+		},
+		{
+			desc:    "receiver and out.Params with the same name",
+			iface:   "github.com/josharian/impl/testdata.Interface6",
+			recv:    "arg3 *Implemented",
+			recvPkg: "testdata",
+			want:    testdata.Interface8Output,
+		},
+	}
+	for _, tt := range cases {
+		t.Run(tt.desc, func(t *testing.T) {
+			fns, err := funcs(tt.iface, ".", tt.recvPkg, WithComments)
+			if err != nil {
+				t.Errorf("funcs(%q).err=%v", tt.iface, err)
+			}
+
+			implemented, err := implementedFuncs(fns, tt.recv, "testdata")
+			if err != nil {
+				t.Errorf("ifuncs.err=%v", err)
+			}
+			src := genStubs(tt.recv, fns, implemented)
+			if string(src) != tt.want {
+				t.Errorf("genStubs(\"r *Implemented\", %+#v).src=\n\n%#v\n\nwant\n\n%#v\n\n", fns, string(src), tt.want)
+			}
+		})
+	}
+}

testdata/interfaces.go

@@ -153,3 +153,22 @@ func (r *Implemented) Method2(arg1 string, arg2 testdata.Interface2, arg3 testda
 }
 
 `
+
+type Interface6 interface {
+	// Method is the first method of Interface6.
+	Method2(arg1 string, arg2 int) (arg3 error)
+}
+
+var Interface7Output = `// Method is the first method of Interface6.
+func (arg1 *Implemented) Method2(_ string, arg2 int) (arg3 error) {
+	panic("not implemented") // TODO: Implement
+}
+
+`
+
+var Interface8Output = `// Method is the first method of Interface6.
+func (arg3 *Implemented) Method2(arg1 string, arg2 int) (_ error) {
+	panic("not implemented") // TODO: Implement
+}
+
+`