go/analysis/passes/unusedresult: update and add test for typeparams

This CL implements  support for setting unused generic funcs via flags.
A test for unused results with generics is added.

Update golang/go#48704

go/analysis/passes/unusedresult/testdata/src/typeparams/typeparams.go

@@ -0,0 +1,36 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+//
+//go:build go1.18
+
+package typeparams
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"typeparams/userdefs"
+)
+
+func _[T any]() {
+	fmt.Errorf("") // want "result of fmt.Errorf call not used"
+	_ = fmt.Errorf("")
+
+	errors.New("") // want "result of errors.New call not used"
+
+	err := errors.New("")
+	err.Error() // want `result of \(error\).Error call not used`
+
+	var buf bytes.Buffer
+	buf.String() // want `result of \(bytes.Buffer\).String call not used`
+
+	fmt.Sprint("")  // want "result of fmt.Sprint call not used"
+	fmt.Sprintf("") // want "result of fmt.Sprintf call not used"
+
+	userdefs.MustUse[int](1) // want "result of typeparams/userdefs.MustUse call not used"
+	_ = userdefs.MustUse[int](2)
+
+	st := userdefs.ST[int]{X: 1}
+	st.String() // want `result of \(typeparams/userdefs.ST\[int\]\).String call not used`
+}

go/analysis/passes/unusedresult/testdata/src/typeparams/userdefs/userdefs.go

@@ -0,0 +1,19 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+//
+//go:build go1.18
+
+package userdefs
+
+func MustUse[T interface{ ~int }](v T) T {
+	return v + 1
+}
+
+type ST[T interface{ ~int }] struct {
+	X T
+}
+
+func (st *ST[T]) String() string {
+	return "st"
+}

go/analysis/passes/unusedresult/unusedresult.go

@@ -70,6 +70,13 @@ func run(pass *analysis.Pass) (interface{}, error) {
 			return // a conversion, not a call
 		}
 
+		index, ok := fun.(*ast.IndexExpr)
+		if ok {
+			if _, ok := pass.TypesInfo.TypeOf(index).(*types.Signature); ok {
+				fun = index.X // get selector of generic function or method call
+			}
+		}
+
 		selector, ok := fun.(*ast.SelectorExpr)
 		if !ok {
 			return // neither a method call nor a qualified ident

go/analysis/passes/unusedresult/unusedresult_test.go

@@ -9,9 +9,15 @@ import (
 
 	"golang.org/x/tools/go/analysis/analysistest"
 	"golang.org/x/tools/go/analysis/passes/unusedresult"
+	"golang.org/x/tools/internal/typeparams"
 )
 
 func Test(t *testing.T) {
 	testdata := analysistest.TestData()
-	analysistest.Run(t, testdata, unusedresult.Analyzer, "a")
+	tests := []string{"a"}
+	if typeparams.Enabled {
+		unusedresult.Analyzer.Flags.Set("funcs", "typeparams/userdefs.MustUse,errors.New,fmt.Errorf,fmt.Sprintf,fmt.Sprint,sort.Reverse,context.WithValue,context.WithCancel,context.WithDeadline,context.WithTimeout")
+		tests = append(tests, "typeparams")
+	}
+	analysistest.Run(t, testdata, unusedresult.Analyzer, tests...)
 }