go/analysis/passes/nilfunc: add typeparams test

testdata/src/typeparams is similar to testdata/src/a but
uses type parameters.

For golang/go#48704

Change-Id: I2d49190b606d6cdcfe79af5f29bba1117b07b94a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/359894
Run-TryBot: Guodong Li <guodongli@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Guodong Li <guodongli@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

go/analysis/passes/nilfunc/nilfunc.go

@@ -14,6 +14,7 @@ import (
 	"golang.org/x/tools/go/analysis"
 	"golang.org/x/tools/go/analysis/passes/inspect"
 	"golang.org/x/tools/go/ast/inspector"
+	"golang.org/x/tools/internal/typeparams"
 )
 
 const Doc = `check for useless comparisons between functions and nil
@@ -59,6 +60,11 @@ func run(pass *analysis.Pass) (interface{}, error) {
 			obj = pass.TypesInfo.Uses[v]
 		case *ast.SelectorExpr:
 			obj = pass.TypesInfo.Uses[v.Sel]
+		case *ast.IndexExpr, *typeparams.IndexListExpr:
+			// Check generic functions such as "f[T1,T2]".
+			if id, ok := typeparams.GetIndexExprData(v).X.(*ast.Ident); ok {
+				obj = pass.TypesInfo.Uses[id]
+			}
 		default:
 			return
 		}

go/analysis/passes/nilfunc/nilfunc_test.go

@@ -5,13 +5,17 @@
 package nilfunc_test
 
 import (
-	"testing"
-
 	"golang.org/x/tools/go/analysis/analysistest"
 	"golang.org/x/tools/go/analysis/passes/nilfunc"
+	"golang.org/x/tools/internal/typeparams"
+	"testing"
 )
 
 func Test(t *testing.T) {
 	testdata := analysistest.TestData()
-	analysistest.Run(t, testdata, nilfunc.Analyzer, "a")
+	tests := []string{"a"}
+	if typeparams.Enabled {
+		tests = append(tests, "typeparams")
+	}
+	analysistest.Run(t, testdata, nilfunc.Analyzer, tests...)
 }

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

@@ -0,0 +1,52 @@
+// Copyright 2021 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.
+
+// This file contains tests for the lostcancel checker.
+
+//go:build go1.18
+
+package typeparams
+
+func f[P any]() {}
+
+func g[P1 any, P2 any](x P1) {}
+
+var f1 = f[int]
+
+type T1[P any] struct {
+	f func() P
+}
+
+type T2[P1 any, P2 any] struct {
+	g func(P1) P2
+}
+
+func Comparison[P any](f2 func()T1[P]) {
+	var t1 T1[P]
+	var t2 T2[P, int]
+	var fn func()
+	if fn == nil || f1 == nil || f2 == nil || t1.f == nil || t2.g == nil {
+		// no error; these func vars or fields may be nil
+	}
+	if f[P] == nil { // want "comparison of function f == nil is always false"
+		panic("can't happen")
+	}
+	if f[int] == nil { // want "comparison of function f == nil is always false"
+		panic("can't happen")
+	}
+	if g[P, int] == nil { // want "comparison of function g == nil is always false"
+		panic("can't happen")
+	}
+}
+
+func Index[P any](a [](func()P)) {
+	if a[1] == nil {
+		// no error
+	}
+	var t1 []T1[P]
+	var t2 [][]T2[P, P]
+	if t1[1].f == nil || t2[0][1].g == nil {
+		// no error
+	}
+}