nilness: add unit test for generic instantiation.

Adds a unit test for generic instantiation to nilness.
Currently the expected behavior is to ignore the instantiation
as it is not added to builssa's SrcFuncs result.

Updates golang/go#52463
Updates golang/go#48525

Change-Id: I7b214aae88c8aa26605abb5019591178f76a7cbb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/402054
Run-TryBot: Tim King <taking@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tim King <taking@google.com>
Reviewed-by: Zvonimir Pavlinovic <zpavlinovic@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

go/analysis/passes/nilness/nilness.go

@@ -62,6 +62,7 @@ var Analyzer = &analysis.Analyzer{
 
 func run(pass *analysis.Pass) (interface{}, error) {
 	ssainput := pass.ResultOf[buildssa.Analyzer].(*buildssa.SSA)
+	// TODO(48525): ssainput.SrcFuncs is missing fn._Instances(). runFunc will be skipped.
 	for _, fn := range ssainput.SrcFuncs {
 		runFunc(pass, fn)
 	}

go/analysis/passes/nilness/nilness_test.go

@@ -9,9 +9,18 @@ import (
 
 	"golang.org/x/tools/go/analysis/analysistest"
 	"golang.org/x/tools/go/analysis/passes/nilness"
+	"golang.org/x/tools/internal/typeparams"
 )
 
 func Test(t *testing.T) {
 	testdata := analysistest.TestData()
 	analysistest.Run(t, testdata, nilness.Analyzer, "a")
 }
+
+func TestInstantiated(t *testing.T) {
+	if !typeparams.Enabled {
+		t.Skip("TestInstantiated requires type parameters")
+	}
+	testdata := analysistest.TestData()
+	analysistest.Run(t, testdata, nilness.Analyzer, "c")
+}

go/analysis/passes/nilness/testdata/src/c/c.go

@@ -0,0 +1,14 @@
+package c
+
+func instantiated[X any](x *X) int {
+	if x == nil {
+		print(*x) // not reported until _Instances are added to SrcFuncs
+	}
+	return 1
+}
+
+var g int
+
+func init() {
+	g = instantiated[int](&g)
+}