go/analysis/passes/unmarshal: allow unmarshalling to a type parameter

We can also unmarshal data to a type parameter, in addition to a pointer
and an interface.

This analyzer probably requires more discussion, but this solution
should be sufficient for now.

Updates golang/go#48704

Change-Id: I333f919109295e80a04e59df131713553cdbe612
Reviewed-on: https://go-review.googlesource.com/c/tools/+/353210
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
Trust: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>

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

@@ -0,0 +1,22 @@
+package typeparams
+
+import (
+	"encoding/json"
+	"fmt"
+)
+
+func unmarshalT[T any](data []byte) T {
+	var x T
+	json.Unmarshal(data, x)
+	return x
+}
+
+func unmarshalT2[T any](data []byte, t T) {
+    json.Unmarshal(data, t)
+}
+
+func main() {
+	x := make(map[string]interface{})
+	unmarshalT2([]byte(`{"a":1}`), &x)
+	fmt.Println(x)
+}

go/analysis/passes/unmarshal/unmarshal.go

@@ -14,6 +14,7 @@ import (
 	"golang.org/x/tools/go/analysis/passes/inspect"
 	"golang.org/x/tools/go/ast/inspector"
 	"golang.org/x/tools/go/types/typeutil"
+	"golang.org/x/tools/internal/typeparams"
 )
 
 const Doc = `report passing non-pointer or non-interface values to unmarshal
@@ -85,7 +86,7 @@ func run(pass *analysis.Pass) (interface{}, error) {
 
 		t := pass.TypesInfo.Types[call.Args[argidx]].Type
 		switch t.Underlying().(type) {
-		case *types.Pointer, *types.Interface:
+		case *types.Pointer, *types.Interface, *typeparams.TypeParam:
 			return
 		}
 

go/analysis/passes/unmarshal/unmarshal_test.go

@@ -9,9 +9,14 @@ import (
 
 	"golang.org/x/tools/go/analysis/analysistest"
 	"golang.org/x/tools/go/analysis/passes/unmarshal"
+	"golang.org/x/tools/internal/typeparams"
 )
 
 func Test(t *testing.T) {
 	testdata := analysistest.TestData()
-	analysistest.Run(t, testdata, unmarshal.Analyzer, "a")
+	tests := []string{"a"}
+	if typeparams.Enabled {
+		tests = append(tests, "typeparams")
+	}
+	analysistest.Run(t, testdata, unmarshal.Analyzer, tests...)
 }