go/analysis/passes/stdmethods: add tests with generic receivers

Ensure that the logic for detecting incorrect signatures of standard
methods applies to methods on generic types.

For golang/go#48704

Change-Id: I1862238ad1ff013137b12674fbc0068a7e8a3c60
Reviewed-on: https://go-review.googlesource.com/c/tools/+/359409
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tim King <taking@google.com>

go/analysis/passes/stdmethods/stdmethods_test.go

@@ -9,11 +9,16 @@ import (
 
 	"golang.org/x/tools/go/analysis/analysistest"
 	"golang.org/x/tools/go/analysis/passes/stdmethods"
+	"golang.org/x/tools/internal/typeparams"
 )
 
 func Test(t *testing.T) {
 	testdata := analysistest.TestData()
-	analysistest.Run(t, testdata, stdmethods.Analyzer, "a")
+	pkgs := []string{"a"}
+	if typeparams.Enabled {
+		pkgs = append(pkgs, "typeparams")
+	}
+	analysistest.Run(t, testdata, stdmethods.Analyzer, pkgs...)
 }
 
 func TestAnalyzeEncodingXML(t *testing.T) {

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

@@ -0,0 +1,41 @@
+// 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.
+
+package typeparams
+
+import "fmt"
+
+type T[P any] int
+
+func (T[_]) Scan(x fmt.ScanState, c byte) {} // want `should have signature Scan\(fmt\.ScanState, rune\) error`
+
+func (T[_]) Format(fmt.State, byte) {} // want `should have signature Format\(fmt.State, rune\)`
+
+type U[P any] int
+
+func (U[_]) Format(byte) {} // no error: first parameter must be fmt.State to trigger check
+
+func (U[P]) GobDecode(P) {} // want `should have signature GobDecode\(\[\]byte\) error`
+
+type V[P any] int // V does not implement error.
+
+func (V[_]) As() T[int]  { return 0 }     // ok - V is not an error
+func (V[_]) Is() bool    { return false } // ok - V is not an error
+func (V[_]) Unwrap() int { return 0 }     // ok - V is not an error
+
+type E[P any] int
+
+func (E[_]) Error() string { return "" } // E implements error.
+
+func (E[P]) As()     {} // want `method As\(\) should have signature As\(interface{}\) bool`
+func (E[_]) Is()     {} // want `method Is\(\) should have signature Is\(error\) bool`
+func (E[_]) Unwrap() {} // want `method Unwrap\(\) should have signature Unwrap\(\) error`
+
+type F[P any] int
+
+func (F[_]) Error() string { return "" } // Both F and *F implement error.
+
+func (*F[_]) As()     {} // want `method As\(\) should have signature As\(interface{}\) bool`
+func (*F[_]) Is()     {} // want `method Is\(\) should have signature Is\(error\) bool`
+func (*F[_]) Unwrap() {} // want `method Unwrap\(\) should have signature Unwrap\(\) error`