go/analysis/passes/printf: check for nil scope of instance methods

Fixes golang/go#55350

Change-Id: I28784b42de72e9d489e40d546e171af23b9e5c13
Reviewed-on: https://go-review.googlesource.com/c/tools/+/433755
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Tim King <taking@google.com>
Run-TryBot: Zvonimir Pavlinovic <zpavlinovic@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

go/analysis/passes/printf/printf.go

@@ -945,11 +945,16 @@ func recursiveStringer(pass *analysis.Pass, e ast.Expr) (string, bool) {
 		return "", false
 	}
 
+	// inScope returns true if e is in the scope of f.
+	inScope := func(e ast.Expr, f *types.Func) bool {
+		return f.Scope() != nil && f.Scope().Contains(e.Pos())
+	}
+
 	// Is the expression e within the body of that String or Error method?
 	var method *types.Func
-	if strOk && strMethod.Pkg() == pass.Pkg && strMethod.Scope().Contains(e.Pos()) {
+	if strOk && strMethod.Pkg() == pass.Pkg && inScope(e, strMethod) {
 		method = strMethod
-	} else if errOk && errMethod.Pkg() == pass.Pkg && errMethod.Scope().Contains(e.Pos()) {
+	} else if errOk && errMethod.Pkg() == pass.Pkg && inScope(e, errMethod) {
 		method = errMethod
 	} else {
 		return "", false

go/analysis/passes/printf/testdata/src/typeparams/diagnostics.go

@@ -121,3 +121,25 @@ func TestTermReduction[T1 interface{ ~int | string }, T2 interface {
 	fmt.Printf("%d", t2)
 	fmt.Printf("%s", t2) // want "wrong type.*contains typeparams.myInt"
 }
+
+type U[T any] struct{}
+
+func (u U[T]) String() string {
+	fmt.Println(u) // want `fmt.Println arg u causes recursive call to \(typeparams.U\[T\]\).String method`
+	return ""
+}
+
+type S[T comparable] struct {
+	t T
+}
+
+func (s S[T]) String() T {
+	fmt.Println(s) // Not flagged. We currently do not consider String() T to implement fmt.Stringer (see #55928).
+	return s.t
+}
+
+func TestInstanceStringer() {
+	// Tests String method with nil Scope (#55350)
+	fmt.Println(&S[string]{})
+	fmt.Println(&U[string]{})
+}