errorsas: handle single-actual case

Handle the case where there is syntactically only one actual argument
passed to errors.As.

Following the unmarshal check, we ignore this case.

Change-Id: Ia7d77d5b3c9eb5416b37a141104c9ad7ed290b5f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/178159
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>

go/analysis/passes/errorsas/errorsas.go

@@ -47,6 +47,9 @@ func run(pass *analysis.Pass) (interface{}, error) {
 		if fn == nil {
 			return // not a static call
 		}
+		if len(call.Args) < 2 {
+			return // not enough arguments, e.g. called with return values of another function
+		}
 		if fn.FullName() == "errors.As" && !pointerToInterfaceOrError(pass, call.Args[1]) {
 			pass.Reportf(call.Pos(), "second argument to errors.As must be a pointer to an interface or a type implementing error")
 		}

go/analysis/passes/errorsas/testdata/src/a/a.go

@@ -18,6 +18,8 @@ type iface interface {
 	m()
 }
 
+func two() (error, interface{}) { return nil, nil }
+
 func _() {
 	var (
 		e  error
@@ -37,4 +39,5 @@ func _() {
 	errors.As(nil, m)   // want `second argument to errors.As must be a pointer to an interface or a type implementing error`
 	errors.As(nil, f)   // want `second argument to errors.As must be a pointer to an interface or a type implementing error`
 	errors.As(nil, &i)  // want `second argument to errors.As must be a pointer to an interface or a type implementing error`
+	errors.As(two())
 }