1 files changed, 21 insertions, 7 deletions
diff --git a/go/analysis/passes/errorsas/errorsas.go b/go/analysis/passes/errorsas/errorsas.go
index 384f02557..96adad3ee 100644
--- a/go/analysis/passes/errorsas/errorsas.go
+++ b/go/analysis/passes/errorsas/errorsas.go
@@ -7,6 +7,7 @@
 package errorsas
 
 import (
+	"errors"
 	"go/ast"
 	"go/types"
 
@@ -50,26 +51,39 @@ func run(pass *analysis.Pass) (interface{}, error) {
 		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.ReportRangef(call, "second argument to errors.As must be a non-nil pointer to either a type that implements error, or to any interface type")
+		if fn.FullName() != "errors.As" {
+			return
+		}
+		if err := checkAsTarget(pass, call.Args[1]); err != nil {
+			pass.ReportRangef(call, "%v", err)
 		}
 	})
 	return nil, nil
 }
 
-var errorType = types.Universe.Lookup("error").Type().Underlying().(*types.Interface)
+var errorType = types.Universe.Lookup("error").Type()
 
 // pointerToInterfaceOrError reports whether the type of e is a pointer to an interface or a type implementing error,
 // or is the empty interface.
-func pointerToInterfaceOrError(pass *analysis.Pass, e ast.Expr) bool {
+
+// checkAsTarget reports an error if the second argument to errors.As is invalid.
+func checkAsTarget(pass *analysis.Pass, e ast.Expr) error {
 	t := pass.TypesInfo.Types[e].Type
 	if it, ok := t.Underlying().(*types.Interface); ok && it.NumMethods() == 0 {
-		return true
+		// A target of interface{} is always allowed, since it often indicates
+		// a value forwarded from another source.
+		return nil
 	}
 	pt, ok := t.Underlying().(*types.Pointer)
 	if !ok {
-		return false
+		return errors.New("second argument to errors.As must be a non-nil pointer to either a type that implements error, or to any interface type")
+	}
+	if pt.Elem() == errorType {
+		return errors.New("second argument to errors.As should not be *error")
 	}
 	_, ok = pt.Elem().Underlying().(*types.Interface)
-	return ok || types.Implements(pt.Elem(), errorType)
+	if ok || types.Implements(pt.Elem(), errorType.Underlying().(*types.Interface)) {
+		return nil
+	}
+	return errors.New("second argument to errors.As must be a non-nil pointer to either a type that implements error, or to any interface type")
 }