From ab13fbe5a39de2081dde10e4d800538871fc4cb1 Mon Sep 17 00:00:00 2001
From: Kamil Kisiel <kamil@kamilkisiel.net>
Date: Fri, 20 May 2022 15:30:18 -0700
Subject: [PATCH] Handle embedded nil interfaces.

Fixes #211
---
 errcheck/embedded_walker.go    |  7 +++----
 testdata/embedded_interface.go | 18 ++++++++++++++++++
 2 files changed, 21 insertions(+), 4 deletions(-)
 create mode 100644 testdata/embedded_interface.go

diff --git a/errcheck/embedded_walker.go b/errcheck/embedded_walker.go
index 3b31925..dff3917 100644
--- a/errcheck/embedded_walker.go
+++ b/errcheck/embedded_walker.go
@@ -73,9 +73,8 @@ func walkThroughEmbeddedInterfaces(sel *types.Selection) ([]types.Type, bool) {
 		// define the method, add it to our list, and loop.
 		namedInterfaceT, ok := getEmbeddedInterfaceDefiningMethod(interfaceT, fn)
 		if !ok {
-			// This should be impossible as long as we type-checked: either the
-			// interface or one of its embedded ones must implement the method...
-			panic(fmt.Sprintf("either %v or one of its embedded interfaces must implement %v", currentT, fn))
+			// Returned a nil interface, we are done.
+			break
 		}
 		result = append(result, namedInterfaceT)
 		interfaceT = namedInterfaceT.Underlying().(*types.Interface)
@@ -102,7 +101,7 @@ func getTypeAtFieldIndex(startingAt types.Type, fieldIndex int) types.Type {
 func getEmbeddedInterfaceDefiningMethod(interfaceT *types.Interface, fn *types.Func) (*types.Named, bool) {
 	for i := 0; i < interfaceT.NumEmbeddeds(); i++ {
 		embedded := interfaceT.Embedded(i)
-		if definesMethod(embedded.Underlying().(*types.Interface), fn) {
+		if embedded != nil && definesMethod(embedded.Underlying().(*types.Interface), fn) {
 			return embedded, true
 		}
 	}
diff --git a/testdata/embedded_interface.go b/testdata/embedded_interface.go
new file mode 100644
index 0000000..5618748
--- /dev/null
+++ b/testdata/embedded_interface.go
@@ -0,0 +1,18 @@
+package main
+
+func embeddedInterface() {
+	t := A(T{})
+	t.X()
+}
+
+type T struct{}
+
+func (t T) X() {}
+
+type A interface {
+	B // embeded
+}
+
+type B = interface { // B is not a defined type
+	X()
+}
\ No newline at end of file