added ExpectImplements() function to test package

gui/sdlimgui/win_rom_select.go

@@ -47,7 +47,6 @@ type winSelectROM struct {
 
 	currPath string
 	entries  []os.DirEntry
-	err      error
 
 	selectedFile           string
 	selectedFileBase       string

test/doc.go

@@ -18,11 +18,14 @@
 //
 // The ExpectEquality() is the most basic and probably the most useful function.
 // It compares like-typed variables for equality and returns true if they match.
+// ExpectInequality() is the inverse function.
 //
 // The ExpectFailure() and ExpectSuccess() functions test for failure and
 // success. These two functions work with bool or error and special handling for
 // nil.
 //
+// ExpectImplements() tests whether an instance implements the specified type.
+//
 // The Writer type meanwhile, implements the io.Writer interface and should be
 // used to capture output. The Writer.Compare() function can then be used to
 // test for equality.

test/expected.go

@@ -34,6 +34,14 @@ func ExpectInequality[T comparable](t *testing.T, value T, expectedValue T) {
 	}
 }
 
+// ExpectImplements tests whether an instance is an implementation of type T
+func ExpectImplements[T comparable](t *testing.T, instance any, implements T) {
+	t.Helper()
+	if _, ok := instance.(T); !ok {
+		t.Fatalf("implementation test of type %T failed: type %T does not implement %T", instance, instance, implements)
+	}
+}
+
 // ExpectFailure tests argument v for a failure condition suitable for it's
 // type. Types bool and error are treated thus:
 //