Callbacks are widely used in google3. Conceptually, a ResultCallback1<bool, T> is just a predicate on argument of type T. Naturally, we sometimes would want to use such a callback as a matcher.
gMock gives you two function templates in namespace testing to turn callbacks into matchers.
The first is Truly(callback). It matches argument iff callback->Run(argument) returns true.
The second is AddressSatisfies(callback), which matches argument whenever callback->Run(&argument) returns true.
The callbacks used in Truly() and AddressSatisfies() must be permanent (e.g. those returned by NewPermanentCallback()), or you‘ll get a run-time error. The matcher takes ownership of the callback, so you don’t need to worry about deleting it.
For examples, see google3/testing/base/internal/gmock_utils/callback-matchers_test.cc.