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gmock-actions: provide a const reference when converting in ReturnAction.

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It doesn't make semantic sense for the conversion to modify the input, and the
fact that it's allowed to do so appears to have just been a historical accident.

PiperOrigin-RevId: 448135555
Change-Id: Id10f17af38cf3947ee25fe10654d97527173ebfc
This commit is contained in:
Aaron Jacobs 2022-05-11 19:00:34 -07:00 committed by Copybara-Service
parent 5e6a533680
commit cbca6bc395
2 changed files with 67 additions and 47 deletions
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47
googlemock/include/gmock/gmock-actions.h
View File

@ -322,6 +322,12 @@ struct is_callable_r_impl<void_t<call_result_t<F, Args...>>, R, F, Args...>
template <typename R, typename F, typename... Args>
using is_callable_r = is_callable_r_impl<void, R, F, Args...>;
// Like std::as_const from C++17.
template <typename T>
typename std::add_const<T>::type& as_const(T& t) {
return t;
}
} // namespace internal
// Specialized for function types below.
@ -872,17 +878,14 @@ class ReturnAction final {
public:
explicit ReturnAction(R value) : value_(std::move(value)) {}
// Support conversion to function types with compatible return types. See the
// documentation on Return for the definition of compatible.
template <typename U, typename... Args>
template <typename U, typename... Args,
typename = typename std::enable_if<conjunction<
// See the requirements documented on Return.
negation<std::is_same<void, U>>, //
negation<std::is_reference<U>>, //
std::is_convertible<const R&, U>, //
std::is_copy_constructible<U>>::value>::type>
operator Action<U(Args...)>() const { // NOLINT
// Check our requirements on the return type.
static_assert(!std::is_reference<U>::value,
"use ReturnRef instead of Return to return a reference");
static_assert(!std::is_void<U>::value,
"Can't use Return() on an action expected to return `void`.");
return Impl<U>(value_);
}
@ -918,27 +921,7 @@ class ReturnAction final {
// that does `return R()` requires R to be implicitly convertible to
// U, and uses that path for the conversion, even U Result has an
// explicit constructor from R.
//
// We provide non-const access to input_value to the conversion
// code. It's not clear whether this makes semantic sense -- what
// would it mean for the conversion to modify the input value? This
// appears to be an accident of history:
//
// 1. Before the first public commit the input value was simply an
// object of type R embedded directly in the Impl object. The
// result value wasn't yet eagerly created, and the Impl class's
// Perform method was const, so the implicit conversion when it
// returned the value was from const R&.
//
// 2. Google changelist 6490411 changed ActionInterface::Perform to
// be non-const, citing the fact that an action can have side
// effects and be stateful. Impl::Perform was updated like all
// other actions, probably without consideration of the fact
// that side effects and statefulness don't make sense for
// Return. From this point on the conversion had non-const
// access to the input value.
//
value(ImplicitCast_<U>(input_value)) {}
value(ImplicitCast_<U>(internal::as_const(input_value))) {}
// A copy of the value originally provided by the user. We retain this in
// addition to the value of the mock function's result type below in case
@ -1763,7 +1746,7 @@ internal::WithArgsAction<typename std::decay<InnerAction>::type> WithoutArgs(
// * U is not void.
// * U is not a reference type. (Use ReturnRef instead.)
// * U is copy-constructible.
// * R& is convertible to U.
// * const R& is convertible to U.
//
// The Action<U(Args)...> object contains the R value from which the U return
// value is constructed (a copy of the argument to Return). This means that the

67
googlemock/test/gmock-actions_test.cc
View File

@ -654,8 +654,8 @@ TEST(ReturnTest, AcceptsStringLiteral) {
TEST(ReturnTest, SupportsReferenceLikeReturnType) {
// A reference wrapper for std::vector<int>, implicitly convertible from it.
struct Result {
std::vector<int>* v;
Result(std::vector<int>& v) : v(&v) {} // NOLINT
const std::vector<int>* v;
Result(const std::vector<int>& v) : v(&v) {} // NOLINT
};
// Set up an action for a mock function that returns the reference wrapper
@ -709,6 +709,56 @@ TEST(ReturnTest, PrefersConversionOperator) {
EXPECT_THAT(mock.AsStdFunction()(), Field(&Out::x, 19));
}
// Return(x) should not be usable with a mock function result type that's
// implicitly convertible from decltype(x) but requires a non-const lvalue
// reference to the input. It doesn't make sense for the conversion operator to
// modify the input.
TEST(ReturnTest, ConversionRequiresMutableLvalueReference) {
// Set up a type that is implicitly convertible from std::string&, but not
// std::string&& or `const std::string&`.
//
// Avoid asserting about conversion from std::string on MSVC, which seems to
// implement std::is_convertible incorrectly in this case.
struct S {
S(std::string&) {} // NOLINT
};
static_assert(std::is_convertible<std::string&, S>::value, "");
#ifndef _MSC_VER
static_assert(!std::is_convertible<std::string&&, S>::value, "");
#endif
static_assert(!std::is_convertible<const std::string&, S>::value, "");
// It shouldn't be possible to use the result of Return(std::string) in a
// context where an S is needed.
using RA = decltype(Return(std::string()));
static_assert(!std::is_convertible<RA, Action<S()>>::value, "");
static_assert(!std::is_convertible<RA, OnceAction<S()>>::value, "");
}
// Return(x) should not be usable with a mock function result type that's
// implicitly convertible from decltype(x) but requires an rvalue reference to
// the input. We don't yet support handing over the value for consumption.
TEST(ReturnTest, ConversionRequiresRvalueReference) {
// Set up a type that is implicitly convertible from std::string&& and
// `const std::string&&`, but not `const std::string&`.
struct S {
S(std::string&&) {} // NOLINT
S(const std::string&&) {} // NOLINT
};
static_assert(std::is_convertible<std::string, S>::value, "");
static_assert(!std::is_convertible<const std::string&, S>::value, "");
// It shouldn't be possible to use the result of Return(std::string) in a
// context where an S is needed.
using RA = decltype(Return(std::string()));
static_assert(!std::is_convertible<RA, Action<S()>>::value, "");
static_assert(!std::is_convertible<RA, OnceAction<S()>>::value, "");
}
// Tests that Return(v) is covaraint.
struct Base {
@ -760,19 +810,6 @@ TEST(ReturnTest, ConvertsArgumentWhenConverted) {
<< "when performed.";
}
class DestinationType {};
class SourceType {
public:
// Note: a non-const typecast operator.
operator DestinationType() { return DestinationType(); }
};
TEST(ReturnTest, CanConvertArgumentUsingNonConstTypeCastOperator) {
SourceType s;
Action<DestinationType()> action(Return(s));
}
// Tests that ReturnNull() returns NULL in a pointer-returning function.
TEST(ReturnNullTest, WorksInPointerReturningFunction) {
const Action<int*()> a1 = ReturnNull();