Merge pull request #30 from Progmasoft/feature/mocking

Add mocks that report through the assertion path

7 files changed, 1235 insertions, 0 deletions

diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 87b19651..43754fff 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -343,6 +343,7 @@ set(PROGMASOFT_RUNNER_FILES
 # Property testing and snapshots.
 set(PROGMASOFT_LIBRARY_FILES
   "${CATCH_DIR}/src/Progmasoft/ChoiceShrinker.cpp"
+  "${CATCH_DIR}/src/Progmasoft/Mock.cpp"
   "${CATCH_DIR}/src/Progmasoft/StrategyContext.cpp"
   "${CATCH_DIR}/src/Progmasoft/PropertyEngine.cpp"
   "${CATCH_DIR}/src/Progmasoft/Snapshot.cpp"
diff --git a/src/Progmasoft/Catch3.hpp b/src/Progmasoft/Catch3.hpp
index cc2b0632..2e04782a 100644
--- a/src/Progmasoft/Catch3.hpp
+++ b/src/Progmasoft/Catch3.hpp
@@ -14,6 +14,7 @@
 #include <Progmasoft/Catch3/Generators/Text.hpp>
 #include <Progmasoft/Catch3/Generators/Tuple.hpp>
 #include <Progmasoft/Catch3/Generators/Vector.hpp>
+#include <Progmasoft/Catch3/Mock.hpp>
 #include <Progmasoft/Catch3/Property.hpp>
 #include <Progmasoft/Catch3/Random.hpp>
 #include <Progmasoft/Catch3/Relations.hpp>
diff --git a/src/Progmasoft/Catch3/Internal/RunContext.cpp b/src/Progmasoft/Catch3/Internal/RunContext.cpp
index e86be2e4..30af4533 100644
--- a/src/Progmasoft/Catch3/Internal/RunContext.cpp
+++ b/src/Progmasoft/Catch3/Internal/RunContext.cpp
@@ -356,6 +356,11 @@ namespace Catch {
     RunContext::~RunContext() {
         updateTotalsFromAtomics();
         m_reporter->testRunEnded(TestRunStats(m_runInfo, m_totals, aborting()));
+        // What outlives the run, such as a mock of static lifetime, must not
+        // find a capture that is gone.
+        if ( getCurrentContext().getResultCapture() == this ) {
+            getCurrentMutableContext().setResultCapture( nullptr );
+        }
     }
 
     Totals RunContext::runTest(TestCaseHandle const& testCase) {
diff --git a/src/Progmasoft/Catch3/Mock.hpp b/src/Progmasoft/Catch3/Mock.hpp
new file mode 100644
index 00000000..59ad8bb9
--- /dev/null
+++ b/src/Progmasoft/Catch3/Mock.hpp
@@ -0,0 +1,1029 @@
+// Copyright (c) 2026 Progmasoft.
+// SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-2.0
+#pragma once
+
+#include <Progmasoft/Catch3/Internal/Enforce.hpp>
+#include <Progmasoft/Catch3/Internal/SourceLineInfo.hpp>
+#include <Progmasoft/Catch3/Matchers/Matchers.hpp>
+#include <Progmasoft/Catch3/Matchers/Templated.hpp>
+#include <Progmasoft/Catch3/Matchers/Vector.hpp>
+#include <Progmasoft/Catch3/ToString.hpp>
+#include <cstddef>
+#include <exception>
+#include <functional>
+#include <limits>
+#include <memory>
+#include <mutex>
+#include <optional>
+#include <string>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+namespace Progmasoft::Catch3
+{
+    template<typename Signature>
+    class MockExpectation;
+    template<typename Signature>
+    class MockFunction;
+
+    namespace Detail
+    {
+        inline constexpr std::size_t kUnlimitedCalls
+            = std::numeric_limits<std::size_t>::max();
+
+        class MockSequence;
+
+        /// What the checks need to know of an expectation, whatever the
+        /// signature of its mock.
+        struct MockExpectationState final
+        {
+            std::string Description;
+            Catch::SourceLineInfo Line;
+            std::size_t Minimum = 1;
+            std::size_t Maximum = 1;
+            std::size_t Calls = 0;
+            bool Retired = false;
+            std::vector<std::shared_ptr<MockSequence>> Sequences;
+
+            MockExpectationState(std::string description,
+                                 const Catch::SourceLineInfo &line)
+                : Description(std::move(description))
+                , Line(line)
+            {}
+
+            [[nodiscard]] bool
+            Satisfied() const noexcept
+            {
+                return Calls >= Minimum;
+            }
+
+            /// Whether the expectation can take one more call.
+            [[nodiscard]] bool
+            Available() const noexcept
+            {
+                return !Retired && Calls < Maximum;
+            }
+        };
+
+        /// The steps of a sequence. It does not keep them alive: a step whose
+        /// mock is gone was verified with that mock.
+        class MockSequence final
+        {
+        public:
+            void
+            Add(const std::shared_ptr<MockExpectationState> &step);
+
+            /// Moves to `step`, retiring the steps before it. Returns what
+            /// was left undone, empty when the call came in order.
+            [[nodiscard]] std::string
+            Enter(const MockExpectationState &step);
+
+        private:
+            std::vector<std::weak_ptr<MockExpectationState>> steps;
+            std::size_t current = 0;
+        };
+
+        /// All mocks share one lock, since a sequence can span them.
+        [[nodiscard]] std::unique_lock<std::recursive_mutex>
+        LockMocks();
+
+        /// Fails the running test case at `line` and never throws, since a
+        /// mock can stand in for a `noexcept` function or be in a destructor.
+        /// Outside of a test case the message goes to standard error.
+        void
+        ReportMockFailure(const Catch::SourceLineInfo &line,
+                          const std::string &mock,
+                          std::string message);
+
+        /// Fails the running test case and leaves it, for a call nothing can
+        /// return from. In a `noexcept` function that ends the process.
+        [[noreturn]] void
+        AbortMockCall(const Catch::SourceLineInfo &line,
+                      const std::string &mock,
+                      std::string message);
+
+        /// Counts a met expectation as a passed assertion.
+        void
+        ReportMockSuccess(const Catch::SourceLineInfo &line,
+                          const std::string &mock,
+                          std::string message);
+
+        /// "exactly 2 calls", "at least 1 call", "any number of calls".
+        [[nodiscard]] std::string
+        DescribeCalls(std::size_t minimum, std::size_t maximum);
+
+        /// "called once" and the like, for a report.
+        [[nodiscard]] std::string
+        DescribeCount(std::size_t calls);
+
+        template<std::size_t Index, typename Signature>
+        struct MockArgumentOf;
+
+        template<std::size_t Index, typename... Arguments>
+        struct MockArgumentOf<Index, void(Arguments...)>
+        {
+            using Type = std::tuple_element_t<Index, std::tuple<Arguments...>>;
+        };
+
+        /// The type of a parameter of a mocked method.
+        template<std::size_t Index, typename Signature>
+        using MockArgument = typename MockArgumentOf<Index, Signature>::Type;
+
+        template<typename T>
+        struct IsComposedMatcher: std::false_type
+        {
+        };
+        template<typename T>
+        struct IsComposedMatcher<Catch::Matchers::Detail::MatchAllOf<T>>
+            : std::true_type
+        {
+        };
+        template<typename T>
+        struct IsComposedMatcher<Catch::Matchers::Detail::MatchAnyOf<T>>
+            : std::true_type
+        {
+        };
+        template<typename T>
+        struct IsComposedMatcher<Catch::Matchers::Detail::MatchNotOf<T>>
+            : std::true_type
+        {
+        };
+        template<typename... Ts>
+        struct IsComposedMatcher<
+            Catch::Matchers::Detail::MatchAllOfGeneric<Ts...>>: std::true_type
+        {
+        };
+        template<typename... Ts>
+        struct IsComposedMatcher<
+            Catch::Matchers::Detail::MatchAnyOfGeneric<Ts...>>: std::true_type
+        {
+        };
+        template<typename T>
+        struct IsComposedMatcher<Catch::Matchers::Detail::MatchNotOfGeneric<T>>
+            : std::true_type
+        {
+        };
+
+        /// Catch's vector matchers refer to what they compare with.
+        template<typename T>
+        struct HoldsReference: std::false_type
+        {
+        };
+        template<typename T, typename Alloc>
+        struct HoldsReference<
+            Catch::Matchers::VectorContainsElementMatcher<T, Alloc>>
+            : std::true_type
+        {
+        };
+        template<typename T, typename AllocComp, typename AllocMatch>
+        struct HoldsReference<
+            Catch::Matchers::ContainsMatcher<T, AllocComp, AllocMatch>>
+            : std::true_type
+        {
+        };
+        template<typename T, typename AllocComp, typename AllocMatch>
+        struct HoldsReference<
+            Catch::Matchers::EqualsMatcher<T, AllocComp, AllocMatch>>
+            : std::true_type
+        {
+        };
+        template<typename T, typename AllocComp, typename AllocMatch>
+        struct HoldsReference<
+            Catch::Matchers::ApproxMatcher<T, AllocComp, AllocMatch>>
+            : std::true_type
+        {
+        };
+        template<typename T, typename AllocComp, typename AllocMatch>
+        struct HoldsReference<
+            Catch::Matchers::UnorderedEqualsMatcher<T, AllocComp, AllocMatch>>
+            : std::true_type
+        {
+        };
+
+        /// Whether two integers have the same value, whatever their types.
+        template<typename Left, typename Right>
+        [[nodiscard]] constexpr bool
+        SameInteger(Left left, Right right) noexcept
+        {
+            if constexpr (std::is_signed_v<Left> == std::is_signed_v<Right>)
+            {
+                return left == right;
+            }
+            else if constexpr (std::is_signed_v<Left>)
+            {
+                return left >= 0
+                       && static_cast<std::make_unsigned_t<Left>>(left)
+                              == right;
+            }
+            else
+            {
+                return right >= 0
+                       && left
+                              == static_cast<std::make_unsigned_t<Right>>(
+                                  right);
+            }
+        }
+
+        /// How one argument of a call is judged.
+        template<typename Argument>
+        struct ArgumentMatcher final
+        {
+            std::function<bool(const Argument &)> Test;
+            std::string Description;
+        };
+    } // namespace Detail
+
+    /// Accepts any value of an argument.
+    struct AnyArgument final
+    {};
+    inline constexpr AnyArgument Any{};
+
+    /// Accepts the values `predicate` accepts; `description` names it in
+    /// reports.
+    template<typename Predicate>
+    struct WhereArgument final
+    {
+        Predicate Test;
+        std::string Description;
+    };
+
+    template<typename Predicate>
+    [[nodiscard]] WhereArgument<std::decay_t<Predicate>>
+    Where(Predicate &&predicate, std::string description)
+    {
+        return { std::forward<Predicate>(predicate), std::move(description) };
+    }
+
+    namespace Detail
+    {
+        template<typename Argument, typename Expected>
+        [[nodiscard]] ArgumentMatcher<Argument>
+        MakeArgumentMatcher(Expected &&expected)
+        {
+            using Stored = std::decay_t<Expected>;
+            if constexpr (std::is_same_v<Stored, AnyArgument>)
+            {
+                return { [](const Argument &) {
+                            return true;
+                        },
+                         "any" };
+            }
+            else if constexpr (std::is_base_of_v<
+                                   Catch::Matchers::MatcherUntypedBase,
+                                   Stored>)
+            {
+                static_assert(!IsComposedMatcher<Stored>::value,
+                              "A composed matcher refers to temporaries that "
+                              "are gone when the mock is called; use Where.");
+                static_assert(!HoldsReference<Stored>::value,
+                              "A vector matcher refers to its vector, which "
+                              "can be gone when the mock is called; use "
+                              "Where.");
+                auto matcher = std::make_shared<Stored>(
+                    std::forward<Expected>(expected));
+                auto description = matcher->toString();
+                return { [matcher](const Argument &value) {
+                            return matcher->match(value);
+                        },
+                         std::move(description) };
+            }
+            else if constexpr (!std::is_null_pointer_v<Stored>
+                               && std::is_constructible_v<std::string, Stored>
+                               && std::is_constructible_v<std::string,
+                                                          Argument>)
+            {
+                // Strings are compared by value, also as `const char *`.
+                if constexpr (std::is_pointer_v<Stored>)
+                {
+                    if (expected == nullptr)
+                    {
+                        return { []([[maybe_unused]] const Argument &value) {
+                                    if constexpr (std::is_pointer_v<Argument>)
+                                    {
+                                        return value == nullptr;
+                                    }
+                                    else
+                                    {
+                                        return false;
+                                    }
+                                },
+                                 "nullptr" };
+                    }
+                }
+                std::string text{ std::forward<Expected>(expected) };
+                auto description = Catch::Detail::stringify(text);
+                return { [text = std::move(text)](const Argument &value) {
+                            if constexpr (std::is_pointer_v<Argument>)
+                            {
+                                return value != nullptr && text == value;
+                            }
+                            else
+                            {
+                                return text == value;
+                            }
+                        },
+                         std::move(description) };
+            }
+            else if constexpr (std::is_integral_v<Argument>
+                               && std::is_integral_v<Stored>
+                               && !std::is_same_v<Argument, bool>
+                               && !std::is_same_v<Stored, bool>)
+            {
+                // A literal is an int, whatever the type of the parameter.
+                const Stored value = expected;
+                return { [value](const Argument &argument) {
+                            return SameInteger(argument, value);
+                        },
+                         Catch::Detail::stringify(value) };
+            }
+            else
+            {
+                auto description = Catch::Detail::stringify(expected);
+                return { [value = std::forward<Expected>(expected)](
+                             const Argument &argument) {
+                            return argument == value;
+                        },
+                         std::move(description) };
+            }
+        }
+
+        template<typename Argument, typename Predicate>
+        [[nodiscard]] ArgumentMatcher<Argument>
+        MakeArgumentMatcher(WhereArgument<Predicate> where)
+        {
+            return { std::move(where.Test), std::move(where.Description) };
+        }
+    } // namespace Detail
+
+    /// Calls that have to come in the order their expectations were declared
+    /// in, across mocks.
+    class Sequence final
+    {
+    public:
+        Sequence()
+            : state(std::make_shared<Detail::MockSequence>())
+        {}
+
+    private:
+        template<typename Signature>
+        friend class MockExpectation;
+
+        std::shared_ptr<Detail::MockSequence> state;
+    };
+
+    /// One expected kind of call of a mock. It expects one call until told
+    /// otherwise.
+    template<typename Result, typename... Arguments>
+    class MockExpectation<Result(Arguments...)> final
+    {
+    public:
+        MockExpectation &
+        Times(std::size_t count)
+        {
+            return Between(count, count);
+        }
+
+        MockExpectation &
+        AtLeast(std::size_t count)
+        {
+            return Between(count, Detail::kUnlimitedCalls);
+        }
+
+        MockExpectation &
+        AtMost(std::size_t count)
+        {
+            return Between(0, count);
+        }
+
+        MockExpectation &
+        AnyTimes()
+        {
+            return Between(0, Detail::kUnlimitedCalls);
+        }
+
+        MockExpectation &
+        Never()
+        {
+            return Between(0, 0);
+        }
+
+        MockExpectation &
+        Between(std::size_t minimum, std::size_t maximum)
+        {
+            CATCH_ENFORCE(minimum <= maximum,
+                          "A mock expectation needs its minimum at most its "
+                          "maximum");
+            const auto lock = Detail::LockMocks();
+            state->Minimum = minimum;
+            state->Maximum = maximum;
+            return *this;
+        }
+
+        /// Takes this step in `sequence`, after the ones declared before it.
+        MockExpectation &
+        InSequence(const Sequence &sequence)
+        {
+            const auto lock = Detail::LockMocks();
+            sequence.state->Add(state);
+            state->Sequences.push_back(sequence.state);
+            return *this;
+        }
+
+        /// Returns `value`. A type that cannot be copied is moved out on the
+        /// first call; a reference result refers to `value`, which has to
+        /// outlive the mock.
+        template<typename Value>
+        MockExpectation &
+        Returns(Value &&value)
+        {
+            static_assert(!std::is_void_v<Result>,
+                          "A mock of a void function returns nothing.");
+            if constexpr (std::is_reference_v<Result>)
+            {
+                static_assert(std::is_lvalue_reference_v<Value>,
+                              "A reference result needs an object that "
+                              "outlives the mock.");
+                auto *target = &value;
+                return Does([target](Arguments...) -> Result {
+                    return static_cast<Result>(*target);
+                });
+            }
+            else if constexpr (std::is_copy_constructible_v<Result>)
+            {
+                return Does([held = static_cast<Result>(std::forward<Value>(
+                                 value))](Arguments...) -> Result {
+                    return held;
+                });
+            }
+            else
+            {
+                auto held
+                    = std::make_shared<Result>(std::forward<Value>(value));
+                return Does([held](Arguments...) -> Result {
+                    // Calls can come from several threads.
+                    const auto lock = Detail::LockMocks();
+                    return std::move(*held);
+                });
+            }
+        }
+
+        /// Runs `action` with the arguments of each call; what it returns is
+        /// what the call returns.
+        template<typename Action>
+        MockExpectation &
+        Does(Action &&work)
+        {
+            const auto lock = Detail::LockMocks();
+            action = std::make_shared<std::function<Result(Arguments...)>>(
+                std::forward<Action>(work));
+            return *this;
+        }
+
+        /// Throws `exception` from each call.
+        template<typename Exception>
+        MockExpectation &
+        Throws(Exception exception)
+        {
+            return Does(
+                [exception = std::move(exception)](Arguments...) -> Result {
+                    Catch::throw_exception(exception);
+                });
+        }
+
+        /// What the arguments of a call have to match.
+        using Matchers
+            = std::tuple<Detail::ArgumentMatcher<std::decay_t<Arguments>>...>;
+
+    private:
+        template<typename Signature>
+        friend class MockFunction;
+
+        MockExpectation(std::string description,
+                        const Catch::SourceLineInfo &line,
+                        std::unique_ptr<Matchers> accepted)
+            : state(std::make_shared<Detail::MockExpectationState>(
+                  std::move(description),
+                  line))
+            , matchers(std::move(accepted))
+        {}
+
+        template<std::size_t... Indices>
+        [[nodiscard]] bool
+        Accepts([[maybe_unused]] const std::tuple<
+                    const std::decay_t<Arguments> &...> &values,
+                std::index_sequence<Indices...>) const
+        {
+            return (std::get<Indices>(*matchers).Test(std::get<Indices>(values))
+                    && ...);
+        }
+
+        [[nodiscard]] bool
+        Accepts(
+            const std::tuple<const std::decay_t<Arguments> &...> &values) const
+        {
+            return matchers == nullptr
+                   || Accepts(values, std::index_sequence_for<Arguments...>{});
+        }
+
+        std::shared_ptr<Detail::MockExpectationState> state;
+        // Empty when any arguments will do.
+        std::unique_ptr<Matchers> matchers;
+        std::shared_ptr<std::function<Result(Arguments...)>> action;
+    };
+
+    /// A mock of one function: a callable that checks its calls against the
+    /// expectations set on it and fails the running test case when they do
+    /// not hold. It is what `CATCH3_MOCK_METHOD` declares, and on its own it
+    /// stands in for a free function behind a seam. Its destructor verifies
+    /// it.
+    template<typename Result, typename... Arguments>
+    class MockFunction<Result(Arguments...)> final
+    {
+        using Expectation = MockExpectation<Result(Arguments...)>;
+
+    public:
+        /// Declares an expectation at `line`; called with what the arguments
+        /// have to match. See `CATCH3_EXPECT_CALL`.
+        class ExpectationStart final
+        {
+        public:
+            template<typename... Expected>
+            Expectation &
+            operator()(Expected &&...expected)
+            {
+                constexpr auto arguments = sizeof...(Arguments);
+                static_assert(sizeof...(Expected) == 0
+                                  || sizeof...(Expected) == arguments,
+                              "Match every argument of the mock, or none to "
+                              "accept any.");
+                if constexpr (sizeof...(Expected) == arguments
+                              && arguments != 0)
+                {
+                    return mock.Add(
+                        line,
+                        std::make_unique<typename Expectation::Matchers>(
+                            Detail::MakeArgumentMatcher<
+                                std::decay_t<Arguments>>(
+                                std::forward<Expected>(expected))...));
+                }
+                else
+                {
+                    return mock.Add(line, nullptr);
+                }
+            }
+
+        private:
+            friend class MockFunction;
+
+            ExpectationStart(MockFunction &function,
+                             const Catch::SourceLineInfo &place)
+                : mock(function)
+                , line(place)
+            {}
+
+            MockFunction &mock;
+            Catch::SourceLineInfo line;
+        };
+
+        /// `place` is where a call no expectation takes is reported when the
+        /// mock has no expectations to point at.
+        explicit MockFunction(std::string mockName = "mock",
+                              const Catch::SourceLineInfo &place
+                              = CATCH_INTERNAL_LINEINFO)
+            : name(std::move(mockName))
+            , declared(place)
+            , exceptions(std::uncaught_exceptions())
+        {}
+
+        MockFunction(const MockFunction &) = delete;
+        MockFunction &
+        operator=(const MockFunction &) = delete;
+
+        ~MockFunction()
+        {
+            // A test case that is being left has failed for its own reason,
+            // and what it did not get to call says nothing more.
+            if (std::uncaught_exceptions() <= exceptions)
+            {
+                VerifyAll();
+            }
+        }
+
+        [[nodiscard]] ExpectationStart
+        Expect(const Catch::SourceLineInfo &line)
+        {
+            return { *this, line };
+        }
+
+        /// Checks the expectations now and forgets them, so that a test case
+        /// can go on with new ones.
+        void
+        Verify()
+        {
+            VerifyAll();
+        }
+
+        Result
+        operator()(Arguments... arguments)
+        {
+            const Decision decision = Decide(
+                std::tuple<const std::decay_t<Arguments> &...>{ arguments... });
+            if (!decision.line)
+            {
+                return Refuse(decision.message);
+            }
+            if (!decision.message.empty())
+            {
+                Detail::ReportMockFailure(*decision.line,
+                                          name,
+                                          decision.message);
+            }
+            if (decision.action != nullptr)
+            {
+                return (*decision.action)(
+                    std::forward<Arguments>(arguments)...);
+            }
+            if constexpr (std::is_void_v<Result>)
+            {
+                return;
+            }
+            else if constexpr (std::is_default_constructible_v<Result>)
+            {
+                return Result{};
+            }
+            else
+            {
+                Detail::AbortMockCall(*decision.line,
+                                      name,
+                                      "nothing to return from " + name
+                                          + ": give the expectation Returns "
+                                            "or Does");
+            }
+        }
+
+        /// A function that calls this mock, which has to outlive it.
+        [[nodiscard]] std::function<Result(Arguments...)>
+        AsFunction()
+        {
+            return [this](Arguments... arguments) -> Result {
+                return (*this)(std::forward<Arguments>(arguments)...);
+            };
+        }
+
+    private:
+        struct Decision final
+        {
+            // Empty when the call is refused.
+            std::optional<Catch::SourceLineInfo> line;
+            std::shared_ptr<std::function<Result(Arguments...)>> action;
+            std::string message;
+        };
+
+        Expectation &
+        Add(const Catch::SourceLineInfo &line,
+            std::unique_ptr<typename Expectation::Matchers> matchers)
+        {
+            auto description = Describe(matchers.get());
+            const auto lock = Detail::LockMocks();
+            expectations.push_back(std::unique_ptr<Expectation>(
+                new Expectation(std::move(description),
+                                line,
+                                std::move(matchers))));
+            return *expectations.back();
+        }
+
+        template<std::size_t... Indices>
+        [[nodiscard]] std::string
+        DescribeMatchers(
+            [[maybe_unused]] const typename Expectation::Matchers &matchers,
+            std::index_sequence<Indices...>) const
+        {
+            std::string text;
+            ((text += (Indices == 0 ? "" : ", ")
+                      + std::get<Indices>(matchers).Description),
+             ...);
+            return text;
+        }
+
+        [[nodiscard]] std::string
+        Describe(const typename Expectation::Matchers *matchers) const
+        {
+            if (matchers != nullptr)
+            {
+                return name + "("
+                       + DescribeMatchers(
+                           *matchers,
+                           std::index_sequence_for<Arguments...>{})
+                       + ")";
+            }
+            return sizeof...(Arguments) == 0 ? name + "()" : name + "(...)";
+        }
+
+        template<std::size_t... Indices>
+        [[nodiscard]] std::string
+        DescribeCall([[maybe_unused]] const std::tuple<
+                         const std::decay_t<Arguments> &...> &values,
+                     std::index_sequence<Indices...>) const
+        {
+            std::string text = name + "(";
+            ((text += (Indices == 0 ? "" : ", ")
+                      + Catch::Detail::stringify(std::get<Indices>(values))),
+             ...);
+            return text + ")";
+        }
+
+        /// Picks the first declared expectation that accepts the call and can
+        /// take it, and counts the call.
+        [[nodiscard]] Decision
+        Decide(const std::tuple<const std::decay_t<Arguments> &...> &values)
+        {
+            const auto lock = Detail::LockMocks();
+            const Expectation *used = nullptr;
+            for (const auto &expectation : expectations)
+            {
+                if (!expectation->Accepts(values))
+                {
+                    continue;
+                }
+                if (expectation->state->Available())
+                {
+                    return Take(*expectation);
+                }
+                used = expectation.get();
+            }
+            const auto call
+                = DescribeCall(values, std::index_sequence_for<Arguments...>{});
+            Decision refused;
+            refused.message = used != nullptr ? DescribeUsed(call, *used->state)
+                                              : DescribeUnexpected(call);
+            return refused;
+        }
+
+        [[nodiscard]] Decision
+        Take(const Expectation &expectation)
+        {
+            auto &state = *expectation.state;
+            ++state.Calls;
+            Decision decision;
+            decision.line = state.Line;
+            decision.action = expectation.action;
+            for (const auto &sequence : state.Sequences)
+            {
+                decision.message += sequence->Enter(state);
+            }
+            return decision;
+        }
+
+        [[nodiscard]] static std::string
+        DescribeUsed(const std::string &call,
+                     const Detail::MockExpectationState &state)
+        {
+            return state.Retired ? "call out of order: " + call
+                                       + ", its step of the sequence has passed"
+                                 : "too many calls: " + call + ", expected "
+                                       + Detail::DescribeCalls(state.Minimum,
+                                                               state.Maximum)
+                                       + " of " + state.Description;
+        }
+
+        [[nodiscard]] std::string
+        DescribeUnexpected(const std::string &call) const
+        {
+            std::string text = "unexpected call: " + call;
+            if (expectations.empty())
+            {
+                return text + ", nothing was expected of this mock";
+            }
+            text += ", expected";
+            for (const auto &expectation : expectations)
+            {
+                text += "\n  " + expectation->state->Description;
+            }
+            return text;
+        }
+
+        /// Reports a call no expectation takes, and returns what the call
+        /// can return.
+        Result
+        Refuse(const std::string &message)
+        {
+            const auto line = FirstLine();
+            if constexpr (std::is_void_v<Result>)
+            {
+                Detail::ReportMockFailure(line, name, message);
+            }
+            else if constexpr (std::is_default_constructible_v<Result>)
+            {
+                Detail::ReportMockFailure(line, name, message);
+                return Result{};
+            }
+            else
+            {
+                Detail::AbortMockCall(line, name, message);
+            }