# Testing

ISO C++ test testsuite performance conformance ABI exception safety

The libstdc++ testsuite includes testing for standard conformance, regressions, ABI, and performance.

## Test Organization

### Directory Layout

The directory `gccsrcdir/libstdc++-v3/testsuite` contains the individual test cases organized in sub-directories corresponding to clauses of the C++ standard (detailed below), the DejaGnu test harness support files, and sources to various testsuite utilities that are packaged in a separate testing library.

All test cases for functionality required by the runtime components of the C++ standard (ISO 14882) are files within the following directories:

        17_intro
        18_support
        19_diagnostics
        20_util
        21_strings
        22_locale
        23_containers
        24_iterators
        25_algorithms
        26_numerics
        27_io
        28_regex
        29_atomics
        30_threads

In addition, the following directories include test files:

`tr1`
  Tests for components as described by the Technical Report on Standard Library Extensions (TR1).
`backward`
  Tests for backwards compatibility and deprecated features.
`demangle`
  Tests for `__cxa_demangle`, the IA-64 C++ ABI demangler.
`ext`
  Tests for extensions.
`performance`
  Tests for performance analysis, and performance regressions.

Some directories don't have test files, but instead contain auxiliary information:

`config`
  Files for the DejaGnu test harness.
`lib`
  Files for the DejaGnu test harness.
`libstdc++*`
  Files for the DejaGnu test harness.
`data`
  Sample text files for testing input and output.
`util`
  Files for libtestc++, utilities and testing routines.

Within a directory that includes test files, there may be additional subdirectories, or files. Originally, test cases were appended to one file that represented a particular section of the chapter under test, and was named accordingly. For instance, to test items related to `21.3.6.1 - `basic_string::find` [lib.string::find]` in the standard, the following was used:

        21_strings/find.cc

However, that practice soon became a liability as the test cases became huge and unwieldy, and testing new or extended functionality (like wide characters or named locales) became frustrating, leading to aggressive pruning of test cases on some platforms that covered up implementation errors. Now, the test suite has a policy of one file, one test case, which solves the above issues and gives finer grained results and more manageable error debugging. As an example, the test case quoted above becomes:

        21_strings/basic_string/find/char/1.cc
        21_strings/basic_string/find/char/2.cc
        21_strings/basic_string/find/char/3.cc
        21_strings/basic_string/find/wchar_t/1.cc
        21_strings/basic_string/find/wchar_t/2.cc
        21_strings/basic_string/find/wchar_t/3.cc

All new tests should be written with the policy of "one test case, one file" in mind.

### Naming Conventions

In addition, there are some special names and suffixes that are used within the testsuite to designate particular kinds of tests.

`_xin.cc`
  This test case expects some kind of interactive input in order to finish or pass. At the moment, the interactive tests are not run by default. Instead, they are run by hand, like:

      g++ 27_io/objects/char/3_xin.cc
      cat 27_io/objects/char/3_xin.in | a.out

`.in`
  This file contains the expected input for the corresponding _xin.cc test case.
`_neg.cc`
  This test case is expected to fail: it's a negative test. At the moment, these are almost always compile time errors.
`char`
  This can either be a directory name or part of a longer file name, and indicates that this file, or the files within this directory are testing the `char` instantiation of a template.
`wchar_t`
  This can either be a directory name or part of a longer file name, and indicates that this file, or the files within this directory are testing the `wchar_t` instantiation of a template. Some hosts do not support `wchar_t` functionality, so for these targets, all of these tests will not be run.
`thread`
  This can either be a directory name or part of a longer file name, and indicates that this file, or the files within this directory are testing situations where multiple threads are being used.
`performance`
  This can either be an enclosing directory name or part of a specific file name. This indicates a test that is used to analyze runtime performance, for performance regression testing, or for other optimization related analysis. At the moment, these test cases are not run by default.

## Running the Testsuite

### Basic

You can check the status of the build without installing it using the DejaGnu harness, much like the rest of the gcc tools, i.e. `make check` in the `libbuilddir` directory, or `make check-target-libstdc++-v3` in the `gccbuilddir` directory.

These commands are functionally equivalent and will create a '`testsuite`' directory underneath `libbuilddir` containing the results of the tests. Two results files will be generated: `libstdc++.sum`, which is a PASS/FAIL summary for each test, and `libstdc++.log` which is a log of the exact command-line passed to the compiler, the compiler output, and the executable output (if any) for each test.

Archives of test results for various versions and platforms are available on the GCC website in the build status section of each individual release, and are also archived on a daily basis on the gcc-testresults mailing list. Please check either of these places for a similar combination of source version, operating system, and host CPU.

### Variations

There are several options for running tests, including testing the regression tests, testing a subset of the regression tests, testing the performance tests, testing just compilation, testing installed tools, etc. In addition, there is a special rule for checking the exported symbols of the shared library.

To debug the DejaGnu test harness during runs, try invoking with a specific argument to the variable `RUNTESTFLAGS`, like so:

        make check-target-libstdc++-v3 RUNTESTFLAGS="-v"

or

        make check-target-libstdc++-v3 RUNTESTFLAGS="-v -v"

To run a subset of the library tests, you can either generate the `testsuite_files` file (described below) by running `make testsuite_files` in the `libbuilddir/testsuite` directory, then edit the file to remove the tests you don't want and then run the testsuite as normal, or you can specify a testsuite and a subset of tests in the `RUNTESTFLAGS` variable.

For example, to run only the tests for containers you could use:

        make check-target-libstdc++-v3 RUNTESTFLAGS="conformance.exp=23_containers/*"

When combining this with other options in `RUNTESTFLAGS` the `testsuite.exp=testfiles` options must come first.

There are two ways to run on a simulator: set up `DEJAGNU` to point to a specially crafted `site.exp`, or pass down `--target_board` flags.

Example flags to pass down for various embedded builds are as follows:

      --target=powerpc-eabisim (libgloss/sim)
      make check-target-libstdc++-v3 RUNTESTFLAGS="--target_board=powerpc-sim"

      --target=calmrisc32 (libgloss/sid)
      make check-target-libstdc++-v3 RUNTESTFLAGS="--target_board=calmrisc32-sid"

      --target=xscale-elf (newlib/sim)
      make check-target-libstdc++-v3 RUNTESTFLAGS="--target_board=arm-sim"

Also, here is an example of how to run the libstdc++ testsuite for a multilibed build directory with different ABI settings:

        make check-target-libstdc++-v3 RUNTESTFLAGS='--target_board \"unix{-mabi=32,,-mabi=64}\"'

You can run the tests with a compiler and library that have already been installed. Make sure that the compiler (e.g., `g++`) is in your `PATH`. If you are using shared libraries, then you must also ensure that the directory containing the shared version of libstdc++ is in your `LD_LIBRARY_PATH`, or equivalent. If your GCC source tree is at `/path/to/gcc`, then you can run the tests as follows:

        runtest --tool libstdc++ --srcdir=/path/to/gcc/libstdc++-v3/testsuite

The testsuite will create a number of files in the directory in which you run this command,. Some of those files might use the same name as files created by other testsuites (like the ones for GCC and G++), so you should not try to run all the testsuites in parallel from the same directory.

In addition, there are some testing options that are mostly of interest to library maintainers and system integrators. As such, these tests may not work on all CPU and host combinations, and may need to be executed in the `libbuilddir/testsuite` directory. These options include, but are not necessarily limited to, the following:

`make testsuite_files`
>

  Five files are generated that determine what test files are run. These files are:

  `testsuite_files`
    This is a list of all the test cases that will be run. Each test case is on a separate line, given with an absolute path from the `libsrcdir/testsuite` directory.
  `testsuite_files_interactive`
    This is a list of all the interactive test cases, using the same format as the file list above. These tests are not run by default.
  `testsuite_files_performance`
    This is a list of all the performance test cases, using the same format as the file list above. These tests are not run by default.
  `testsuite_thread`
    This file indicates that the host system can run tests which involved multiple threads.
  `testsuite_wchar_t`
    This file indicates that the host system can run the `wchar_t` tests, and corresponds to the macro definition `_GLIBCXX_USE_WCHAR_T` in the file `c++config.h`.
`make check-abi`
>

  The library ABI can be tested. This involves testing the shared library against a baseline list of symbol exports that defines the previous version of the ABI. The tests require that no exported symbols are removed, no new symbols are added to the old symbol versions, and any new symbols have the latest symbol version. See Versioning for more details of the ABI version history.

`make new-abi-baseline`
>

  Generate a new baseline set of symbols exported from the library (written to a file under `libsrcdir/config/abi/post/target/`). A different baseline symbols file is needed for each architecture and is used by the `check-abi` target described above. The files are usually re-generated by target maintainers for releases.

`make check-compile`
>

  This rule compiles, but does not link or execute, the `testsuite_files` test cases and displays the output on stdout.

`make check-performance`
>

  This rule runs through the `testsuite_files_performance` test cases and collects information for performance analysis and can be used to spot performance regressions. Various timing information is collected, as well as number of hard page faults, and memory used. This is not run by default, and the implementation is in flux.

`make check-debug`
>

  This rule runs through the test suite under the debug mode.

`make check-parallel`
>

  This rule runs through the test suite under the parallel mode.

We are interested in any strange failures of the testsuite; please email the main libstdc++ mailing list if you see something odd or have questions.

### Permutations

The tests will be compiled with a set of default compiler flags defined by the `libbuilddir/scripts/testsuite_flags` file, as well as options specified in individual tests. You can run the tests with different options by adding them to the output of the `--cxxflags` option of that script, or by setting the `CXXFLAGS` variable when running `make`, or via options for the DejaGnu test framework (described below). The latter approach uses the `--target_board` option that was shown earlier, but requires DejaGnu version 1.5.3 or newer to work reliably, so that the `dg-options` in the test aren't overridden. For example, to run the tests with `-O1 -D_GLIBCXX_ASSERTIONS` you could use:

        make check RUNTESTFLAGS=--target_board=unix/-O1/-D_GLIBCXX_ASSERTIONS

The `--target_board` option can also be used to run the tests multiple times in different variations. For example, to run the entire testsuite three times using `-O3` but with different `-std` options:

        make check 'RUNTESTFLAGS=--target_board=unix/-O3\"{-std=gnu++98,-std=gnu++11,-std=gnu++14}\"'

N.B. that set of variations could also be written as `unix/-O3\"{-std=gnu++98,-std=gnu++11,}\"` so that the third variation would use the default for `-std` (which is `-std=gnu++14` as of GCC 6).

To run the libstdc++ test suite under the debug mode, use `make check-debug`. Alternatively, edit `libbuilddir/scripts/testsuite_flags` to add the compile-time flag `-D_GLIBCXX_DEBUG` to the result printed by the `--cxxflags` option. Additionally, add the `-D_GLIBCXX_DEBUG_PEDANTIC` flag to turn on pedantic checking. The libstdc++ test suite should produce the same results under debug mode that it does under release mode: any deviation indicates an error in either the library or the test suite. Note, however, that the number of tests that PASS may change, because some test cases are skipped in normal mode, and some are skipped in debug mode, as determined by the `dg-require-support` directives described below.

The parallel mode can be tested using `make check-parallel`, or in much the same manner as the debug mode, substituting `-D_GLIBCXX_PARALLEL` for `-D_GLIBCXX_DEBUG` in the previous paragraph.

Or, just run the testsuite `-D_GLIBCXX_DEBUG` or `-D_GLIBCXX_PARALLEL` in `CXXFLAGS` or `RUNTESTFLAGS`.

## Writing a new test case

The first step in making a new test case is to choose the correct directory and file name, given the organization as previously described.

All files are copyright the FSF, and GPL'd: this is very important. The first copyright year should correspond to the date the file was checked in to version control. If a test is copied from an existing file it should retain the copyright years from the original file.

The DejaGnu instructions say to always return `0` from `main` to indicate success. Strictly speaking this is redundant in C++, since returning from `main` is defined to return `0`. Most tests still have an explicit return.

A bunch of utility functions and classes have already been abstracted out into the testsuite utility library, `libtestc++`. To use this functionality, just include the appropriate header file: the library or specific object files will automatically be linked in as part of the testsuite run.

Tests that need to perform runtime checks should use the `VERIFY` macro, defined in the `<testsuite_hooks.h>` header. This usually expands to the standard `assert` macro, but allows targets to define it to something different. In order to support the alternative expansions of `VERIFY`, before any use of the macro there must be a variable called `test` in scope, which is usually defined like so (the attribute avoids warnings about an unused variable):

        bool test __attribute__((unused)) = true;

The testsuite uses the DejaGnu framework to compile and run the tests. Test cases are normal C++ files which contain special directives in comments. These directives look like `{ dg-* ... }` and tell DejaGnu what to do and what kinds of behavior are to be expected for a test. The core DejaGnu directives are documented in the `dg.exp` file installed by DejaGnu. The GCC testsuites support additional directives as described in the GCC internals documentation, see Syntax and Descriptions of test directives. GCC also defines many Keywords describing target attributes (a.k.a effective targets) which can be used where a target selector can appear.

Some directives commonly used in the libstdc++ testsuite are:

`{ dg-do do-what-keyword [{ target/xfail selector }] }`
  Where do-what-keyword is usually one of `run` (which is the default), `compile`, or `link`, and typical selectors are targets such as `*-*-gnu*` or an effective target such as `c++11`.
`{ dg-require-support args }`
  Skip the test if the target does not provide the required support. See below for values of support.
`{ dg-options options [{ target selector }] }`
`{ dg-error regexp [ comment [{ target/xfail selector } [line] ]] }`
`{ dg-excess-errors comment [{ target/xfail selector }] }`

For full details of these and other directives see the main GCC DejaGnu documentation in the internals manual.

Test cases that use features of a particular C++ standard should specify the minimum required standard as an effective target:

        // { dg-do run { target c++11 } }

or

        // { dg-require-effective-target c++11 }

Specifying the minimum required standard for a test allows it to be run using later standards, so that we can verify that C++11 components still work correctly when compiled as C++14 or later. Specifying a minimum also means the test will be skipped if the test is compiled using an older standard, e.g. using `RUNTESTFLAGS=--target_board=unix/-std=gnu++98`.

It is possible to indicate that a test should only be run for a specific standard (and not later standards) using an effective target like `c++11_only`. However, this means the test will be skipped by default (because the default mode is `gnu++14`), and so will only run when `-std=gnu++11` or `-std=c++11` is used explicitly. For tests that require a specific standard it is better to use a `dg-options` directive:

        // { dg-options "-std=gnu++11" }

This means the test will not get skipped by default, and will always use the specific standard dialect that the test requires. This isn't needed often, and most tests should use an effective target to specify a minimum standard instead, to allow them to be tested for all possible variations.

Similarly, tests which depend on a newer standard than the default must use `dg-options` instead of (or in addition to) an effective target, so that they are not skipped by default. For example, tests for C++17 features should use

        // { dg-options "-std=gnu++17" }

before any `dg-do` such as:

        // { dg-do run "c++17" }

The `dg-options` directive must come first, so that the `-std` flag has already been added to the options before checking the `c++17` target.

### Examples of Test Directives

Example 1: Testing compilation only:

    // { dg-do compile }

Example 2: Testing for expected warnings on line 36, which all targets fail:

    // { dg-warning "string literals" "" { xfail *-*-* } 36 }

Example 3: Testing for expected warnings on line 36:

    // { dg-warning "string literals" "" { target *-*-* } 36 }

Example 4: Testing for compilation errors on line 41:

    // { dg-do compile }
    // { dg-error "no match for" "" { target *-*-* } 41 }

Example 5: Testing with special command line settings, or without the use of pre-compiled headers, in particular the `stdc++.h.gch` file. Any options here will override the `DEFAULT_CXXFLAGS` and `PCH_CXXFLAGS` set up in the `normal.exp` file:

    // { dg-options "-O0" { target *-*-* } }

Example 6: Compiling and linking a test only for C++14 and later, and only if Debug Mode is active:

    // { dg-do link { target c++14 } }
    // { dg-require-debug-mode "" }

Example 7: Running a test only on x86 targets, and only for C++11 and later, with specific options, and additional options for 32-bit x86:

    // { dg-options "-fstrict-enums" }
    // { dg-additional-options "-march=i486" { target ia32 } }
    // { dg-do run { target { ia32 || x86_64-*-* } } }
    // { dg-require-effective-target "c++11" }

More examples can be found in the `libstdc++-v3/testsuite/*/*.cc` files.

### Directives Specific to Libstdc++ Tests

In addition to the usual Variants of `dg-require-support` several more directives are available for use in libstdc++ tests, including the following:

`dg-require-namedlocale` name

  The named locale must be available.

`dg-require-debug-mode ""`

  Skip the test if the Debug Mode is not active (as determined by the `_GLIBCXX_DEBUG` macro).

`dg-require-parallel-mode ""`

  Skip the test if the Parallel Mode is not active (as determined by the `_GLIBCXX_PARALLEL` macro).

`dg-require-profile-mode ""`

  Skip the test if the Profile Mode is not active (as determined by the `_GLIBCXX_PROFILE` macro).

`dg-require-normal-mode ""`

  Skip the test if any of Debug, Parallel or Profile Mode is active.

`dg-require-atomic-builtins ""`

  Skip the test if atomic operations on bool and int are not lock-free.

`dg-require-gthreads ""`

  Skip the test if the C++11 thread library is not supported, as determined by the `_GLIBCXX_HAS_GTHREADS` macro.

`dg-require-gthreads-timed ""`

  Skip the test if C++11 timed mutexes are not supported, as determined by the `_GLIBCXX_HAS_GTHREADS` and `_GTHREAD_USE_MUTEX_TIMEDLOCK` macros.

`dg-require-string-conversions ""`

  Skip the test if the C++11 `to_string` and `stoi`, `stod` etc. functions are not fully supported (including wide character versions).

`dg-require-filesystem-ts ""`

  Skip the test if the Filesystem TS is not supported.

## Test Harness and Utilities

### DejaGnu Harness Details

Underlying details of testing for conformance and regressions are abstracted via the GNU DejaGnu package. This is similar to the rest of GCC.

This is information for those looking at making changes to the testsuite structure, and/or needing to trace DejaGnu's actions with `--verbose`. This will not be useful to people who are "merely" adding new tests to the existing structure.

The first key point when working with DejaGnu is the idea of a "tool". Files, directories, and functions are all implicitly used when they are named after the tool in use. Here, the tool will always be "libstdc++".

The `lib` subdir contains support routines. The `lib/libstdc++.exp` file ("support library") is loaded automagically, and must explicitly load the others. For example, files can be copied from the core compiler's support directory into `lib`.

Some routines in `lib/libstdc++.exp` are callbacks, some are our own. Callbacks must be prefixed with the name of the tool. To easily distinguish the others, by convention our own routines are named "v3-*".

The next key point when working with DejaGnu is "test files". Any directory whose name starts with the tool name will be searched for test files. (We have only one.) In those directories, any `.exp` file is considered a test file, and will be run in turn. Our main test file is called `normal.exp`; it runs all the tests in testsuite_files using the callbacks loaded from the support library.

The `config` directory is searched for any particular "target board" information unique to this library. This is currently unused and sets only default variables.

### Utilities

The testsuite directory also contains some files that implement functionality that is intended to make writing test cases easier, or to avoid duplication, or to provide error checking in a way that is consistent across platforms and test harnesses. A stand-alone executable, called abi_check, and a static library called libtestc++ are constructed. Both of these items are not installed, and only used during testing.

These files include the following functionality:

- testsuite_abi.h, testsuite_abi.cc, testsuite_abi_check.cc

  Creates the executable abi_check. Used to check correctness of symbol versioning, visibility of exported symbols, and compatibility on symbols in the shared library, for hosts that support this feature. More information can be found in the ABI documentation here

- testsuite_allocator.h, testsuite_allocator.cc

  Contains specialized allocators that keep track of construction and destruction. Also, support for overriding global new and delete operators, including verification that new and delete are called during execution, and that allocation over max_size fails.

- testsuite_character.h

  Contains `std::char_traits` and `std::codecvt` specializations for a user-defined POD.

- testsuite_hooks.h, testsuite_hooks.cc

  A large number of utilities, including:

  - VERIFY

  - set_memory_limits

  - verify_demangle

  - run_tests_wrapped_locale

  - run_tests_wrapped_env

  - try_named_locale

  - try_mkfifo

  - func_callback

  - counter

  - copy_tracker

  - copy_constructor

  - assignment_operator

  - destructor

  - pod_char, pod_int and associated char_traits specializations

- testsuite_io.h

  Error, exception, and constraint checking for `std::streambuf, std::basic_stringbuf, std::basic_filebuf`.

- testsuite_iterators.h

  Wrappers for various iterators.

- testsuite_performance.h

  A number of class abstractions for performance counters, and reporting functions including:

  - time_counter

  - resource_counter

  - report_performance

## Special Topics

### Qualifying Exception Safety Guarantees

Test Exception Safety

#### Overview

Testing is composed of running a particular test sequence, and looking at what happens to the surrounding code when exceptions are thrown. Each test is composed of measuring initial state, executing a particular sequence of code under some instrumented conditions, measuring a final state, and then examining the differences between the two states.

Test sequences are composed of constructed code sequences that exercise a particular function or member function, and either confirm no exceptions were generated, or confirm the consistency/coherency of the test subject in the event of a thrown exception.

Random code paths can be constructed using the basic test sequences and instrumentation as above, only combined in a random or pseudo-random way.

To compute the code paths that throw, test instruments are used that throw on allocation events (__gnu_cxx::throw_allocator_random and __gnu_cxx::throw_allocator_limit) and copy, assignment, comparison, increment, swap, and various operators (__gnu_cxx::throw_type_random and __gnu_cxx::throw_type_limit). Looping through a given test sequence and conditionally throwing in all instrumented places. Then, when the test sequence completes without an exception being thrown, assume all potential error paths have been exercised in a sequential manner.

#### Existing tests

- Ad Hoc

  For example, `testsuite/23_containers/list/modifiers/3.cc`.

- Policy Based Data Structures

  For example, take the test functor rand_reg_test in in `testsuite/ext/pb_ds/regression/tree_no_data_map_rand.cc`. This uses container_rand_regression_test in `testsuite/util/regression/rand/assoc/container_rand_regression_test.h`.

  Which has several tests for container member functions, Includes control and test container objects. Configuration includes random seed, iterations, number of distinct values, and the probability that an exception will be thrown. Assumes instantiating container uses an extension allocator, __gnu_cxx::throw_allocator_random, as the allocator type.

- C++11 Container Requirements.

  Coverage is currently limited to testing container requirements for exception safety, although __gnu_cxx::throw_type meets the additional type requirements for testing numeric data structures and instantiating algorithms.

  Of particular interest is extending testing to algorithms and then to parallel algorithms. Also io and locales.

  The test instrumentation should also be extended to add instrumentation to iterator and const_iterator types that throw conditionally on iterator operations.

#### C++11 Requirements Test Sequence Descriptions

- Basic

  Basic consistency on exception propagation tests. For each container, an object of that container is constructed, a specific member function is exercised in a `try` block, and then any thrown exceptions lead to error checking in the appropriate `catch` block. The container's use of resources is compared to the container's use prior to the test block. Resource monitoring is limited to allocations made through the container's allocator_type, which should be sufficient for container data structures. Included in these tests are member functions are iterator and const_iterator operations, `pop_front`, `pop_back`, `push_front`, `push_back`, `insert`, `erase`, `swap`, `clear`, and `rehash`. The container in question is instantiated with two instrumented template arguments, with __gnu_cxx::throw_allocator_limit as the allocator type, and with __gnu_cxx::throw_type_limit as the value type. This allows the test to loop through conditional throw points.

  The general form is demonstrated in `testsuite/23_containers/list/requirements/exception/basic.cc` . The instantiating test object is __gnu_test::basic_safety and is detailed in `testsuite/util/exception/safety.h`.

- Generation Prohibited

  Exception generation tests. For each container, an object of that container is constructed and all member functions required to not throw exceptions are exercised. Included in these tests are member functions are iterator and const_iterator operations, `erase`, `pop_front`, `pop_back`, `swap`, and `clear`. The container in question is instantiated with two instrumented template arguments, with __gnu_cxx::throw_allocator_random as the allocator type, and with __gnu_cxx::throw_type_random as the value type. This test does not loop, an instead is sudden death: first error fails.

  The general form is demonstrated in `testsuite/23_containers/list/requirements/exception/generation_prohibited.cc` . The instantiating test object is __gnu_test::generation_prohibited and is detailed in `testsuite/util/exception/safety.h`.

- Propagation Consistent

  Container rollback on exception propagation tests. For each container, an object of that container is constructed, a specific member function that requires rollback to a previous known good state is exercised in a `try` block, and then any thrown exceptions lead to error checking in the appropriate `catch` block. The container is compared to the container's last known good state using such parameters as size, contents, and iterator references. Included in these tests are member functions are `push_front`, `push_back`, `insert`, and `rehash`. The container in question is instantiated with two instrumented template arguments, with __gnu_cxx::throw_allocator_limit as the allocator type, and with __gnu_cxx::throw_type_limit as the value type. This allows the test to loop through conditional throw points.

  The general form demonstrated in `testsuite/23_containers/list/requirements/exception/propagation_coherent.cc` . The instantiating test object is __gnu_test::propagation_coherent and is detailed in `testsuite/util/exception/safety.h`.