Commit Graph

54 Commits

Author SHA1 Message Date
Louis Dionne aeecef08c3 [libc++] Remove default definition of std::char_traits
This patch removes the base template implementation for std::char_traits.
If my reading of http://eel.is/c++draft/char.traits is correct, the
Standard mandates that the library provides specializations for several
types like char and wchar_t, but not any implementation in the base
template. Indeed, such an implementation is bound to be incorrect for
most types anyways, since things like `eof()` and `int_type` will definitely
have to be customized.

Since the base template implementation should not have worked for anyone,
this shouldn't be a breaking change (I expect that anyone defining a
custom character type today will already have to provide their own
specialization of char_traits). However, since we're aware of some users
of char_traits for unsigned char and signed char, we're keeping those two
specializations around for two releases to give people some time to migrate.

Differential Revision: https://reviews.llvm.org/D138307
2022-11-23 09:51:01 -05:00
Louis Dionne 9fa8f113f7 [libc++] Add test for bug that had been introduced in D98573 and fixed in D119633
Differential Revision: https://reviews.llvm.org/D135297
2022-10-12 15:05:04 -04:00
Nikolas Klauser 786366b18f [libc++][NFC] Remove some of the code duplication in the string tests
Reviewed By: ldionne, #libc, huixie90

Spies: huixie90, libcxx-commits, arphaman

Differential Revision: https://reviews.llvm.org/D131856
2022-08-26 21:57:42 +02:00
Jake Egan 1cf4113952 [libcxx][AIX] Switch build compiler to clang
This patch switches the build compiler for AIX from ibm-clang to clang. ibm-clang++_r has `-pthread` by default, but clang for AIX doesn't, so `-pthread` had to be added to the test config. A bunch of tests now pass, so the `XFAIL` was removed. This patch also switch the build to use the visibility support available in clang-15 to control symbols exported by the shared library (AIX traditionally uses explicit export lists for this purpose).

Reviewed By: #libc, #libc_abi, daltenty, #libunwind, ldionne

Differential Revision: https://reviews.llvm.org/D127470
2022-06-13 21:45:18 -04:00
Nikolas Klauser ef8e918261 [libc++] Forward more often to memmove in copy
In D122982 I accidentally disabled the memmove optimization. This re-enables it and adds more cases where copy forwards to memmove.
Fixes https://github.com/llvm/llvm-project/issues/33687

Reviewed By: var-const, #libc, ldionne

Spies: pkasting, ayzhao, dcheng, xbolva00, libcxx-commits

Differential Revision: https://reviews.llvm.org/D124328
2022-06-03 20:29:18 +02:00
Nikolas Klauser 425620ccdd [libc++] Implement P0980R1 (constexpr std::string)
Reviewed By: #libc, ldionne

Spies: daltenty, sdasgup3, ldionne, arichardson, MTC, ChuanqiXu, mehdi_amini, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, stephenneuendorffer, Joonsoo, grosul1, Kayjukh, jurahul, msifontes, tatianashp, rdzhabarov, teijeong, cota, dcaballe, Chia-hungDuan, wrengr, wenzhicui, arphaman, Mordante, miscco, Quuxplusone, smeenai, libcxx-commits

Differential Revision: https://reviews.llvm.org/D110598
2022-04-27 12:25:34 +02:00
Nikolas Klauser 85e9b2687a [libc++] Prepare string tests for constexpr
These are the last™ changes to the tests for constexpr preparation.

Reviewed By: Quuxplusone, #libc, Mordante

Spies: Mordante, EricWF, libcxx-commits

Differential Revision: https://reviews.llvm.org/D120951
2022-03-19 18:48:14 +01:00
Nikolas Klauser dcffa7d3e1 [libc++] Prepare string.modifiers tests for constexpr
Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D119329
2022-02-10 21:43:18 +01:00
Louis Dionne 4a47ac7d51 [libc++] Remove incorrect default constructor in cpp17_input_iterator
AFAICT, Cpp17InputIterators are not required to be default constructible,
since that requirement is added in Cpp17ForwardIterator. Hence, our
archetype for Cpp17InputIterator should not be default constructible.
Removing that constructor has a ripple effect on a couple of tests that
were making incorrect assumptions. Notably:

- Some tests were using cpp17_input_iterator as a sentinel for itself.
  That is not valid, because a cpp17_input_iterator is not semiregular
  anymore after the change (and hence it doesn't satisfy sentinel_for).

- Some tests were using a stride-counted cpp17_input_iterator as the
  sentinel for a range. This doesn't work anymore because of the problem
  above, so these tests were changed not to check stride counts for
  input iterators.

- Some tests were default constructing cpp17_input_iterator when a simple
  alternative was available -- those have been changed to use that alternative.

Differential Revision: https://reviews.llvm.org/D115806
2022-01-04 14:33:51 -05:00
Arthur O'Dwyer db9425cb06 [libc++] [LIBCXX-DEBUG-FIXME] Fix an iterator-invalidation issue in string::assign.
This appears to be a bug in our string::assign: when assigning into
a longer string, from a shorter snippet of itself, we invalidate
iterators before doing the copy. We should invalidate them afterward.
Also drive-by improve the formatting of a function header.

Differential Revision: https://reviews.llvm.org/D101675
2021-05-05 16:20:53 -04:00
Christopher Di Bella 773ae44124 [libcxx][nfc] prefixes test type `input_iterator` with `cpp17_`
C++20 revised the definition of what it means to be an iterator. While
all _Cpp17InputIterators_ satisfy `std::input_iterator`, the reverse
isn't true. D100271 introduces a new test adaptor to accommodate this
new definition (`cpp20_input_iterator`).

In order to help readers immediately distinguish which input iterator
adaptor is _Cpp17InputIterator_, the current `input_iterator` adaptor
has been prefixed with `cpp17_`.

Differential Revision: https://reviews.llvm.org/D101242
2021-05-02 05:02:59 +00:00
Arthur O'Dwyer 86d1f590c2 [libc++] [test] Add a debug-mode CI.
To run llvm-lit manually from the command line:

    ./bin/llvm-lit -sv --param std=c++2b --param cxx_under_test=`pwd`/bin/clang \
        --param debug_level=1 ../libcxx/test/

Tests that currently fail with `debug_level=1` are marked `LIBCXX-DEBUG-FIXME`,
but my intent is to deal with all of them and leave no such annotations in
the codebase within the next couple weeks. (I have patches for all of them
in my local checkout.)

Differential Revision: https://reviews.llvm.org/D100866
2021-04-30 18:08:09 -04:00
Arthur O'Dwyer 036b80fcbb [libc++] [test] Improve test_exceptions() in each string.modifiers test.
When checking the strong exception guarantee, also check that
iterators haven't been invalidated.

Reviewed as part of https://reviews.llvm.org/D98573
2021-04-26 16:22:43 -04:00
Arthur O'Dwyer e87479b00f [libc++] Remove the special logic for "noexcept iterators" in basic_string.
This reverts a large chunk of http://reviews.llvm.org/D15862 ,
and also fixes bugs in `insert`, `append`, and `assign`, which are now regression-tested.
(Thanks to Tim Song for pointing out the bug in `append`!)

Before this patch, we did a special dance in `append`, `assign`, and `insert`
(but not `replace`). All of these require the strong exception guarantee,
even when the user-provided InputIterator might have throwing operations.

The naive way to accomplish this is to construct a temporary string and
then append/assign/insert from the temporary; i.e., finish all the potentially
throwing and self-inspecting InputIterator operations *before* starting to
modify self. But this is slow, so we'd like to skip it when possible.

The old code (D15682) attempted to check that specific iterator operations
were nothrow: it assumed that if the iterator operations didn't throw, then
it was safe to iterate the input range multiple times and therefore it was
safe to use the fast-path non-naive version. This was wrong for two reasons:
(1) the old code checked the wrong operations (e.g. checked noexceptness of `==`,
but the code that ran used `!=`), and (2) the conversion of value_type to char
could still throw, or inspect the contents of self.

The new code is much simpler, although still much more complicated than it
really could be. We'll likely revisit this codepath at some point, but for now
this patch suffices to get it passing all the new regression tests.

The added tests all fail before this patch, and succeed afterward.
See https://quuxplusone.github.io/blog/2021/04/17/pathological-string-appends/

Differential Revision: https://reviews.llvm.org/D98573
2021-04-26 16:22:43 -04:00
Louis Dionne 3c62198c61 [libc++] NFC: Normalize links to bug reports 2021-03-03 13:45:29 -05:00
Louis Dionne a506a66bd9 [libc++] Fix several debug mode tests
These tests were only being run when _LIBCPP_DEBUG was defined, which
isn't the case by default when we run the test suite. In other words,
all these debug mode tests were never being run. This commit makes sure
they are run, and in some cases, extracts them into a file under test/libcxx
to separate them from the Standard tests.

Differential Revision: https://reviews.llvm.org/D88836
2020-10-05 16:15:32 -04:00
Louis Dionne 31cbe0f240 [libc++] Remove the c++98 Lit feature from the test suite
C++98 and C++03 are effectively aliases as far as Clang is concerned.
As such, allowing both std=c++98 and std=c++03 as Lit parameters is
just slightly confusing, but provides no value. It's similar to allowing
both std=c++17 and std=c++1z, which we don't do.

This was discovered because we had an internal bot that ran the test
suite under both c++98 AND c++03 -- one of which is redundant.

Differential Revision: https://reviews.llvm.org/D80926
2020-06-03 09:37:22 -04:00
Louis Dionne 7149bb7068 [libc++] NFC: Clean up a lot of old Lit features
The libc++ test suite has a lot of old Lit features used to XFAIL tests
and mark them as UNSUPPORTED. Many of them are to workaround problems on
old compilers or old platforms. As time goes by, it is good to go and
clean those up to simplify the configuration of the test suite, and also
to reflect the testing reality. It's not useful to have markup that gives
the impression that e.g. clang-3.3 is supported, when we don't really
test on it anymore (and hence several new tests probably don't have the
necessary markup on them).
2020-04-10 17:20:29 -04:00
Louis Dionne aaaa25e23d [libc++] Remove useless nothing_to_do.pass.cpp tests
The testing script used to test libc++ historically did not like directories
without any testing files, so these tests had been added. Since this is
not necessary anymore, we can now remove these files. This has the benefit
that the total number of tests reflects the real number of tests more
closely, and we also skip some unnecessary work (especially relevant when
running tests over SSH).

However, some nothing_to_do.pass.cpp tests actually serve the purpose of
documenting that an area of the Standard doesn't need to be tested, or is
tested elsewhere. These files are not removed by this commit.

Removal done with:

  import os
  import itertools
  for (dirpath, dirnames, filenames) in itertools.chain(os.walk('./libcxx/test'),
                                                        os.walk('./libcxxabi/test')):
      if len(filenames + dirnames) > 1 and \
         any(p == 'nothing_to_do.pass.cpp' for p in filenames):
          os.remove(os.path.join(dirpath, 'nothing_to_do.pass.cpp'))
2020-04-03 13:48:34 -04:00
Marshall Clow 7fc6a55688 Add include for 'test_macros.h' to all the tests that were missing them. Thanks to Zoe for the (big, but simple) patch. NFC intended.
llvm-svn: 362252
2019-05-31 18:35:30 +00:00
Louis Dionne 25838c6dac [libc++] Mark several tests as XFAIL on macosx10.7
Those tests fail when linking against a new dylib but running against
macosx10.7. I believe this is caused by a duplicate definition of the
RTTI for exception classes in libc++.dylib and libc++abi.dylib, but
this matter still needs some investigation.

This issue was not caught previously because all the tests always linked
against the same dylib used for running (because LIT made it impossible
to do otherwise before r349171).

rdar://problem/46809586

llvm-svn: 354940
2019-02-27 00:57:57 +00:00
JF Bastien 2df59c5068 Support tests in freestanding
Summary:
Freestanding is *weird*. The standard allows it to differ in a bunch of odd
manners from regular C++, and the committee would like to improve that
situation. I'd like to make libc++ behave better with what freestanding should
be, so that it can be a tool we use in improving the standard. To do that we
need to try stuff out, both with "freestanding the language mode" and
"freestanding the library subset".

Let's start with the super basic: run the libc++ tests in freestanding, using
clang as the compiler, and see what works. The easiest hack to do this:

In utils/libcxx/test/config.py add:

  self.cxx.compile_flags += ['-ffreestanding']

Run the tests and they all fail.

Why? Because in freestanding `main` isn't special. This "not special" property
has two effects: main doesn't get mangled, and main isn't allowed to omit its
`return` statement. The first means main gets mangled and the linker can't
create a valid executable for us to test. The second means we spew out warnings
(ew) and the compiler doesn't insert the `return` we omitted, and main just
falls of the end and does whatever undefined behavior (if you're luck, ud2
leading to non-zero return code).

Let's start my work with the basics. This patch changes all libc++ tests to
declare `main` as `int main(int, char**` so it mangles consistently (enabling us
to declare another `extern "C"` main for freestanding which calls the mangled
one), and adds `return 0;` to all places where it was missing. This touches 6124
files, and I apologize.

The former was done with The Magic Of Sed.

The later was done with a (not quite correct but decent) clang tool:

  https://gist.github.com/jfbastien/793819ff360baa845483dde81170feed

This works for most tests, though I did have to adjust a few places when e.g.
the test runs with `-x c`, macros are used for main (such as for the filesystem
tests), etc.

Once this is in we can create a freestanding bot which will prevent further
regressions. After that, we can start the real work of supporting C++
freestanding fairly well in libc++.

<rdar://problem/47754795>

Reviewers: ldionne, mclow.lists, EricWF

Subscribers: christof, jkorous, dexonsmith, arphaman, miyuki, libcxx-commits

Differential Revision: https://reviews.llvm.org/D57624

llvm-svn: 353086
2019-02-04 20:31:13 +00:00
Chandler Carruth 57b08b0944 Update more file headers across all of the LLVM projects in the monorepo
to reflect the new license. These used slightly different spellings that
defeated my regular expressions.

We understand that people may be surprised that we're moving the header
entirely to discuss the new license. We checked this carefully with the
Foundation's lawyer and we believe this is the correct approach.

Essentially, all code in the project is now made available by the LLVM
project under our new license, so you will see that the license headers
include that license only. Some of our contributors have contributed
code under our old license, and accordingly, we have retained a copy of
our old license notice in the top-level files in each project and
repository.

llvm-svn: 351648
2019-01-19 10:56:40 +00:00
Marshall Clow 76b26852b6 Implement LWG 2946, 3075 and 3076. Reviewed as https://reviews.llvm.org/D48616
llvm-svn: 336132
2018-07-02 18:41:15 +00:00
Stephan T. Lavavej c1fcd97ede [libcxx] [test] Fix MSVC warnings and errors.
test/std/numerics/numeric.ops/exclusive.scan/exclusive_scan.pass.cpp
test/std/numerics/numeric.ops/exclusive.scan/exclusive_scan_init_op.pass.cpp
test/std/numerics/numeric.ops/inclusive.scan/inclusive_scan.pass.cpp
test/std/numerics/numeric.ops/inclusive.scan/inclusive_scan_op.pass.cpp
test/std/numerics/numeric.ops/inclusive.scan/inclusive_scan_op_init.pass.cpp
test/std/numerics/numeric.ops/transform.exclusive.scan/transform_exclusive_scan_init_bop_uop.pass.cpp
test/std/numerics/numeric.ops/transform.inclusive.scan/transform_inclusive_scan_bop_uop.pass.cpp
test/std/numerics/numeric.ops/transform.inclusive.scan/transform_inclusive_scan_bop_uop_init.pass.cpp
Fix MSVC x64 truncation warnings.
warning C4267: conversion from 'size_t' to 'int', possible loss of data

test/std/strings/basic.string/string.modifiers/string_append/push_back.pass.cpp
Fix MSVC uninitialized memory warning.
warning C6001: Using uninitialized memory 'vl'.

test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp
Include <cassert> for the assert() macro.

Fixes D43273.

llvm-svn: 326120
2018-02-26 20:47:46 +00:00
Marshall Clow fc940277cb Fix PR#31454 - 'basic_string<T>::push_back() crashes if sizeof(T)>sizeof(long long)'. We were mishandling the small-string optimization calculations for very large 'characters'. This may be an ABI change (change the size of) strings of very large 'characters', but since they never worked, I'm not too concerned.
llvm-svn: 324531
2018-02-07 21:30:17 +00:00
Marshall Clow 936a2fd4b4 Ooops. I checked in a test for a bug I haven't fixed yet. Temporrarily commented it out.
llvm-svn: 319693
2017-12-04 20:46:38 +00:00
Marshall Clow 800259c98d Implement P0457R2: 'String Prefix and Suffix Checking' for c++2a
llvm-svn: 319687
2017-12-04 20:11:38 +00:00
Stephan T. Lavavej 326df3d819 [libcxx] [test] Strip trailing whitespace.
llvm-svn: 314258
2017-09-26 23:08:41 +00:00
Marshall Clow b6d73126c8 Add more string_view tests
llvm-svn: 312690
2017-09-07 02:46:09 +00:00
Stephan T. Lavavej 4159db7698 [libcxx] [test] Untabify, NFC.
llvm-svn: 309464
2017-07-29 00:55:10 +00:00
Billy Robert O'Neal III 4deabc97a1 Add asserts that the nullptr is maintained in string erase operations.
llvm-svn: 303899
2017-05-25 19:01:14 +00:00
Eric Fiselier 414d17c67c Cleanup _LIBCPP_HAS_NO_<c++11-feature> in the string library.
llvm-svn: 300633
2017-04-19 00:28:44 +00:00
Eric Fiselier e84fcb5f3d Fix PR32642 - string::insert and string::append don't work with move_iterator.
llvm-svn: 300397
2017-04-15 06:49:02 +00:00
Eric Fiselier ae85a922d5 Work around test failure on 32 bit OS X
llvm-svn: 296840
2017-03-02 23:18:40 +00:00
Stephan T. Lavavej d72ece6462 [libcxx] [test] D27027: Strip trailing whitespace.
llvm-svn: 287829
2016-11-23 22:03:28 +00:00
Stephan T. Lavavej 4dc0ed8390 [libcxx] [test] D26314: Fix MSVC warning C4189 "local variable is initialized but not referenced".
test/std/depr/depr.c.headers/inttypes_h.pass.cpp
test/std/input.output/file.streams/c.files/cinttypes.pass.cpp
test/std/input.output/iostream.forward/iosfwd.pass.cpp
Add test() to avoid a bunch of void-casts, although we still need a few.

test/std/input.output/iostream.format/quoted.manip/quoted.pass.cpp
skippingws was unused (it's unclear to me whether this was mistakenly copy-pasted from round_trip() below).

test/std/localization/locale.categories/category.collate/locale.collate/types.pass.cpp
test/std/localization/locale.categories/category.ctype/facet.ctype.special/types.pass.cpp
test/std/localization/locale.categories/category.ctype/locale.codecvt/types_char.pass.cpp
test/std/localization/locale.categories/category.ctype/locale.codecvt/types_wchar_t.pass.cpp
test/std/localization/locale.categories/category.ctype/locale.ctype/types.pass.cpp
test/std/localization/locale.categories/facet.numpunct/locale.numpunct/types.pass.cpp
test/std/localization/locales/locale.global.templates/use_facet.pass.cpp
When retrieving facets, the references are unused.

test/std/localization/locale.categories/category.numeric/locale.nm.put/facet.num.put.members/put_long.pass.cpp
test/std/localization/locale.categories/category.numeric/locale.nm.put/facet.num.put.members/put_unsigned_long.pass.cpp
"std::ios_base::iostate err = ios.goodbit;" was completely unused here.

test/std/localization/locale.categories/category.time/locale.time.get/time_base.pass.cpp
test/std/numerics/c.math/ctgmath.pass.cpp
test/std/numerics/rand/rand.device/entropy.pass.cpp
test/std/numerics/rand/rand.device/eval.pass.cpp
test/std/strings/basic.string/string.modifiers/string_copy/copy.pass.cpp
test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/eof.pass.cpp
test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/eof.pass.cpp
test/std/thread/futures/futures.promise/dtor.pass.cpp
test/std/thread/futures/futures.task/futures.task.members/dtor.pass.cpp
test/std/thread/thread.condition/thread.condition.condvar/wait_for_pred.pass.cpp
These variables are verifying types but are otherwise unused.

test/std/strings/basic.string/string.capacity/reserve.pass.cpp
old_cap was unused (it's unclear to me whether it was intended to be used).

test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/eq.pass.cpp
test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/eq.pass.cpp
test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/lt.pass.cpp
test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/eq.pass.cpp
test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/lt.pass.cpp
test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/eq.pass.cpp
test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/lt.pass.cpp
These tests contained unused characters.

llvm-svn: 286847
2016-11-14 17:35:14 +00:00
Roger Ferrer Ibanez 8a915ed644 Protect exceptional paths under libcpp-no-exceptions
These tests are of the form

try {
   action-that-may-throw
   assert(!exceptional-condition)
   assert(some-other-facts)
 } catch (relevant-exception) {
   assert(exceptional-condition)
 }

Under libcpp-no-exceptions there is still value in verifying
some-other-facts while avoiding the exceptional case. So for these tests
just conditionally check some-other-facts if exceptional-condition is
false. When exception are supported make sure that a true
exceptional-condition throws an exception

Differential Revision: https://reviews.llvm.org/D26136

llvm-svn: 285697
2016-11-01 15:46:16 +00:00
Eric Fiselier 009fb08d14 Fix shadow warnings in string_view tests. Patch from STL@microsoft.com
llvm-svn: 285011
2016-10-24 20:10:00 +00:00
Marshall Clow 67be6ff839 Add another append test for basic_string
llvm-svn: 283331
2016-10-05 15:47:13 +00:00
Marshall Clow a8fd1e4be0 Fix a few static_asserts that need extra parens on -03
llvm-svn: 282343
2016-09-24 23:52:21 +00:00
Marshall Clow 54f0981ebd Implement proposed resolution for LWG#2758. Reviewed as D24446. Normally, I would wait for these to be voted upon at a committee meeting (November), but the current draft standard is broken, and this should fix it. (And if it doesn't, we want to know about it soonest)
llvm-svn: 282342
2016-09-24 22:45:42 +00:00
Marshall Clow a77bb8ef34 Fix PR#30303 - no matching function for call to '__ptr_in_range'
llvm-svn: 280779
2016-09-07 03:32:06 +00:00
Marshall Clow 7e1a23001d Fix Bug 30240 - std::string: append(first, last) error when aliasing. Add test cases for append/insert/assign/replace while we're at it, and fix a similar bug in insert.
llvm-svn: 280643
2016-09-05 01:54:30 +00:00
Marshall Clow 053d81ceeb Implement std::string_view as described in http://wg21.link/P0254R1. Reviewed as https://reviews.llvm.org/D21459
llvm-svn: 276238
2016-07-21 05:31:24 +00:00
Eric Fiselier 1f4231f8cf Guard libc++ specific c.__invariants() tests in LIBCPP_ASSERT macros
llvm-svn: 267947
2016-04-28 22:28:23 +00:00
Marshall Clow ffc888bc09 Implement LWG#2579: Inconsistency wrt Allocators in basic_string assignment vs. basic_string::assign
llvm-svn: 263042
2016-03-09 18:08:29 +00:00
Marshall Clow e612a8877a More string fixes for noexcept cases. Apparently I didn't get them all in r258281.
llvm-svn: 258291
2016-01-20 05:41:24 +00:00
Marshall Clow 9d10d27c67 Fix up the tests I added for string exceptions to be skipped when exceptions are disabled
llvm-svn: 258279
2016-01-20 03:19:15 +00:00
Marshall Clow 76b4afc040 Fix PR#25973 : 'basic_string::assign(InputIt, InputIt) doesn't provide the strong exception safety guarantee'. This turned out to be a pervasive problem in <string>, which required a fair amount of rework. Add in an optimization for when iterators provide noexcept increment/comparison/assignment/dereference (which covers many of the iterators in libc++). Reviewed as http://reviews.llvm.org/D15862
llvm-svn: 257682
2016-01-13 21:54:34 +00:00