[libc++] Make `<map>` `std::map` constexpr as part of P3372R3 #134330

vinay-deshmukh · 2025-04-04T00:41:50Z

Depends on:

Summary:

Apply _LIBCPP_CONSTEXPR_SINCE_CXX26 to map, __tree, node-handle, __libcpp_erase_if, __lazy_synth_three_way_comparator, __lazy_compare_result, libcxx/include/__utility/try_key_extraction.h
Skip test erase_if.pass.cpp for GCC during constant evaluation. AFAICT this appears to be a g++ bug, as the test passes with clang
Disable test for node-handle::key() during constant evaluation (CWG2514). It is annotated with a // FIXME
map.modifiers/try.emplace.pass.cpp : Start using the previously unused mv3. (Should this be a separate patch?)
Has a TODO for multimap and ohters
a. libcxx/test/std/containers/associative/map/map.ops/contains.pass.cpp
b. libcxx/test/std/containers/associative/map/map.ops/contains_transparent.pass.cpp
a. libcxx/test/std/containers/container.node/node_handle.pass.cpp
~~Fix typo in libcxx/include/__memory/pointer_traits.h~~ -> [NFC][libc++] Fix typo in libcxx/include/__memory/pointer_traits.h #157304
pair<const MoveOnly, ...>
a. move_assign.pass.cpp
b. move_alloc.pass.cpp
c. Fails to compile if static_assert(test()); is called in the test file
d. Has a // FIXME with commented code
~~Change __tree_node to construct it's __node_value_type during construction to avoid the: note: member call on object outside its lifetime is not allowed in a constant expression issue.~~ This became -> [libc++] Remove UB from std::__tree_node construction #153908

github-actions · 2025-04-04T00:43:27Z

⚠️ We detected that you are using a GitHub private e-mail address to contribute to the repo.
Please turn off Keep my email addresses private setting in your account.
See LLVM Discourse for more information.

github-actions · 2025-04-04T00:44:15Z

✅ With the latest revision this PR passed the C/C++ code formatter.

libcxx/include/__utility/pair.h

vinay-deshmukh · 2025-04-10T12:09:13Z

At this point, I have fixed all the trivially fixable issues as far as I can tell, but naturally trying to fix the lifetime issue I saw when trying to fix it in:

cec33fb

(need explicit constructors for std::__value_type and __tree_node_base to start the lifetime of a node at

https://github.com/llvm/llvm-project/blame/900d44976c89477607946fad4493e4b9ac09346f/libcxx/include/__tree#L1956)

And that has resulted in a im-perfect forwarding (I think)

I did find this commit that mentions that the code is UB (slightly less than previously), so maybe constexpr flagging that is expected

f52318b | https://reviews.llvm.org/D47607

@frederick-vs-ja

frederick-vs-ja · 2025-04-11T03:24:55Z

I did find this commit that mentions that the code is UB (slightly less than previously), so maybe constexpr flagging that is expected

f52318b | https://reviews.llvm.org/D47607

This is clearly UB in the core language. Per current rules we can't assign the first data member of a pair<const int, int>, even when the complete object is non-const.

IIUC we need to perform different operations (e.g. reconstructing the node) in constant evaluation.

vinay-deshmukh · 2025-04-14T23:54:18Z

Update:

Need to wait for #134819 to be merged before this PR can proceed.

That PR should get rid of some more UB, and later I should be able to:

There's still some UB left, but that' only in the __assign_value and __insert_from_orphaned_node functions, where you should be able to just destroy/construct instead of assigning to remove any UB during constant evaluation.

From Discord by philnik777

…660-P3372-constexpr-map

…P3372-constexpr-map

libcxx/test/std/containers/associative/map/map.access/at.pass.cpp

H-G-Hristov · 2025-10-19T03:44:30Z

libcxx/docs/ReleaseNotes/22.rst

 - P3060R3: Add ``std::views::indices(n)`` (`Github <https://llvm.org/PR148175>`__)
 - P2835R7: Expose ``std::atomic_ref``'s object address (`Github <https://llvm.org/PR118377>`__)
 - P3168R2: Give ``std::optional`` Range Support (`Github <https://llvm.org/PR105430>`__)
+- P3372R3: ``constexpr map`` (`Github <https://llvm.org/PR134330>`__) (The paper is partially implemented. ``constexpr map`` is implemented in this release)


You don't need to add this release note for a partially implemented paper. You can update the status page though.

I was looking at this PR for a reference:
https://github.com/llvm/llvm-project/pull/137453/files

So Cxx2cPapers.csv already has an entry for it:

llvm-project/libcxx/docs/Status/Cxx2cPapers.csv

Line 107 in f5ae102

"`P3372R3 <https://wg21.link/P3372R3>`__","constexpr containers and adaptors","2025-02 (Hagenberg)","|In Progress|","","`#127876 <https://github.com/llvm/llvm-project/issues/127876>`__",""

and I originally added it here using this as a reference:

llvm-project/libcxx/docs/ReleaseNotes/21.rst

Line 51 in f5ae102

- P3372R3: ``constexpr`` containers and adaptors (`Github <https://github.com/llvm/llvm-project/issues/127876>`__) (``forward_list``, ``list``, ``priority_queue``, ``flat_map``, and ``flat_set`` are implemented)

~~So perhaps should I just remove this line for this review?~~

removed in 483a33b

libcxx/include/__tree

H-G-Hristov · 2025-10-19T03:54:39Z

libcxx/include/__tree

+      // within __insert_unique_from_orphaned_node, the code will fail to compile where the value is not
+      // copy_constructible for runtime execution as well; unless we use `if constexpr`. Given the copy-constructible
+      // code path will be a performance regression, we want to restrict it to only execute during constant evaluation
+      //, we need to delay the template instantiation


Suggested change

//, we need to delay the template instantiation

//, we need to delay the template instantiation.

Nit. Please do this everywhere too.

Added a few . and capitalized first letters of sentences. Rewrote some parts.

69bb9d3

let me know if this is better!

H-G-Hristov · 2025-10-19T03:58:22Z

libcxx/include/__tree

+      // we use copy, and not "move" as the constraint
+      // because we can NOT move from `const key_type`, which is how `value_type` is defined
+      // atleast for map
+      // typedef pair<const key_type, mapped_type> value_type;
+      // so we must copy it
+
+      // const_cast is not allowed at constexpr time.
+      // we get around this by deleting __lhs and creating a new node in-place
+      // to avoid const_cast __lhs.first
+
+      // We create a sfinae wrapper method here, because if the body of the true_type overload for
+      // __assign_value__sfinae() gets template instantiated within __assign_value, the code will fail to compile where
+      // the value is not copy_constructible for runtime execution as well; unless we use `if constexpr`. Given the
+      // copy-constructible code path will be a performance regression, we want to restrict it to only execute during
+      // constant evaluation
+      //, we need to delay the template instantiation


This is a bit formatted oddly, could you please update it and add punctuation, e.g (not complete).

Suggested change

// we use copy, and not "move" as the constraint

// because we can NOT move from `const key_type`, which is how `value_type` is defined

// atleast for map

// typedef pair<const key_type, mapped_type> value_type;

// so we must copy it

// const_cast is not allowed at constexpr time.

// we get around this by deleting __lhs and creating a new node in-place

// to avoid const_cast __lhs.first

// We create a sfinae wrapper method here, because if the body of the true_type overload for

// __assign_value__sfinae() gets template instantiated within __assign_value, the code will fail to compile where

// the value is not copy_constructible for runtime execution as well; unless we use `if constexpr`. Given the

// copy-constructible code path will be a performance regression, we want to restrict it to only execute during

// constant evaluation

//, we need to delay the template instantiation

// We use copy, and not "move" as the constraint because we can NOT move from `const key_type`, which is how `value_type` is defined

// atleast for map

// typedef pair<const key_type, mapped_type> value_type;

// so we must copy it

// const_cast is not allowed at constexpr time.

// we get around this by deleting __lhs and creating a new node in-place

// to avoid const_cast __lhs.first

// We create a sfinae wrapper method here, because if the body of the true_type overload for

// __assign_value__sfinae() gets template instantiated within __assign_value, the code will fail to compile where

// the value is not copy_constructible for runtime execution as well; unless we use `if constexpr`. Given the

// copy-constructible code path will be a performance regression, we want to restrict it to only execute during

// constant evaluation, we need to delay the template instantiation.

Added a few . and capitalized first letters of sentences. Rewrote some parts.

69bb9d3

let me know if this is better!

libcxx/test/std/containers/associative/map/map.ops/equal_range_transparent.pass.cpp

H-G-Hristov · 2025-10-19T04:05:35Z

libcxx/include/map

+        constexpr iterator emplace_hint(const_iterator position, Args&&... args); // constexpr since C++26
+    constexpr pair<iterator, bool> insert(const value_type& v); // constexpr since C++26
+    constexpr pair<iterator, bool> insert(      value_type&& v);                                // C++17, constexpr
+since C++26


Suggested change

constexpr iterator emplace_hint(const_iterator position, Args&&... args); // constexpr since C++26

constexpr pair<iterator, bool> insert(const value_type& v); // constexpr since C++26

constexpr pair<iterator, bool> insert( value_type&& v); // C++17, constexpr

since C++26

constexpr iterator emplace_hint(const_iterator position, Args&&... args); // constexpr since C++26

constexpr pair<iterator, bool> insert(const value_type& v); // constexpr since C++26

constexpr pair<iterator, bool> insert( value_type&& v); // C++17, constexpr since C++26

Can you align these comments a bit?

Done in
66b4d6f

Aligned it to each "stanza" / section, let me know if this looks good

libcxx/include/__tree

libcxx/test/std/containers/associative/map/map.nonmember/compare.three_way.pass.cpp

missing double //

…P3372-constexpr-map

vinay-deshmukh · 2025-10-25T18:06:09Z

The latest failure:

  diff -u /home/gha/actions-runner/_work/llvm-project/llvm-project/build/generic-cxx26/libcxx/test/libcxx/module_std_compat.gen.py/Output/module_std_compat.sh.cpp.dir/t.tmp.cuchar.module /home/gha/actions-runner/_work/llvm-project/llvm-project/build/generic-cxx26/libcxx/test/libcxx/module_std_compat.gen.py/Output/module_std_compat.sh.cpp.dir/t.tmp.cuchar.include
  # executed command: diff -u /home/gha/actions-runner/_work/llvm-project/llvm-project/build/generic-cxx26/libcxx/test/libcxx/module_std_compat.gen.py/Output/module_std_compat.sh.cpp.dir/t.tmp.cuchar.module /home/gha/actions-runner/_work/llvm-project/llvm-project/build/generic-cxx26/libcxx/test/libcxx/module_std_compat.gen.py/Output/module_std_compat.sh.cpp.dir/t.tmp.cuchar.include
  # .---command stdout------------
  # | --- /home/gha/actions-runner/_work/llvm-project/llvm-project/build/generic-cxx26/libcxx/test/libcxx/module_std_compat.gen.py/Output/module_std_compat.sh.cpp.dir/t.tmp.cuchar.module
  # | +++ /home/gha/actions-runner/_work/llvm-project/llvm-project/build/generic-cxx26/libcxx/test/libcxx/module_std_compat.gen.py/Output/module_std_compat.sh.cpp.dir/t.tmp.cuchar.include
  # | @@ -1,9 +1,7 @@
  # |  using ::c16rtomb;
  # |  using ::c32rtomb;
  # | -using ::c8rtomb;
  # |  using ::mbrtoc16;
  # |  using ::mbrtoc32;
  # | -using ::mbrtoc8;
  # |  using ::std::c16rtomb;
  # |  using ::std::c32rtomb;
  # |  using ::std::c8rtomb;
  # `-----------------------------
  # error: command failed with exit status: 1

seems to be related to this: #152724

not sure why that is failing however

…P3372-constexpr-map

vinay-deshmukh · 2025-10-30T13:53:54Z

@frederick-vs-ja @philnik777 @H-G-Hristov @Zingam

Can you take a look at this pull request when you have a chance? Thank you!

…P3372-constexpr-map

vinay-deshmukh · 2025-11-06T13:05:14Z

@frederick-vs-ja @philnik777 @H-G-Hristov @Zingam

Ping

vinay-deshmukh force-pushed the vinay-issue-128660-P3372-constexpr-map branch 2 times, most recently from 46d0bb9 to 9ea718f Compare April 7, 2025 22:25

vinay-deshmukh added 5 commits April 9, 2025 20:35

clang-format

27f2bfe

Generate macro version correctly

10f536f

fix unrelated CI failures

1270c3e

pass map.modifiers/emplace.pass.cpp

97b00cd

pass map.modifiers/emplace_hint.pass.cpp

900d449

vinay-deshmukh commented Apr 10, 2025

View reviewed changes

libcxx/include/__utility/pair.h Outdated Show resolved Hide resolved

vinay-deshmukh added 16 commits June 8, 2025 13:09

WIP:Merge remote-tracking branch 'upstream/main' into vinay-issue-128…

71b4ffb

…660-P3372-constexpr-map

at.pass.cpp passes again

f470158

iterator.pass.cpp passes

687ca5f

move_assign.pass.cpp constexpr fixed

eb3c0f8

move_alloc.pass.cpp constexpr fixed, other remains

b5577a6

copy_assign.pass.cpp constexpr fixed, other remains

dda93ff

erase_iter.pass.cpp

ab52bea

fix typo

f9a4e70

map.ops pass

37110f1

Merge remote-tracking branch 'upstream/main' into vinay-issue-128660-…

fedf39b

…P3372-constexpr-map

Partially fix assign_initializer_list.pass.cpp

f3b5970

Fix assign_initializer_list.pass.cpp

1d9e310

fix placement new to use std::addressof

d8095a7

fix copy_assign.pass.cpp

01d8633

move alloc failure location

e2f3b8f

git-clang-format upstream/main

0cce96e

vinay-deshmukh force-pushed the vinay-issue-128660-P3372-constexpr-map branch from 7906780 to 0cce96e Compare June 9, 2025 01:07

vinay-deshmukh requested a review from Zingam October 18, 2025 19:24