[libc++][ranges] Fix the LWG 3568 test for basic_istream_view

[iskandarem]

The previous test was dereferencing begin() iterator on empty view which is UB, because begin() == end().

A new test case was suggested in a post-merge feedback in #193891, which verifies LWG3568 through constant evaluation. Such a constexpr basic_istream_view variable can only be created if it is completely initialized, including its exposition-only value_ member.

The existing test case is changed to use a non-empty stream and a testing class type for which operator>> is no-op. The state of the class object stored in basic_istream_view is unchanged even after the initial operator>> call.

This avoids dereferencing a past-the-end iterator while testing LWG3568.

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[llvmorg-github-actions]

@llvm/pr-subscribers-libcxx

Author: Iskandar Emomzoda (iskandarem)

Changes

The previous test was dereferencing begin() iterator on empty view which is UB, because begin() == end().

This solution was suggested by @frederick-vs-ja in #193891, which verifies lwg3568 through constant evaluation. A constexpr basic_istream_view can only be created if its complete state, including value_.

This avoids dereferencing a past-the-end iterator while testing lwg 3568.

---

Full diff: https://github.com/llvm/llvm-project/pull/215589.diff

1 Files Affected:

• (modified) libcxx/test/std/ranges/range.factories/range.istream.view/ctor.pass.cpp (+2-4)

diff --git a/libcxx/test/std/ranges/range.factories/range.istream.view/ctor.pass.cpp b/libcxx/test/std/ranges/range.factories/range.istream.view/ctor.pass.cpp
index beeeea4c1bbe1..d15225674049c 100644
--- a/libcxx/test/std/ranges/range.factories/range.istream.view/ctor.pass.cpp
+++ b/libcxx/test/std/ranges/range.factories/range.istream.view/ctor.pass.cpp
@@ -39,10 +39,8 @@ void test() {
 
   // LWG 3568. basic_istream_view needs to initialize value_
   {
-    auto iss = make_string_stream<CharT>("");
-    std::ranges::basic_istream_view<int, CharT> isv{iss};
-    auto iter = isv.begin();
-    assert(*iter == 0);
+    static auto fn_local_iss = make_string_stream<CharT>("");
+    [[maybe_unused]] constexpr auto isv = std::views::istream<int>(fn_local_iss);
   }
 }
 

[iskandarem]

Hello @iskandarem 👋

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Thank you, The LLVM Community

confirmed. i have read the LLVM policies.

[Zingam]

I updated the title to be more in line with our practices.

[github-actions]

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

[frederick-vs-ja]

I reworded the PR description to avoid tagging anybody and explained the modifications to the existing test case. Please double-check the changes.

We generally use the PR title and description as the squashed commit message by default. It will be disturbing if the squashed commit contains tagging and is replicated in some forks in GitHub since GitHub will send mail(s) when the replicated commit is pushed.

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