<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/151081>151081</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            LLVM 21.1.0-rc1 C++ Standard Library Compatibility Issues
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          chatgpt-copypasta
      </td>
    </tr>
</table>

<pre>
    **ISSUE**

LLVM 21.1.0-rc1's libc++ introduces template metaprogramming changes that break compatibility with large C++ codebases and C++ binding generators, specifically around `std::invoke_result_t` and `std::basic_string_view` template instantiations.

**EXPECTED BEHAVIOR**

LLVM 21's libc++ should maintain backward compatibility with existing C++ code patterns used in major projects, or provide clear migration paths.

**STEPS TO REPRODUCE**

**Environment:**
- LLVM 21.1.0-rc1 on macOS aarch64
- Large C++ codebase (Firefox, ~10M+ lines)
- C++ binding generator (rust-bindgen)

**Template Usage Patterns That Break:**
```cpp
// 1. Complex invoke_result_t instantiations
template<typename ResolveFunction, typename ResolveValueT>
struct Promise {
    using resolve_result = std::invoke_result_t<ResolveFunction, ResolveValueT>;
};

// 2. basic_string_view in template contexts
template<typename CharT, typename Traits = std::char_traits<CharT>>
class StringProcessor {
 std::basic_string_view<CharT, Traits> view_;
};
```

**Compilation Context:**
```bash
clang++ -std=c++17 -I/opt/llvm-21.1.0-rc1/include/c++/v1 test.cpp
```

 **ATTEMPTS TO RESOLVE**

**Analysis of Template Changes**
Compared LLVM 21 vs LLVM 19 template definitions:

**LLVM 19 (Working):**
```cpp
template<class _Fn, class... _Args>
struct invoke_result : public __invoke_result<_Fn, _Args...> {};

template<class _Fn, class... _Args>
using invoke_result_t = typename invoke_result<_Fn, _Args...>::type;
```

**LLVM 21 (Problematic):**
```cpp
// More complex SFINAE and template parameter handling
template<class _Fn, class... _Args>
struct invoke_result : public __invoke_result<_Fn, _Args...> {
 // Additional template metaprogramming complexity
};
```

 **Impact on Binding Generators**
Binding generators (bindgen, SWIG, etc.) fail because:
1. **Template Parameter Extraction:** Cannot resolve `_CharT`, `_Traits`, `_Args` in complex instantiations
2. **SFINAE Patterns:** More sophisticated SFINAE in LLVM 21 breaks simple template parsing
3. **Nested Template Resolution:** Deeply nested template hierarchies are not properly resolved

**OUTCOME AND FINDINGS**

**Compatibility Regression**
LLVM 21 introduces template metaprogramming changes that are:
1. **Semantically Equivalent:** Same runtime behavior as LLVM 19
2. **Syntactically Different:** Different template instantiation patterns
3. **Tool-Breaking:** Incompatible with existing C++ tooling ecosystem

**Affected Template Types**
1. **std::invoke_result_t** - Core issue in complex template instantiations
2. **std::basic_string_view** - Template parameter resolution problems
3. **std::enable_if_t patterns** - SFINAE changes break some use cases

**Ecosystem Impact**
This affects:
- **Firefox** (confirmed failing at 58% build completion)
- **Other large C++ projects** using binding generators
- **C++ tooling ecosystem** (bindgen, SWIG, etc.)

## **REQUESTED ACTION**

**Immediate (LLVM 21.1.0 stable)**
1. **Template Compatibility Audit:** Review template metaprogramming changes for backward compatibility
2. **Binding Generator Testing:** Test LLVM 21 against major binding generators (bindgen, SWIG, pybind11)
3. **Documentation:** Document template changes that affect tooling

 **Long Term**
1. **Template Stability Policy:** Consider impact on C++ tooling ecosystem when making template changes
2. **Binding Generator Liaison:** Coordinate with binding generator maintainers on template changes
3. **Compatibility Testing:** Add major C++ projects to LLVM CI for compatibility testing

**Specific Technical Review**
**Files needing review:**
- `libcxx/include/type_traits` (invoke_result_t implementation)
- `libcxx/include/string_view` (basic_string_view template patterns)
- Template SFINAE patterns that changed between LLVM 19 and 21

**Questions for LLVM team:**
1. Were the template metaprogramming changes intentional compatibility breaks?
2. Is there a migration path for binding generators?
3. Can template instantiation patterns be made more tooling-friendly?

**Priority:** This will block adoption of LLVM 21 by major C++ projects that rely on binding generators.

**Note:** This is not a request to revert changes, but to ensure ecosystem compatibility is considered in template metaprogramming decisions.

</pre>
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