Fix: Resolve compilation failure in F14Table.h due to two-phase lookup

[ihb2032]

Hi, this PR fixes a compilation issue in folly/container/detail/F14Table.h.

Bug Description

When building Folly with a modern g++ toolchain, the compilation fails with multiple errors related to occupiedMask.

Environment:

• OS: Ubuntu 24.04
• g++ version: 13.3.0
• CPU: Intel(R) Xeon(R) E-2286M CPU @ 2.40GHz

Key Error Message:

/root/folly/folly/container/detail/F14Table.h:821:37: error: there are no arguments to 'occupiedMask' that depend on a template parameter, so a declaration of 'occupiedMask' must be available [-fpermissive]
   821 |     return DenseMaskIter{&tags_[0], occupiedMask()};
       |                                     ^~~~~~~~~~~~

Root Cause

The error is a classic C++ two-phase name lookup problem. Inside the F14Chunk<ItemType> template, functions like occupiedIter() call occupiedMask() without any qualification. Because occupiedMask is a member function of F14Chunk, its existence depends on the template parameter ItemType. According to the rules, the compiler needs to be explicitly told that occupiedMask is a dependent name, which g++ 13.3.0 correctly enforces.

The Fix

The solution is to explicitly qualify the calls to occupiedMask with this->. This informs the compiler that occupiedMask is a member of the template instance and its lookup should be deferred to the instantiation phase.

This change is applied to all calls to occupiedMask within F14Chunk in folly/container/detail/F14Table.h.

How to Test

The fix can be verified by successfully compiling the folly_base target in the described environment (Ubuntu 24.04, g++ 13.3.0) where the build was previously failing. All existing tests should continue to pass.

Thanks for your review!During compilation with certain toolchains (e.g., g++), the build fails with an "'occupiedMask' was not declared in this scope" error inside the F14Chunk template class.

This is caused by C++'s two-phase name lookup rules for templates, where non-dependent names are resolved during the initial template parsing. The compiler fails to find 'occupiedMask' as it is a member of a dependent base class.

This commit resolves the build failure by explicitly qualifying the member function calls with 'this->', correctly hinting to the compiler that 'occupiedMask' is a dependent name that should be looked up during template instantiation.

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[ihb2032]

Hi, I'd like to gently follow up on this PR. It appears the CI checks are awaiting approval. Could a maintainer please help approve and run the CI workflows? Thank you!