llvm/llvm-project

[C++20] [Modules] Not properly mangling for entities in named modules for MSVC ABI

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#174064 opened on Dec 31, 2025

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ABIclang:moduleshelp wantedplatform:windows

Description

Reproducer: https://godbolt.org/z/5oaM6Yjv5

for

export module a;
export int func() {
    return 43;
}

Clang in Windows ABI generates:

"?func@@YAHXZ"

but MSVC generates:

?func@@YAHXZ::<!a>

This is a compatibility problem. And also this is a correctness problem. Since the design of C++20 Named Modules ask toolchains to treat the entity in different named modules as different entity at link time even they have the same name. So this is a missing feature at ABI level.

Estimated difficulty: medium

Contributor guide

Tech stack
cpp
Domain
backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C++20LLVM internalsMSVC ABIName mangling
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
The issue requires implementing the correct MSVC ABI mangling for entities in C++20 named modules. The current Clang output is missing the module suffix '::<!a>'. The fix likely involves modifying the mangling logic in the Clang codegen, particularly in files related to name mangling for the Microsoft C++ ABI. The contributor should study MSVC's mangling scheme as seen in the provided godbolt link and ensure that the module name is appended appropriately. Additional comments in the issue may provide further guidance on the expected behavior.