Lambdas as non‐type template parameters cause link errors (.rodata._ZTAXtl3$_0EE defined in discarded section)

[ryanofsky]

I am encountering a linker error with Clang 19.1.4 when using lambdas as non‐type template parameters across multiple translation units. The minimal example below compiles without issues using GCC and also does not trigger the link error in Clang when optimizations (-O) are enabled or if I change the parameter from const auto& to auto (i.e., pass by value rather than by const reference).

Error:

`.rodata._ZTAXtl3$_0EE' referenced in section `.text' of pass_b.o: defined in discarded section `.rodata._ZTAXtl3$_0EE[_ZTAXtl3$_0EE]' of pass_b.o 

(_ZTAXtl3$_0EE demangles to template parameter object for $_0{}.)

Command to reproduce:

clang++ -std=c++20 pass_a.cpp pass_b.cpp

Example code:

pass.h

#ifndef PASS_H
#define PASS_H

void PassArg(const auto& arg)
{
}

template<auto object>
void PassTemplate()
{
    PassArg(object);
}

#endif

pass_a.cpp

#include "pass.h"

constexpr auto fn_a = []{};

void pass_a()
{
    PassTemplate<fn_a>();
}

int main(int, char**)
{
    return 0;
}

pass_b.cpp

#include "pass.h"

constexpr auto fn_b = []{};

void pass_b()
{
    PassTemplate<fn_b>();
}

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Ryan Ofsky (ryanofsky)

I am encountering a linker error with Clang 19.1.4 when using lambdas as non‐type template parameters across multiple translation units. The minimal example below compiles without issues using GCC and also _does not_ trigger the link error in Clang when optimizations (`-O`) are enabled or if I change the parameter from `const auto&` to `auto` (i.e., pass by value rather than by const reference).

Error:

`.rodata._ZTAXtl3$_0EE' referenced in section `.text' of pass_b.o: defined in discarded section `.rodata._ZTAXtl3$_0EE[_ZTAXtl3$_0EE]' of pass_b.o 

(_ZTAXtl3$_0EE demangles to template parameter object for $_0{}.)

Command to reproduce:

clang++ -std=c++20 pass_a.cpp pass_b.cpp

Example code:

pass.h

#ifndef PASS_H
#define PASS_H

void PassArg(const auto& arg)
{
}

template<auto object>
void PassTemplate()
{
    PassArg(object);
}

#endif

pass_a.cpp

#include "pass.h"

constexpr auto fn_a = []{};

void pass_a()
{
    PassTemplate<fn_a>();
}

int main(int, char**)
{
    return 0;
}

pass_b.cpp

#include "pass.h"

constexpr auto fn_b = []{};

void pass_b()
{
    PassTemplate<fn_b>();
}

[ryanofsky]

Should also point out there there is no problem building the example code in a single translation unit. The error only happens when there are two translation units with similar symbols that seem to confuse the linker.

[maflcko]

I checked a few clang versions and it looks like it was introduced somewhere between 16 and 17:

# clang++-16 -std=c++20 pass_a.cpp pass_b.cpp
# clang++-17 -std=c++20 pass_a.cpp pass_b.cpp
`.rodata._ZTAXtl3$_0EE' referenced in section `.text' of /tmp/pass_b-7abe35.o: defined in discarded section `.rodata._ZTAXtl3$_0EE[_ZTAXtl3$_0EE]' of /tmp/pass_b-7abe35.o
clang++-17: error: linker command failed with exit code 1 (use -v to see invocation)

It seems to have been fixed between 20 and 21 in commit 8f025f2

According to https://llvm.org/docs/HowToReleaseLLVM.html#annual-release-schedule and https://discourse.llvm.org/t/rfc-llvm-releases-every-two-weeks/84245 there won't be a 19.1.x release (or earlier), since work on 20.1.x has begun. Though, a backport to 20 could be considered.

[ryanofsky]

Really interesting, thanks for identifying the fix. Related PR seems to be #125448. I think this issue can just be closed given the fix and fact that the problem does not seem to be too hard to work around once you can figure out what is causing it.