[clang-tidy] Peculiar behaviour of modernize-use-std-format

[asakovets]

Hi,

I’ve encountered unexpected behavior with the modernize-use-std-format check in clang-tidy. Consider the following code:

#include <format>

namespace foo {
void info__ (const char * s, ...) {}

template <typename... Args> 
void info_fmt (std::format_string<Args...> fmt, Args &&... args) {}
}

int main ()
{
    foo::info__ ("hello, %s!", "world");
}

My .clang-tidy configuration is

Checks: 'modernize-use-std-format'
CheckOptions:
  - key: modernize-use-std-format.StrFormatLikeFunctions
    value: "foo::info"
  - key: modernize-use-std-format.ReplacementFormatFunction
    value: foo::info_fmt

Running clang-tidy:

clang-tidy -checks=-clang-analyzer-* fmt.cc

1 warning generated.
~/fmt.cc:12:5: warning: use 'foo::info_fmt' instead of 'info__' [modernize-use-std-format]
   12 |     foo::info__ ("hello, %s!", "world");
      |     ^~~~~~~~~~~  ~~~~~~~~~~~~
      |     foo::info_fmt"hello, {}!"

The warning suggests replacing info__ with info_fmt, even though StrFormatLikeFunctions only includes foo::info—not foo::info__. It appears that clang-tidy matches function names by prefix rather than requiring an exact match.

After applying the suggested fix and running clang-tidy again, it then suggests replacing foo::info_fmt with foo::info_fmt, creating a “diagnostics loop”:

clang-tidy -checks=-clang-analyzer-* fmt.cc
1 warning generated.
~/fmt.cc:12:5: warning: use 'foo::info_fmt' instead of 'info_fmt' [modernize-use-std-format]
   12 |     foo::info_fmt ("hello, {}!", "world");
      |     ^~~~~~~~~~~~~  ~~~~~~~~~~~~
      |     foo::info_fmt  "hello, {{}}!"

Is it intentional?

clang-tidy version: 21.1.2
Platform: Windows

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Aleksey Sakovets (asakovets)

Hi,

I’ve encountered unexpected behavior with the modernize-use-std-format check in clang-tidy. Consider the following code:

#include <fmt/format.h>

namespace foo {
void info__ (const char * s, ...) {}

template <typename... Args> 
void info_fmt (fmt::format_string<Args...> fmt, Args &&... args) {}
}

int main ()
{
    foo::info__ ("hello, %s!", "world");
}

My .clang-tidy configuration is

Checks: 'modernize-use-std-format'
CheckOptions:
  - key: modernize-use-std-format.StrFormatLikeFunctions
    value: "foo::info"
  - key: modernize-use-std-format.ReplacementFormatFunction
    value: foo::info_fmt

Running clang-tidy:

clang-tidy -checks=-clang-analyzer-* fmt.cc

1 warning generated.
~/fmt.cc:12:5: warning: use 'foo::info_fmt' instead of 'info__' [modernize-use-std-format]
   12 |     foo::info__ ("hello, %s!", "world");
      |     ^~~~~~~~~~~  ~~~~~~~~~~~~
      |     foo::info_fmt"hello, {}!"

The warning suggests replacing info__ with info_fmt, even though StrFormatLikeFunctions only includes foo::info—not foo::info__. It appears that clang-tidy matches function names by prefix rather than requiring an exact match.

After applying the suggested fix and running clang-tidy again, it then suggests replacing foo::info_fmt with foo::info_fmt, creating a “diagnostics loop”:

clang-tidy -checks=-clang-analyzer-* fmt.cc
1 warning generated.
~/fmt.cc:12:5: warning: use 'foo::info_fmt' instead of 'info_fmt' [modernize-use-std-format]
   12 |     foo::info_fmt ("hello, {}!", "world");
      |     ^~~~~~~~~~~~~  ~~~~~~~~~~~~
      |     foo::info_fmt  "hello, {{}}!"

Is it intentional?

clang-tidy version: 21.1.2
Platform: Windows

[flovent]

The warning suggests replacing info__ with info_fmt, even though StrFormatLikeFunctions only includes foo::info—not foo::info__. It appears that clang-tidy matches function names by prefix rather than requiring an exact match.

It's matched by regex, you can change it to foo::info$ to avoid matching of foo::info__.

Edit:
But for some checks (including this one) they didn't explicitly state whether they are using regex to match in doc, maybe that's something can be improve.

[asakovets]

Got it, thanks.

But for some checks (including this one) they didn't explicitly state whether they are using regex to match in doc, maybe that's something can be improve.

Yeah, that's why it confused me.