ambiguous regression

[alephzero]

There was a regression between v3.10.3 and v3.10.4, in the compiler's ability to disambiguate std::function containing nlohmann::json.

In compiler explorer:

#include <https://raw.githubusercontent.com/nlohmann/json/v3.10.3/single_include/nlohmann/json.hpp>
#include <functional>
#include <string>

struct Foo {
  Foo() {}
  Foo(std::string) {}
};

struct Bar {
  Bar(std::function<void(Foo)>) {}
  Bar(std::function<void(nlohmann::json)>) {}
};

int main() {
  Bar bar([](Foo) {});
}

Generates

ASM generation compiler returned: 0
Execution build compiler returned: 0
Program returned: 0

Changing the version to v3.10.4:

#include <https://raw.githubusercontent.com/nlohmann/json/v3.10.4/single_include/nlohmann/json.hpp>
#include <functional>
#include <string>

struct Foo {
  Foo() {}
  Foo(std::string) {}
};

struct Bar {
  Bar(std::function<void(Foo)>) {}
  Bar(std::function<void(nlohmann::json)>) {}
};

int main() {
  Bar bar([](Foo) {});
}

Generates

<source>: In function 'int main()':
<source>:16:21: error: call of overloaded 'Bar(main()::<lambda(Foo)>)' is ambiguous
<source>:12:3: note: candidate: 'Bar::Bar(std::function<void(nlohmann::basic_json<>)>)'
<source>:11:3: note: candidate: 'Bar::Bar(std::function<void(Foo)>)'
ASM generation compiler returned: 1
<source>: In function 'int main()':
<source>:16:21: error: call of overloaded 'Bar(main()::<lambda(Foo)>)' is ambiguous
<source>:12:3: note: candidate: 'Bar::Bar(std::function<void(nlohmann::basic_json<>)>)'
<source>:11:3: note: candidate: 'Bar::Bar(std::function<void(Foo)>)'
Execution build compiler returned: 1

[alephzero]

This is still an issue in v3.10.5

[falbrechtskirchinger]

Like #3384 this one is also caused by 0e694b4.

[falbrechtskirchinger]

@alephzero Unfortunately I don't believe your issue is fixable in the short term. Above mentioned commit enabled the intended behavior of implicit string conversion so your only options are to either turn of implicit conversions (define JSON_USE_IMPLICIT_CONVERSIONS=0) or de-rank an overload in the ambiguous overload set, e.g.:

#include <https://raw.githubusercontent.com/nlohmann/json/develop/single_include/nlohmann/json.hpp>
#include <functional>
#include <string>

struct Foo {
  Foo() {}
  Foo(std::string) {}
};

struct Bar {
  // de-rank overload by making it a template
  template<typename>
  Bar(std::function<void(Foo)>) {}
  Bar(std::function<void(nlohmann::json)>) {}
};

int main() {
  Bar bar([](Foo) {});
}

@nlohmann Thoughts?

[falbrechtskirchinger]

@nlohmann This issue was fixed by #3604.