[Bug c++/124357] New: Type inference depends on the order of arguments

[d7d1cd at mail dot ru via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124357

            Bug ID: 124357
           Summary: Type inference depends on the order of arguments
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: d7d1cd at mail dot ru
  Target Milestone: ---

Here's the code:
    $ cat source.cc
    int bar(int, char) { return 0; }
    int bar(double, long) { return 1; }

    template <typename T, typename U>
    int foo(T t, int (*)(T, U)) { return bar(t, 0); }

    int main() {
        return foo(42, &bar);
    }

It compiles successfully, and the compiler infers the types correctly.


But if you swap the function arguments, the inference stops working, even
though nothing has changed:
    $ cat source.cc
    int bar(int, char) { return 0; }
    int bar(double, long) { return 1; }

    template <typename T, typename U>
    int foo(int (*)(T, U), T t) { return bar(t, 0); }

    int main() {
        return foo(&bar, 42);
    }

    $ g++ source.cc -std=c++23
    <source>: In function 'int main()':
    <source>:8:15: error: no matching function for call to 'foo(<unresolved
overloaded function type>, int)'
        8 |     return foo(&bar, 42);
        |            ~~~^~~~~~~~~~
    • there is 1 candidate
        • candidate 1: 'template<class T, class U> int foo(int (*)(T, U), T)'
        <source>:5:5:
            5 | int foo(int (*)(T, U), T t) { return bar(t, 0); }
                |     ^~~
        • template argument deduction/substitution failed:
            •   couldn't deduce template parameter 'U'
            <source>:8:15:
                8 |     return foo(&bar, 42);
                    |            ~~~^~~~~~~~~~