[llvm-bugs] [Bug 49869] New: Overload resolution with using-declaration and constraints doesn't consider base function template

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49869

            Bug ID: 49869
           Summary: Overload resolution with using-declaration and
                    constraints doesn't consider base function template
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: barry.revzin at gmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

Consider:

#include <concepts>

struct A {};

struct Base {
#ifdef OK
    static constexpr int foo(int) { return 1; }
#else
    static constexpr int foo(auto) { return 1; }
#endif
};

struct Derived : Base {
    using Base::foo;
    template <std::same_as<A> T>
    static constexpr int foo(T) { return 2; }
};

static_assert(Derived::foo(42) == 1);
static_assert(Derived::foo(A{}) == 2);

With -DOK this compiles. Otherwise, the call to Derived::foo(42) fails with

<source>:19:15: error: no matching function for call to 'foo'
static_assert(Derived::foo(42) == 1);
              ^~~~~~~~~~~~
<source>:16:26: note: candidate template ignored: constraints not satisfied
[with T = int]
    static constexpr int foo(T) { return 2; }
                         ^
<source>:15:20: note: because 'std::same_as<int, A>' evaluated to false
    template <std::same_as<A> T>
                   ^

The base candidate is considered and used when it's not a template, but not
considered at all when it is a template. This seems related to whether the
Derived function template uses constraints or not.

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