Spurious "did not produce a valid constant expression" when a consteval function returns the address of a function that calls it

[stevensudit-corvid]

Issue Summary

The front end rejects a consteval member function template that returns the address of another member function template whose body calls it back. It reports the consteval function as "not completely defined yet".

gcc and clang accept the code. I ran into it in a type-erased callable whose consteval dispatch-table builder takes the address of a thunk that installs the same table.

Reproducing Source Code

struct t {
  template<int S> static bool live() { return make<S>() != nullptr; }
  template<int S> static consteval auto make() -> bool (*)() { return &live<S>; }
  template<int S> static auto get() -> bool (*)() { return make<S>(); }
};
bool (*p)() = t::get<1>();

Command-line Options

cpfe --c++23 repro.cpp

Type of Issue

[Front End] This code SHOULD compile with these options.

Additional Details

The diagnostic, at 158320e (version 7.0):

"repro.cpp", line 2: error: call to consteval function "t::make<S>()->bool
          (*)() [with S=1]" did not produce a valid constant expression
    template<int S> static bool live() { return make<S>() != nullptr; }
                                                       ^
"repro.cpp", line 2: note: this call cannot be evaluated because the target
          function function "t::make<S>()->bool (*)() [with S=1]" (declared
          at line 3) is not constexpr or not completely defined yet
    template<int S> static bool live() { return make<S>() != nullptr; }
                                                       ^
          detected during:
            instantiation of "bool t::live<S>() [with S=1]" at line 3
            instantiation of "auto t::make<S>()->bool (*)() [with S=1]" at line 4
            instantiation of "auto t::get<S>()->bool (*)() [with S=1]" at line 6

The trace shows live<1> being instantiated while make<1> is still being instantiated. Evaluating make<1>() needs only the address of live<1>, not its body, so the body can be instantiated after make<1> is complete, and then the call inside it evaluates.

--c++20 and --c++26 give the same error, and so do the GNU, Clang and Microsoft modes.

Any one of these changes makes it compile:

• live does not call make, so there is no cycle.
• live is defined outside the class.
• get is defined outside the class.
• make is constexpr instead of consteval.

It still fails when get is a member of a different class, and when t is a class template whose members are not themselves templates.

A possibly related symptom, in the default mode only: without get, calling t::make<1>() directly from non-template code, for example bool (*p)() = t::make<1>();, gives the same error. In the GNU and Clang modes that form compiles.

nvcc 13.3, which embeds EDG 6.8, rejects the six-line file with the same message, so this is not new in 7.0.

Reproducing (Standard) Configuration(s)

linux-gcc-release

Reproducing (Non-standard) Configuration

No response

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• I have checked open issues and existing discussions before filing this report.
• The reproducing source code that I have provided does not depend on headers that I have not included with my submission.
• I have reduced the reproducing source code to the best of my ability.
• I have selected at least one standard configuration or uploaded a non-standard configuration dump.
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