[clang] diagnostic of not-yet-defined consteval function isn't great

[philnik777]

struct S {

  static bool func2() { return func(); }

  static consteval bool func() { return true; }
};

results in the diagnostic

<source>:4:32: error: call to consteval function 'S::func' is not a constant expression
    4 |   static bool func2() { return func(); }
      |                                ^
<source>:4:32: note: undefined function 'func' cannot be used in a constant expression
<source>:6:25: note: declared here
    6 |   static consteval bool func() { return true; }
      |                         ^
1 error generated.

The note is somewhat confusing IMO, since the consteval function is in fact defined. Something like consteval function that has not yet been defined cannot be used would be better, and an additional note consider moving the definition of `func` above `func2` would be really great.

[shafik]

I believe this is related to cwg1890, cwg2335, cwg1397 etc

At first it seems like expr.const p5.4:

an invocation of an undefined constexpr function;

but once we are in the body of member function we are in a complete class context and so then we should have a definition. All the issues I mentioned are various versions of that.

I posted to core and waiting for more feedback.

[shafik]

See core discussion at: https://lists.isocpp.org/core/2023/10/14932.php

[jwakely]

Is this why the following code fails with clang + libstdc++?

#include <string>
constexpr auto z  = std::string("", 0).size();

There's no consteval function here, but we get a similar undefined function '_M_construct<const char *>' cannot be used in a constant expression diagnostic about a member function defined outside the class body.

[tbaederr]

Is this why the following code fails with clang + libstdc++?

Shouldn't be a problem I think? https://godbolt.org/z/vcbcKK1Ez works, unless you swap lines 10 and 12.

[cor3ntin]

@jwakely see #73232