c++: fix truncated diagnostic in C++23 [PR111272]
In C++23, since P2448, a constexpr function F that calls a non-constexpr
function N is OK as long as we don't actually call F in a constexpr
context. So instead of giving an error in maybe_save_constexpr_fundef,
we only give an error when evaluating the call. Unfortunately, as shown
in this PR, the diagnostic can be truncated:
z.C:10:13: note: 'constexpr Jam::Jam()' is not usable as a 'constexpr' function because:
10 | constexpr Jam() { ft(); }
| ^~~
...because what? With this patch, we say:
z.C:10:13: note: 'constexpr Jam::Jam()' is not usable as a 'constexpr' function because:
10 | constexpr Jam() { ft(); }
| ^~~
z.C:10:23: error: call to non-'constexpr' function 'int Jam::ft()'
10 | constexpr Jam() { ft(); }
| ~~^~
z.C:8:7: note: 'int Jam::ft()' declared here
8 | int ft() { return 42; }
| ^~
Like maybe_save_constexpr_fundef, explain_invalid_constexpr_fn should
also check the body of a constructor, not just the mem-initializer.
PR c++/111272
gcc/cp/ChangeLog:
* constexpr.cc (explain_invalid_constexpr_fn): Also check the body of
a constructor in C++14 and up.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1y/constexpr-diag1.C: New test.