https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118323
Bug ID: 118323
Summary: Unhelpful diagnostic for consteval failure inside
lambda that is immediate-escalating
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Keywords: diagnostic
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
Target Milestone: ---
This C++20 program gives a nice clear diagnostic:
consteval int check(int i) { if (i > 100) throw; return i; }
void funk(int) { }
void func()
{
#ifdef LAMBDA
[]{
funk(check(101));
}();
#else
funk(check(101));
#endif
}
int main()
{
func();
}
But if compiled with -DLAMBDA the diagnostic no longer says what's wrong:
esc.cc: In function 'void func()':
esc.cc:10:4: error: call to consteval function '<lambda closure
object>func()::<lambda()>().func()::<lambda()>()' is not a constant expression
8 | []{
| ~~~
9 | funk(check(101));
| ~~~~~~~~~~~~~~~~~
10 | }();
| ~^~
esc.cc:10:4: error: 'func()::<lambda()>' called in a constant expression
esc.cc:8:3: note: 'func()::<lambda()>' is not usable as a 'constexpr' function
because:
8 | []{
| ^
esc.cc:9:9: error: call to non-'constexpr' function 'void funk(int)'
9 | funk(check(101));
| ~~~~^~~~~~~~~~~~
esc.cc:3:6: note: 'void funk(int)' declared here
3 | void funk(int) { }
| ^~~~
esc.cc:9:15: note: 'func()::<lambda()>' was promoted to an immediate function
because its body contains an immediate-escalating expression 'check(101)'
9 | funk(check(101));
| ~~~~~^~~~~
This is completely unhelpful. It says nothing about the error inside check(101)
which makes it not a constant expression. Obviously the lambda body can't be a
constant expression, because it calls func(int) which is not constexpr.
