[Bug c++/115844] New: [14 Regression] Invalid diagnostics for correctly-rejected non-consteval call in consteval function called in a regular lambda

[ali.mpfard at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115844

            Bug ID: 115844
           Summary: [14 Regression] Invalid diagnostics for
                    correctly-rejected non-consteval call in consteval
                    function called in a regular lambda
           Product: gcc
           Version: 14.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ali.mpfard at gmail dot com
  Target Milestone: ---

(Presumably post-<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107687>)
Diagnostics are completely wrong about consteval-ness of surrounding context
when an immediate function evaluation fails due to calling a non-consteval
function.
Furthermore, the actual cause is nowhere to be seen in the diagnostics.

Reproducer (CE link: https://gcc.godbolt.org/z/hdjecKsEn)
     template<typename>                                                         
     void point_at_me();                                                        

     template<typename T>                                                       
     consteval void point_at_that() {                                           
         point_at_me<T>();                                                      
     }                                                                          

     int main() {                                                               
         auto x = 42;                                                           
         auto l = [&x] {                                                        
             x += 1;                                                            
             point_at_that<int>();                                              
         };                                                                     
         l();                                                                   
     }    



Diagnostics on trunk:
     repro.cpp: In function 'int main()':                                       
     repro.cpp:16:6: error: call to consteval function 'l.main()::<lambda()>()'
is not a constant expression                                                    
        16 |     l();                                                           
           |     ~^~                                                            
     repro.cpp:16:6:   in 'constexpr' expansion of 'l.main()::<lambda()>()'     
     repro.cpp:12:11: error: the value of 'l' is not usable in a constant
expression                                                                      
        12 |         x += 1;                                                    
           |         ~~^~~~                                                     
     repro.cpp:11:10: note: 'l' was not declared 'constexpr'                    
        11 |     auto l = [&x] {                                                
           |          ^                                                         
     repro.cpp:13:27: note: 'main()::<lambda()>' was promoted to an immediate
function because its body contains an immediate-escalating expression
'point_at_that<int>()'                                          
        13 |         point_at_that<int>();                                      
           |         ~~~~~~~~~~~~~~~~~~^~ 



Diagnostics on 13.3 (expected):
     repro.cpp: In lambda function:                                             
     repro.cpp:13:27: error: 'consteval void point_at_that() [with T = int]'
called in a constant expression                                                 
        13 |         point_at_that<int>();                                      
           |         ~~~~~~~~~~~~~~~~~~^~                                       
     repro.cpp:5:16: note: 'consteval void point_at_that() [with T = int]' is
not usable as a 'constexpr' function because:                                   
         5 | consteval void point_at_that() {                                   
           |                ^~~~~~~~~~~~~                                       
     repro.cpp:6:19: error: call to non-'constexpr' function 'void
point_at_me() [with <template-parameter-1-1> = int]'                            
         6 |     point_at_me<T>();                                              
           |     ~~~~~~~~~~~~~~^~                                               
     repro.cpp:2:6: note: 'void point_at_me() [with <template-parameter-1-1> =
int]' declared here                                                             
         2 | void point_at_me();                                                
           |      ^~~~~~~~~~~