https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120017
Bug ID: 120017
Summary: Nested diagnostics could be slightly improved for
concepts-related diagnostics
Product: gcc
Version: unknown
Status: UNCONFIRMED
Keywords: diagnostic
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ensadc at mailnesia dot com
CC: dmalcolm at gcc dot gnu.org
Target Milestone: ---
This issue is specific to the new `experimental-nesting` option
(`-fdiagnostics-set-output=text:experimental-nesting=yes`).
====
https://godbolt.org/z/Wc1fqzoan
====
template<class T>
struct A {};
template<class T>
using B = A<T>::B;
template<class T>
concept C = false;
template<class T>
concept D = requires(T t) {
typename B<T>;
requires C<T>;
};
static_assert(D<int>);
====
<source>:16:15: error: static assertion failed
<source>:16:15: note: constraints not satisfied
• (level 1):required by the constraints of 'template<class T> concept D'
• (level 1):in requirements with 'T t' [with T = int]
• (level 1):the required type 'B<T>' is invalid, because
• (level 1):In substitution of 'template<class T> using B = typename A::B
[with T = int]':
• (level 1):required from here
• (level 1):error: no type named 'B' in 'struct A<int>'
• (level 1):nested requirement 'C<T>' is not satisfied, because
• (level 1):required for the satisfaction of 'C<T>' [with T = int]
• (level 1):the expression 'false' evaluated to 'false'
====
I'm using `experimental-nesting-show-locations=no`,
`experimental-nesting-show-levels=yes` and `-fno-diagnostics-show-caret` to
make it clearer to see the issue, and `-fconcepts-diagnostics-depth=3` to make
GCC not truncate the concepts-related diagnostics. So the full command-line is:
g++ -std=c++20
-fdiagnostics-set-output=text:experimental-nesting=yes,experimental-nesting-show-locations=no,experimental-nesting-show-levels=yes
-fno-diagnostics-show-caret -fconcepts-diagnostics-depth=3 example.cpp
I expect the messages after "because" to be more nested than the ones that end
with "because". That is, I expect the nesting levels to be:
```
<source>:16:15: error: static assertion failed
<source>:16:15: note: constraints not satisfied
• (level 1):required by the constraints of 'template<class T> concept D'
• (level 1):in requirements with 'T t' [with T = int]
• (level 1):the required type 'B<T>' is invalid, because
• (level 2):In substitution of 'template<class T> using B = typename A::B
[with T = int]':
• (level 2):required from here
• (level 2):error: no type named 'B' in 'struct A<int>'
• (level 1):nested requirement 'C<T>' is not satisfied, because
• (level 2):required for the satisfaction of 'C<T>' [with T = int]
• (level 2):the expression 'false' evaluated to 'false'
```
By the way, the nested messages are already a huge improvement to the old, flat
structure. I really appreciate it.
