https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97751
Bug ID: 97751
Summary: C++20 NTTP: class template argument deduction failed
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: janpmoeller at gmx dot de
Target Milestone: ---
The following code results in a hard compile error in gcc-10.2 and the latest
trunk:
/////////////////////////////////////////////////////////////////////////////
template <int N>
struct use_as_nttp {};
template <use_as_nttp Value>
struct has_nttp {};
template <use_as_nttp Value>
using has_nttp_2 = has_nttp<Value>;
/////////////////////////////////////////////////////////////////////////////
This is the output:
#############################################################################
<source>:8:34: error: class template argument deduction failed:
8 | using has_nttp_2 = has_nttp<Value>;
| ^
<source>:8:34: error: no matching function for call to
'use_as_nttp(use_as_nttp<...auto...>)'
<source>:2:8: note: candidate: 'template<int N> use_as_nttp()-> use_as_nttp<N>'
2 | struct use_as_nttp {};
| ^~~~~~~~~~~
<source>:2:8: note: template argument deduction/substitution failed:
<source>:8:34: note: candidate expects 0 arguments, 1 provided
8 | using has_nttp_2 = has_nttp<Value>;
| ^
<source>:2:8: note: candidate: 'template<int N> use_as_nttp(use_as_nttp<N>)->
use_as_nttp<N>'
2 | struct use_as_nttp {};
| ^~~~~~~~~~~
<source>:2:8: note: template argument deduction/substitution failed:
<source>:8:34: note: mismatched types 'use_as_nttp<N>' and
'use_as_nttp<...auto...>'
8 | using has_nttp_2 = has_nttp<Value>;
| ^
Compiler returned: 1
#############################################################################
gcc was invoked with "-std=c++20 -Wall -Wextra -Wpedantic". The error can also
be observed on compiler explorer: https://godbolt.org/z/xbPrjj
I believe that this is valid C++20 and should compile (but not generate any
assembly, since none of the types are actually instantiated).
