https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118187
Bug ID: 118187
Summary: Constraints of template template parameter not checked
for non-dependent arguments despite [temp.names]/8
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Keywords: accepts-invalid
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: hstong at ca dot ibm.com
Target Milestone: ---
https://wg21.link/temp.names#8 specifies that when
"""
all _template-argument_s in the _simple-template-id_ are non-dependent
([temp.dep.temp]), the associated constraints ([temp.constr.decl]) of the
constrained template shall be satisfied
"""
where the "constrained template" may be a template template parameter.
Even when template is instantiated, GCC and Clang both fail to emit a
diagnostic for a violation of this rule.
EDG and MSVC both diagnose without needing an instantiation.
See also: https://github.com/cplusplus/CWG/issues/658
Online compiler link: https://godbolt.org/z/fTzMnEPG8
### SOURCE (<stdin>)
template <typename T>
concept C = false;
template <typename> struct Q;
template <template <C> class TT>
struct A {
TT<int> *p;
};
A<Q> a;
### COMPILER INVOCATION
g++ -fsyntax-only -std=c++20 -Wall -Wextra -Werror -pedantic-errors -xc++ -
### ACTUAL COMPILER OUTPUT
(clean compile)
### EXPECTED COMPILER OUTPUT
(error)
### COMPILER VERSION INFO (g++ -v)
Using built-in specs.
COLLECT_GCC=/opt/wandbox/gcc-head/bin/g++
COLLECT_LTO_WRAPPER=/opt/wandbox/gcc-head/libexec/gcc/x86_64-pc-linux-gnu/15.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../source/configure --prefix=/opt/wandbox/gcc-head
--enable-languages=c,c++ --disable-multilib --without-ppl --without-cloog-ppl
--enable-checking=release --disable-nls --enable-lto
LDFLAGS=-Wl,-rpath,/opt/wandbox/gcc-head/lib,-rpath,/opt/wandbox/gcc-head/lib64,-rpath,/opt/wandbox/gcc-head/lib32
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 15.0.0 20241222 (experimental) (GCC)
