[Bug c++/96959] New: GCC allows ill-formed explicit capture of requires-expression local parameter

[hstong at ca dot ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96959

            Bug ID: 96959
           Summary: GCC allows ill-formed explicit capture of
                    requires-expression local parameter
           Product: gcc
           Version: 11.0
            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: ---

A local parameter of a requires-expression is not a local entity (the name is
not introduced into a block scope and is, therefore, not a variable with
automatic storage duration). It is required that a simple-capture names a local
entity.

GCC appears to accept simple-captures that name local parameters of
requires-expressions.

Note: Even the IFNDR interpretation of this should not consider the concept to
be satisfied.

### SOURCE (<stdin>):
template <typename, typename> concept C0 = true;

template <typename T>
concept C =
requires(T t) {
  [t] { };
};

static_assert(C<int>);


### COMPILER INVOCATION:
g++ -fsyntax-only -std=c++20 -Wall -Wextra -pedantic-errors -xc++ -


### ACTUAL OUTPUT:
(clean compile)


### EXPECTED OUTPUT:
(error diagnostic)


### 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/11.0.0/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
gcc version 11.0.0 20200906 (experimental) (GCC)