[Bug c++/111008] New: '>' in a lambda as a template argument causes a syntax error

[Bug c++/111008] New: '>' in a lambda as a template argument causes a syntax error

[shin1_morita at yahoo dot co.jp]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111008

            Bug ID: 111008
           Summary: '>' in a lambda as a template argument causes a syntax
                    error
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: shin1_morita at yahoo dot co.jp
  Target Milestone: ---

Created attachment 55732
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55732&action=edit
Complete code for reproducing the bug

The following code (a lambda as a template argument) produces a syntax error,
which is compiled successfully by clang or msvc:
    return F<[](auto x)
    {
        return x > 0 ? x : 0;
    }>()(0);

'>' in the lambda seems to cause the error.

A workaround is surrounding it with parentheses:
        return (x > 0) ? x : 0;

See the attachment for complete code.

$ g++ -std=c++20 lambda.cc
lambda.cc: In lambda function:
lambda.cc:14:17: error: expected ';' before '>' token
   14 |         return x > 0 ? x : 0;
      |                 ^~
      |                 ;
lambda.cc:14:18: error: expected primary-expression before '>' token
   14 |         return x > 0 ? x : 0;
      |                  ^

$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/13.2.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure
--enable-languages=ada,c,c++,d,fortran,go,lto,objc,obj-c++ --enable-bootstrap
--prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/
--with-build-config=bootstrap-lto --with-linker-hash-style=gnu
--with-system-zlib --enable-__cxa_atexit --enable-cet=auto
--enable-checking=release --enable-clocale=gnu --enable-default-pie
--enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object
--enable-libstdcxx-backtrace --enable-link-serialization=1
--enable-linker-build-id --enable-lto --enable-multilib --enable-plugin
--enable-shared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch
--disable-werror
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 13.2.1 20230801 (GCC)

[Bug c++/111008] '>' in a lambda as a template argument causes a syntax error

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111008

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The most obvious workaround is to add parentheses around `x > 0`.

[Bug c++/111008] '>' in a lambda as a template argument causes a syntax error

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111008

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2023-08-13

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed reduced testcase:
template<bool f()>struct F{};
 F<[]() { return (1 > 0);  }> a;
 F<[]() { return 1 > 0;  }> a0;

The line containing a0 is rejected.

[Bug c++/111008] '>' in a lambda as a template argument causes a syntax error

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111008

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
A related non-lambda testcase:
```
 struct b{int a;};
 template<b>struct F1{};
 F1<b{(1>0)}> a;
 F1<b{1>0}> a0;
```

[Bug c++/111008] '>' in a lambda as a template argument causes a syntax error

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111008

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Here is a C++11 testcase that also fails in a similar way:
```
typedef int b;
template<b>struct F1{};
F1<b{(1>0)}> a;
F1<b{1>0}> a0;
```

[Bug c++/111008] '>' in a lambda as a template argument causes a syntax error

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111008

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=33744

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I suspect the fix will look similar to the fix of PR 33744 .

[Bug c++/111008] '>' in a lambda as a template argument causes a syntax error

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111008

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Here is another valid C++20 which is also causing issues, this time with `>>`
rather than `>`:
```
typedef int b;
template<b>struct F1{};
template<auto>struct F2{};
F1<b{(1>0)}> a;
constexpr int t = 3;
F2<F1<b{t>>0}>{}> a0;
```