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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/88557] Lambda in template parameter list compiler
 segmentation fault (ICE)
Date: Sun, 14 Apr 2024 00:08:00 +0000
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X-Bugzilla-Keywords: c++-lambda, ice-on-valid-code, rejects-valid
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D88557

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |c++-lambda, rejects-valid
   Last reconfirmed|2021-08-07 00:00:00         |2024-4-13

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Now we are rejecting this valid C++17 code:
```
    template<int =3D  []()constexpr{ return 0; }()>
    void f() { }
```

With:
```
<source>:3:21: error: lambda-expression in template-argument only available
with '-std=3Dc++2a' or '-std=3Dgnu++2a'
    3 |     template<int =3D  []()constexpr{ return 0; }()>
      |                     ^
```

Because I would have assumed calling a constexpr lambda would be a valid C+=
+17
code.

We even reject this and cause an ICE:
```
    template<int =3D [](auto)constexpr{ return 0; }.operator()(1)>
    void f() { }
```

But those examples are all valid C++17 due to constexpr'ness.=