Bad recommended fix for nodiscard + enum class

[xTachyon]

Link.

Code:

[[nodiscard]] enum class E1 {};

enum [[nodiscard]] class E2 {};

Error:

<source>:1:19: error: misplaced attributes; expected attributes here
    1 | [[nodiscard]] enum class E1 {};
      | ~~~~~~~~~~~~~     ^
      |                   [[nodiscard]]
<source>:3:6: error: an attribute list cannot appear here
    3 | enum [[nodiscard]] class E2 {};
      |      ^~~~~~~~~~~~~

For E1, clang says to put [[nodiscard]] after the enum keyword, but if you do that, it errors on not being valid there either.

My guess is that works with old enums, but was not tested with enum class.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Damian Andrei (xTachyon)

[Link](https://godbolt.org/z/vb1dYo73q).

Code:

[[nodiscard]] enum class E1 {};

enum [[nodiscard]] class E2 {};

Error:

<source>:1:19: error: misplaced attributes; expected attributes here
    1 | [[nodiscard]] enum class E1 {};
      | ~~~~~~~~~~~~~     ^
      |                   [[nodiscard]]
<source>:3:6: error: an attribute list cannot appear here
    3 | enum [[nodiscard]] class E2 {};
      |      ^~~~~~~~~~~~~

For E1, clang says to put [[nodiscard]] after the enum keyword, but if you do that, it errors on not being valid there either.

My guess is that works with old enums, but was not tested with enum class.

[erichkeane]

The source of this diagnostic is the call to ProhibitAttributes in Parser::ParseDeclOrFunctionDefInternal in Parser.cpp. See ~1127.

We probably have to do a little parser-look-ahead there. Right now we're doing it with 'string length', but we probably want to do the 'next token' location (unless of course it is enum class/struct, where we have to do TWO tokens ahead!).

But this is distinctly parser problems, so @AaronBallman :)

Perhaps you have a better enough idea to mark this as 'good first issue'?