[[maybe_unused]] attribute used inside a template argument specifying a type of a function causes a compilation error

[jarzec]

If a specification of a type of a function in a template argument uses the [[maybe_unused]] attribute an error is raised - the initial [ is interpreted as a beginning of a lambda capture.
This description is quite a mouthful, so let me give an example. The following code:

//...
std::function<int([[maybe_unused]] int)> f = fun;
//...

Causes the following error:

<source>:9:24: error: expected variable name or 'this' in lambda capture list
    9 |     std::function<int([[maybe_unused]] int)> f = fun;
      |                        ^
<source>:11:2: error: expected '>'
   11 | }
      |  ^
<source>:9:18: note: to match this '<'
    9 |     std::function<int([[maybe_unused]] int)> f = fun;
      |                  ^
<source>:11:2: error: expected unqualified-id
   11 | }
      |  ^
<source>:11:2: error: expected '}'
<source>:8:1: note: to match this '{'
    8 | {
      | ^
4 errors generated.
Compiler returned: 1

The quoted error is for x86-64 clang 18.1.0 from godbolt, but it is almost identical for all versions (at least) since 12.0.0.

I agree the [[maybe_unused]] attribute makes little sense in the presented context. However, at least to my understanding, the mentioned example is valid C++ syntax. I also believe a valid use case where it should be accepted is generated code. This, in fact, is how the issue was identified.
Other compilers (GCC and MSVC) correctly compile the code.

[shafik]

Looks like only clang rejects this: https://godbolt.org/z/Kbh44KxKn

CC @cor3ntin @erichkeane