https://issues.dlang.org/show_bug.cgi?id=13845

Peter Alexander <peter.alexander.au@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |peter.alexander.au@gmail.co
                   |                            |m
          Component|Phobos                      |DMD
            Summary|std.variant.Algebraic       |Error using alias of
                   |"__lambda2 is not a         |typeof(null) as unnamed
                   |function or delegate" when  |lambda parameter
                   |using alias vs. bare type   |
                 OS|Windows                     |All

--- Comment #1 from Peter Alexander <peter.alexander.au@gmail.com> ---
Reduced:

alias Null = typeof(null);
auto a = (int) => 0;           // ok
auto b = (typeof(null)) => 0;  // ok
auto c = (Null x) => 0;        // ok
auto d = (Null) => 0;          // error

Error: variable bug.d type void is inferred from initializer (Null) => 0, and variables cannot be of type void

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https://issues.dlang.org/show_bug.cgi?id=13845

Kenji Hara <k.hara.pg@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |INVALID

--- Comment #2 from Kenji Hara <k.hara.pg@gmail.com> ---
(In reply to Peter Alexander from comment #1)
> alias Null = typeof(null);
> auto a = (int) => 0;           // ok
> auto b = (typeof(null)) => 0;  // ok

`int` and `typeof(null)` are definitely parsed as types.

> auto c = (Null x) => 0;        // ok

`Null x` is parsed as a parameter, typed Null and parameter name is `x`.

> auto d = (Null) => 0;          // error
> 
> Error: variable bug.d type void is inferred from initializer (Null) => 0, and variables cannot be of type void

If a lambda parameter is an identifier, it's treated as the "parameter name". Therefore it is equivalent with:

  auto d = (x) => 0;

and compiler cannot infer the lambda parameter type so there's no "target" type.

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