Bug 13845 – Error using alias of typeof(null) as unnamed lambda parameter

import std.variant; import std.stdio; alias Null = typeof(null); alias Maybe(T) = Algebraic!(T, typeof(null)); void main() { Maybe!int n = 0; writeln(n); //Prints "0" //Prints "I'm Null!" n.visit!( (int) => writeln("I'm an int!"), //Ok (typeof(null)) => writeln("I'm Null!"), //std/variant.d(1966): Error: static assert "__lambda2 is not a //function or delegate" /opt/compilers/dmd2/include/std/variant.d(1766): //instantiated from here: //visitImpl!(true, // VariantN!(8LU, int, typeof(null)), // function (int _param_0) //(Null) => writeln("I'm Null!"), ); }

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

(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.