Type of string literal concatenated with non-immutable char array

[Per Nordlöw]

Given

     char x[];

why is

     typeof("a" ~ x)

`char[]` when

     typeof("a" ~ x.idup)

is

     `string`?

My case is

class NameLookupException : Exception
{
     this(string name) {
         super("Name " ~ name ~ " could not be found");
     }
     this(scope const(char)[] name) {
         super("Name " ~ name.idup ~ " could not be found");
     }
}

where I instead would like to only need

class NameLookupException : Exception
{
     this(scope const(char)[] name) {
         super("Name " ~ name ~ " could not be found");
     }
}

Why isn't

     "Name " ~ name ~ " could not be found"

implicitly convertible to `string`?

Would

class NameLookupException : Exception
{
     this(scope const(char)[] name) @trusted {
         super("Name " ~ cast(string)name ~ " could not be found");
     }
}

be ok?