gistfile1.txt

//$ zig test opaquetype.zig 
//opaquetype.zig:8:30: error: parameter of type '(anonymous opaque at opaquetype.zig:12:30)' not allowed
//    fn getValue(pOpaqueSelf: @OpaqueType()) u32 {
//                             ^
const assert = @import("std").debug.assert;

const S = struct {
    const Self = @This();

    value: u32,

    fn getValue(pOpaqueSelf: @OpaqueType()) u32 {
        pSelf = @ptrCast(*Self, pOpaqueSelf);
        return pSelf.value;
    }
};

test "opaquetype" {
    var s = S {
        .value = 1,
    };
    var pS = &s;
    assert(pS.value == 1);
    assert(pS.getValue() == 1);
}

Below is something I got working with help from Andrew and MajorLag.

$ zig test opaquetype.zig 
Test 1/1 opaquetype...OK
All tests passed.

My first mistake was getValue parameter pOpaqueSelf needed to be a pointer; pOpaqueSelf: *@OpaqueType. But that still
didn't work, there was still an error as the types didn't match because the call site of getValue had it's own @OpaqueType and apparently
the location is part of the type. So I had to have a single defintion for the type, but that also didn't work because @ptrCast forces
an alignment. So I finally created fn OpaquePtr which returns a pointer with proper alignment and is an @OpaqueType. And then
use that when defining fn getValue(pOpaqueSelf: OpaquePtr()) u32 and also when calling getValue; assert(S.getValue(@ptrCast(S.OpaquePtr(), pS)) == 1);

An interesting note: when an @OpaqueType() is used with extern C code @ptrCast at the call site isn't needed.
And example is that pthread_t = @OpaqueType() and Thread.Handle = c.pthread_t. There are then two call sites
which pass thread_ptr.data.handle, a call to c.pthread_create and posix.close in std/os/index.zig. Neither of these need @ptrCasts.

But when using @OpaqueType with zig it seems to require @ptrCast as can be seen below:

    assert(S.getValue(@ptrCast(S.OpaquePtr(), pS)) == 1);

Here is the full code that "works":

const assert = @import("std").debug.assert;

const S = struct {
    const Self = @This();

    value: u32,

    fn getValue(pOpaqueSelf: OpaquePtr()) u32 {
        var pSelf = @ptrCast(*Self, pOpaqueSelf);
        return pSelf.value;
    }

    /// OpaquePtr to S with appropriate alignment
    fn OpaquePtr() type {
        return (*align(@alignOf(S)) @OpaqueType());
    }
};

test "opaquetype" {
    var s = S {
        .value = 1,
    };
    var pS = &s;
    assert(pS.value == 1);
    assert(S.getValue(@ptrCast(S.OpaquePtr(), pS)) == 1);
}