[Issue 22648] New: [std.variant.Variant] Incorrectly written unittests

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22648

          Issue ID: 22648
           Summary: [std.variant.Variant] Incorrectly written unittests
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: trivial
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: jlourenco5691 at gmail.com

The unittests written from the line 2055 to 2058 inclusive are wrong. Their
logic is correct but they're passing by the wrong reasons. Currently, they fail
because of the compile-time error "Error: has no effect". The result of the
comparison is discarded and that results in an error, failing the
'__traits(compiles, ...)' tests. This is unittest in question:

```
@system unittest
{
    // check comparisons incompatible with AllowedTypes
    Algebraic!int v = 2;

    assert(v == 2);
    assert(v < 3);
    static assert(!__traits(compiles, {v == long.max;}));
    static assert(!__traits(compiles, {v == null;}));
    static assert(!__traits(compiles, {v < long.max;}));
    static assert(!__traits(compiles, {v > null;}));
}
```

To correctly write the test, the value should be explicitly discarded by using
a `cast(void)` in the whole expression, or by returning the result either by
adding a `return` or transforming the lambda in an inline lambda `() =>`;

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