[Issue 23666] Recognize template opApply pattern

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23666

--- Comment #1 from Bolpat <qs.il.paperinik at gmail.com> ---
Generalization:

To be “aptly constrained” means that the constraint is a type of the following
form:
```d
Int delegate ParameterList MemberFunctionAttributes`
```
where `ParameterList` and `MemberFunctionAttributes` [3, 4] are defined in the
D grammar, and Int is `int` with any type constructors (`const`, `inout`,
`shared`) applied to it (they don’t matter anyways), but it shouldn’t fail
because some type inference made it e.g. `const`.

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