[Issue 15623] is(M!N) evaluates to true for M!N that fails to instantiate.

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=15623

--- Comment #2 from Adrian Matoga <epi at atari8.info> ---
It's not only module scope - it's broken also inside templates, e.g.:

---
template canBeInstantiated(alias StructTemplate, Parameters...)
{
    static if (is(StructTemplate!Parameters))
        enum canBeInstantiated = true;
    else
        enum canBeInstantiated = false;
}
static assert( canBeInstantiated!(CallsFoo, HasFoo));
//static assert(!canBeInstantiated!(CallsFoo, NoFoo)); //fails
---

or:

---
template canBeInstantiated(alias StructTemplate, Parameters...)
{
    enum canBeInstantiated = is(StructTemplate!Parameters);
}
static assert( canBeInstantiated!(CallsFoo, HasFoo));
//static assert(!canBeInstantiated!(CallsFoo, NoFoo)); //fails
---


However, the function trick indeed works:

---
bool canBeInstantiated(alias StructTemplate, Parameters...)()
{
    return is(StructTemplate!Parameters);
}

static assert( canBeInstantiated!(CallsFoo, HasFoo));
static assert(!canBeInstantiated!(CallsFoo, NoFoo));  // hooray!
---

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