@nogc check fails when there are both nested @nogc function **and** a closure inside __trait(compiles)

[yanok]

Here is an example:

auto test(int a) @nogc
{
    int foo() @nogc { return a + 3; }  // (1)
    enum yes = __traits(compiles, { return a; });  // (2)
    return 1;
}

This produces an error:

<source>(1): Error: function `example.test` is `@nogc` yet allocates closure for `test()` with the GC
auto test(int a) @nogc
     ^
<source>(4):        delegate `() => a` closes over variable `a`
    enum yes = __traits(compiles, { return a; });
                                  ^
<source>(1):        `a` declared here
auto test(int a) @nogc
              ^

... but if I comment out either line (1) or line (2) it works fine. It also seems to be important that both foo and a closure in __trait(compiles) close over the same variable.

It's probably expected, but also worth mentioning that it's not just static analysis, codegen is also affected. If I remove @nogc from test declaration, the code compiles, but it does call _d_allocmemory, while commenting either line (1) or line (2) makes it work without memory allocation.

The example in godbolt: https://godbolt.org/z/Wzaq1Tzo7

Kudos to @EyalIO for spotting the bug and to @ljmf00 for making a minimal example.