[Issue 14771] New: Hidden @nogc violation around closure creation

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=14771

          Issue ID: 14771
           Summary: Hidden @nogc violation around closure creation
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: accepts-invalid
          Severity: major
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: k.hara.pg at gmail.com

The variable x in makeS() function is actually placed in closure object on
heap, but it's not checked by the @nogc.

struct S(alias f)
{
    auto foo()() { return f(0); }
    void dummy() {}
}

auto makeS() @nogc
{
    int x = 10;
    S!(a => x) s;

    // instantiating foo inside makeS will raise the @nogc violation error.
    //assert(s.foo() == 10);

    return s;
}

void main() @nogc
{
    auto s = makeS();

    // the hidden field of s is actually non-null,
    // that points closure object on heap.
    assert(s.tupleof[$-1] !is null);

    // instantiating foo outside makeS will place the variable x in closure
    // *after* the semantic3 completion of the function.
    // --> @nogc attribute on makeS() is ignored!
    // --> @nogc on main() has no effect, so foo itself has no GC-allocation.
    assert(s.foo() == 10);
}

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