[Issue 17645] New: `pure` is transitively applied to all delegates inside a pure function

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=17645

          Issue ID: 17645
           Summary: `pure` is transitively applied to all delegates inside
                    a pure function
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: major
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: tomer at weka.io

DMD64 D Compiler v2.074.0. Consider the following:

static int x;
void f() pure {
    auto dg = (){x++;};
}

src/dtest.d(45,19): Error: pure delegate 'dtest.f.__lambda1' cannot access 
mutable static data 'x'

Had I invoked dg() inside f(), this error would be correct. But I'm only
creating a delegate, which I will pass to some external, non-pure entity, which
is allowed to call it. 

The use-case here is that sometimes we need the compiler to "just trust us" and
assume something is pure. It's used in exception cases which would soon after
reach an abort() so we don't consider this impure. And this restriction is too
broad for us

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