[Issue 20050] New: pure function should be able to return function pointer to impure static nested function

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20050

          Issue ID: 20050
           Summary: pure function should be able to return function
                    pointer to impure static nested function
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: rejects-valid
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: ag0aep6g at gmail.com

Found while looking into issue 20047.

Should compile:

----
int g = 0;
void impure_function_1() { ++g; }
void function() get_impure_function() pure
{
    static void impure_function_2()
    {
        ++g; /* Error: pure function test.get_impure_function cannot access
mutable static data g */
        impure_function_1(); /* Error: pure function test.get_impure_function
cannot call impure function test.impure_function_1 */
    }
    return &impure_function_2;
}
void main()
{
    auto f = get_impure_function();
    f();
    assert(g == 2);
}
----

Just because `get_impure_function` is pure doesn't mean that
`impure_function_2` also has to be pure.

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