Bug 14781 – [REG2.067] impure delegate to pure function context should be able to modify context

In a followup to issue 9148, which made this illegal: int impureFuncCall() { static int g; return g; } auto foo(out int delegate() pure pureDg) pure { int x; auto bar()() { impureFuncCall(); x = 1; // Error: impure function 'bar' cannot access variable 'x' // declared in enclosing pure function 'foo' } pureDg = &(bar!()); int dg() pure { return x;} return &dg; } This should be allowed. This doesn't violate purity of the returned delegate because the context pointer is mutable -- it's weak pure. This is similar to how a class can have pure and impure functions both accessing the same data.

Thanks.

(In reply to Steven Schveighoffer from comment #0) > In a followup to issue 9148, which made this illegal: > > int impureFuncCall() { static int g; return g; } > auto foo(out int delegate() pure pureDg) pure { > int x; > auto bar()() { > impureFuncCall(); > x = 1; // Error: impure function 'bar' cannot access variable > 'x' > // declared in enclosing pure function 'foo' > } > > pureDg = &(bar!()); > > int dg() pure { return x;} > return &dg; > } > > > This should be allowed. > ... Do you mean this code should compile? If so, either pure function `bar` can call an impure function or impure delegate is implicitly convertible to a pure one. If you are opening an issue with an enhancement please provide a clear and concise testcase. Thanks.

(In reply to Denis Shelomovskij from comment #3) > Do you mean this code should compile? Yes. > If so, either pure function `bar` can > call an impure function or impure delegate is implicitly convertible to a > pure one. No, but I made an error in the example from what I meant. should be: auto foo(out int delegate() impureDg) pure { ... impureDg = &(bar!()); > If you are opening an issue with an enhancement please provide a clear and > concise testcase. Thanks. It's hard to make a test case that "should compile" without making an error with no compiler to help you :D Really, the issue is that bar doesn't compile, not that the impure delegate can't be assigned.

(In reply to Steven Schveighoffer from comment #4) > (In reply to Denis Shelomovskij from comment #3) > > > Do you mean this code should compile? > > Yes. > > > If so, either pure function `bar` can > > call an impure function or impure delegate is implicitly convertible to a > > pure one. > > No, but I made an error in the example from what I meant. > > should be: > > auto foo(out int delegate() impureDg) pure { > > ... > > impureDg = &(bar!()); > > > If you are opening an issue with an enhancement please provide a clear and > > concise testcase. Thanks. > > It's hard to make a test case that "should compile" without making an error > with no compiler to help you :D > > Really, the issue is that bar doesn't compile, not that the impure delegate > can't be assigned. Then this function isn't strongly pure anymore: --- T f() pure; --- if `T` is `int delegate()`. Is everybody OK with this?

(In reply to Denis Shelomovskij from comment #5) > Then this function isn't strongly pure anymore: > --- > T f() pure; > --- > if `T` is `int delegate()`. > > Is everybody OK with this? I think that's already not strongly pure. `int delegate()` has a mutable indirection in the context pointer. So a function that returns such a delegate can't be strongly pure. See http://klickverbot.at/blog/2012/05/purity-in-d/#indirections_in_the_return_type An example with the delegate type: ---- struct S { int x = 1; int method() {return x++;} } int delegate() f() pure { return &(new S).method; } void main() { auto dg1 = f(); version(all) auto dg2 = f(); else auto dg2 = dg1; /* This would be equivalent to the above if f were strongly pure. */ import std.stdio; writeln(dg1()); /* "1" */ writeln(dg1()); /* "2" */ writeln(dg2()); /* "1" or "3", depending on the version above */ } ----

I change the importance to regression, because the introduced behavior since 2.067 rejects valid code.

New PR targeting 2.068.1 release: https://github.com/D-Programming-Language/dmd/pull/4970

Commits pushed to stable at https://github.com/D-Programming-Language/dmd https://github.com/D-Programming-Language/dmd/commit/922d79a43ff60ed324e32e2f4be1430de209746f fix Issue 14781 - impure delegate to pure function context should be able to modify context https://github.com/D-Programming-Language/dmd/commit/b30556bf2ce1b4e49fdba51ea759725c2fc6e48f Merge pull request #4970 from 9rnsr/fix14781 [REG2.067/2.068] Issue 14781 & 14962 - fix problematic purity inference introduced in #3626

Commits pushed to master at https://github.com/D-Programming-Language/dmd https://github.com/D-Programming-Language/dmd/commit/922d79a43ff60ed324e32e2f4be1430de209746f fix Issue 14781 - impure delegate to pure function context should be able to modify context https://github.com/D-Programming-Language/dmd/commit/b30556bf2ce1b4e49fdba51ea759725c2fc6e48f Merge pull request #4970 from 9rnsr/fix14781