[Issue 20437] New: Transitive immutable/shared does not apply to variables captured by delegates

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20437

          Issue ID: 20437
           Summary: Transitive immutable/shared does not apply to
                    variables captured by delegates
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: rightfold+bugzilla+dlang at gmail.com

The following compiles:

    int x;
    void delegate() d = delegate() { x = 1; };
    immutable(void delegate()) bad1 = d;
    shared(void delegate()) bad2 = d;

And the following, as a result, does too:

    void delegate() shared {
        bad1();
        bad2();
    }

This allows you to transfer bad1 or bad2 to another thread, to call it, and
modify x without synchronization.

* * *

Theoretically, a type like void delegate() immutable could be seen as the
following:

    struct dgt_ {
        int* x;
        void opCall() immutable;
    }

Whereas a type like immutable(void delegate()) could be seen as the following:

    struct dgt_ {
        int* x;
        void opCall();
    };
    alias dgt = immutable(dgt_);

In the latter case, immutable should apply to x too (similar for shared) but it
seemingly doesn’t for actual delegate types.

* * *

Verified with:

 - DMD64 D Compiler v2.085.1
 - LDC 1.16.0 based on DMD v2.086.1 and LLVM 8.0.1

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