dmd accepts conversion from shared(int)* to int* when value comes from method

[dlangBugzillaToGithub]

ag0aep6g reported this on 2017-08-20T18:09:10Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=17769

CC List

• Lucia Cojocaru

Description

----
struct S()
{
    shared(int)* method() { return ptr; }
    shared(int)* ptr;
}

void main()
{
    S!() s;
    int* foo = s.method(); /* accepted; should be rejected */
}
----

These variations are correctly rejected:

----
    auto foo = s.method();
    int* bar = foo; // rejected as expected
    int* baz = s.ptr; // ditto
----

[dlangBugzillaToGithub]

lucia.mcojocaru commented on 2017-11-02T09:06:44Z

The reason for the bug is that there are different overloads in dcast.d for implicitConvTo for CallExp (int* foo = s.method() falling here) and Expression (where the other examples in this issue go).
https://github.com/dlang/dmd/blob/master/src/ddmd/dcast.d#L179

Expression: https://github.com/dlang/dmd/blob/master/src/ddmd/dcast.d#L194
CallExp: https://github.com/dlang/dmd/blob/master/src/ddmd/dcast.d#L809

For: int* foo = s.method(); 
Code goes into visit(CallExp) and calls visit(Expression) which defaults to NOMATCH(false).
https://github.com/dlang/dmd/blob/master/src/ddmd/dcast.d#L817

On NOMATCH, the checks continue in visit(CallExp) and if nothing is found, it dafaults to success (true).
https://github.com/dlang/dmd/blob/master/src/ddmd/dcast.d#L940


Everything else hits the default in visit(Expression) which is NOMATCH - hence the rejecting.

Now I have a fix here disallowing pointer conversions from shared to nonshared
https://github.com/somzzz/dmd/commit/6fdead01822071b464016f1c0313358f40494de2

But DIP29 allows this kind of conversions as long as the pointers are unique.
https://wiki.dlang.org/DIP29

This results in my fix breaking the following compiler tests for DIP29:
https://github.com/dlang/dmd/blob/master/test/runnable/implicit.d#L217

I'm not sure how to check if the pointer is unique. Also, are there any other subtleties related to DIP29 that these changes could be disregarding?