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digitalmars.D.bugs - [Issue 7542] New: inout parameter contravariant should be allowed

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=7542

           Summary: inout parameter contravariant should be allowed
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: k.hara.pg gmail.com
        Depends on: 7105



After fixing issue 7105, we should allow inout parameter covariant.

Test cases:
----
void function(int* p) mfp;
void function(const(int)* p) cfp;
void function(immutable(int)* p) ifp;
void function(inout(int)* p) wfp;
wfp = mfp;
wfp = cfp;
wfp = ifp;
static assert(!__traits(compiles, wfp = function void(int**){}));
static assert(!__traits(compiles, wfp = function void(int[]){}));

void delegate(int* p) mdg;
void delegate(const(int)* p) cdg;
void delegate(immutable(int)* p) idg;
void delegate(inout(int)* p) wdg;
wdg = mdg;
wdg = cdg;
wdg = idg;
static assert(!__traits(compiles, wdg = delegate void(int**){}));
static assert(!__traits(compiles, wdg = delegate void(int[]){}));

void foo(T)(void delegate(inout(int)** p, inout(int)* v) dg)
{
    T* pm;
    T m;
    dg(&pm, &m);
    assert(pm == &m);
}
foo!(          int)((          int ** p,           int * v){ *p = v; });
foo!(    const int)((    const(int)** p,     const(int)* v){ *p = v; });
foo!(immutable int)((immutable(int)** p, immutable(int)* v){ *p = v; });

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next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=7542


Kenji Hara <k.hara.pg gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |pull



https://github.com/D-Programming-Language/dmd/pull/735

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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timon.gehr gmx.ch changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |timon.gehr gmx.ch



This pull breaks the type system:

void main(){
    // conversion from void function(int*) to void function(inout(int)*):
    void function(inout(int)*) wfp = function(int*)(*p = 1;}
    immutable int x = 0;
    wfp(&x); // mutates x
}

The title of the bug report is correct though, contravariance is safe.

Those are the safe conversions:
mfp = wfp; // match inout as mutable
wfp = cfp; // inout is a subtype of const
cfp = wfp; // match inout as const
ifp = wfp; // match inout as immutable

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prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=7542






 This pull breaks the type system:
 
 void main(){
     // conversion from void function(int*) to void function(inout(int)*):
     void function(inout(int)*) wfp = function(int*)(*p = 1;}
     immutable int x = 0;
     wfp(&x); // mutates x
 }
 
 The title of the bug report is correct though, contravariance is safe.
 
 Those are the safe conversions:
 mfp = wfp; // match inout as mutable
 wfp = cfp; // inout is a subtype of const
 cfp = wfp; // match inout as const
 ifp = wfp; // match inout as immutable


You are right, and this fix doesn't block 7543.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Boscop <kingboscop gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kingboscop gmail.com





 This pull breaks the type system:
 
 void main(){
     // conversion from void function(int*) to void function(inout(int)*):
     void function(inout(int)*) wfp = function(int*)(*p = 1;}


You can't do that because void function(int*) is a supertype of void
function(inout(int)*).
(as I described in comment 4 of bug 7543).

One wants to be able to do:
void fooM(int*);
void fooC(const(int)*);
void fooI(immutable(int)*);
void function(const(int)*) wfp;
wfp = &fooM;
wfp = &fooC;
wfp = &fooI;

This is only possible when the following subtyping rules are obeyed:
T <: const(T)
immutable(T) <: const(T)
And we also have:
T* <: const(T)*
immutable(T)* <: const(T)*
(
And btw:
T* <: const(T)* <: const(T*)
immutable(T)* <: immutable(T*) <: const(T*)
)
And of course the usual function subtyping rules (types of in-arguments are
contravariant and types of out-arguments and return types are covariant).

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prev sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=7542






 void fooM(int*);
 void fooC(const(int)*);
 void fooI(immutable(int)*);
 void function(const(int)*) wfp;
 wfp = &fooM;
 wfp = &fooC;
 wfp = &fooI;


Sorry, this was wrong, that would allow modifying the arg in fooM. And it
wouldn't work because int* is not a supertype of const(int)* but a subtype.
If you want the arg to be mutable, you'd want to do:
void fooM(int*);
void fooC(const(int)*);
void fooI(immutable(int)*);
void function(int*) wfp;
wfp = &fooM;
wfp = &fooC;
wfp = &fooI;
The last case doesn't work because immutable(T) is not a supertype of T.
But if you have a
void fooIO(inout(int)*);
you could do
wfp = &fooIO;
because inout(T) is a supertype of T.

If you want
void function(const(int)*) wfp;
only these work:
wfp = &fooC;
wfp = &fooIO;

fooM doesn't work because T is not a supertype of const(T)
Makes sense because you don't want to modify a const object.
fooI doesn't work because immutable(T) is not a supertype of const(T)
Makes sense because immutable functions assume that the arg is not modified
elsewhere.

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