Bug 4860 – Taking delegates to a member function broken if method is also aliased in from a base class

Consider the following example (the alias directive in there might seem strange, but is needed if there would be another, non-overridden overload of foo (see http://www.digitalmars.com/d/2.0/function.html#function-inheritance): --- import std.stdio; class Base { void foo() { } } class Derived : Base { alias Base.foo foo; override void foo() { } } void main() { auto d = new Derived(); void delegate() dg = &d.foo; writefln("dg: (%s, %s)", dg.ptr, dg.funcptr); } --- As long as the alias is present, the delegate created via »&d.foo« is invalid – its funcptr part is null (and indeed, trying to call the delegate yields an access violation). If the alias directive is removed, everything works as expected, but as explained above, this is not an option. What may be also relevant is that DMD fails to infer the type for »&d.foo« – replacing »void delegate() dg« with »auto dg« produces »cannot infer type from overloaded function symbol &d.foo«. I would also be happy to learn about any workarounds, since this blocks the release of my D SWIG module.

Iain pointed out on #d that the example above works if you put the alias directive *after* the last overload: --- import std.stdio; class Base { void foo() { } } class Derived : Base { override void foo() { } alias Base.foo foo; } void main() { auto d = new Derived(); void delegate() dg = &d.foo; writefln("dg: (%s, %s)", dg.ptr, dg.funcptr); } ---

In order to make this bug report valid, you should cite a better example. In your example, you are overriding the base function, and then also aliasing it. Yes, in the case you reference, it's valid, but your trivial example is nonsensical -- you get nothing by aliasing Base.foo in this case. I am pretty sure you have a better example :)

Steven, I am not quite sure if I see why a non-minimal code snippet in a bug report would be useful, but here you go: --- import std.stdio; class Base { void foo( int i ) {} void foo( string s ) {} } class Derived : Base { alias Base.foo foo; override void foo( int i ) {} } void main() { auto d = new Derived(); void delegate( int ) dg = &d.foo; writefln("dg: (%s, %s)", dg.ptr, dg.funcptr); } --- Feel free to reduce that to the above test case again. ;)

(In reply to comment #3) > Steven, I am not quite sure if I see why a non-minimal code snippet in a bug > report would be useful Because it avoids an argument against fixing the bug because the use case is completely useless. Your new example is perfect, thanks!

What? A bug is a bug, it doesn't matter if the code causing it is nonsensical. A code snippet reproducing a bug should be as minimal as possible.

Wrong-code bugs are always important. Here's a mitigation patch to turn it into a rejects-valid bug. Haven't tracked down the root cause yet, but there certainly should be an assert in this function -- if it's a virtual function, it should have a non-negative vtable index, not -1. e2ir.c, line 3275. DelegateExp::toElem() Symbol *sfunc; int directcall = 0; printf("DelegateExp::toElem() '%s'\n", toChars()); + if (func->isVirtual() && func->vtblIndex < 0) + error("Internal compiler error: malformed delegate. See Bugzilla 4860"); sfunc = func->toSymbol(); if (func->isNested())

That patch was a bit too early in the function. Should be e2ir.c, line 3308. Still in DelegateExp::toElem(). { // Get pointer to function out of virtual table unsigned vindex; assert(ethis); ep = el_same(&ethis); ep = el_una(OPind, TYnptr, ep); vindex = func->vtblIndex; + if (vindex < 0) + error("Internal compiler error: malformed delegate. See Bugzilla 4860"); // Build *(ep + vindex * 4) ep = el_bin(OPadd,TYnptr,ep,el_long(TYsize_t, vindex * 4)); ep = el_una(OPind,TYnptr,ep); }

Don's mitigation patch: http://www.dsource.org/projects/dmd/changeset/824

Downgrading to rejects-valid, now that the patch is in place.

This is a symptom of issue 3359 - overload sets are not searched properly in some cases. *** This issue has been marked as a duplicate of issue 3359 ***

This is a symptom of issue 3559 - overload sets are not searched properly in some cases. *** This issue has been marked as a duplicate of issue 3559 ***