[release/7.0] [metadata] Skip null vtable entries when checking covariant return overrides #76324
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When there are covariant return overrides, we check the vtable slot in every
parent class for signature compatability with the proposed override.
However if one of the ancestor classes is abstract, it could have an
abstract override for hte method:
class Base {
public virtual Base Method() => this;
}
public abstract Intermediate : Base {
public override abstract Base Method();
}
public Leaf : Intermediate {
public override Leaf Method() => this;
}
In this case when we're checking that Leaf.Method is compatible with
the vtable slot in Intermediate, we will see a null method (since
Intermediate is abstract). In such cases we can skip over the class
and continue with its parent.
Fixes
#76312
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@lambdageek if this is ready, can you please send an email to Tactics requesting approval? |
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Sep 29, 2022
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Approved by Tactics, signed off, green CI. This is fully baked. |
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Backport of #76323 to release/7.0
/cc @lambdageek
Customer Impact
Customers using code that uses C# covariant returns in mobile or WebAssembly applications experienced crashes when using classes that re-abstracted virtual methods that had covariant returns in child subclasses. At least one commercial NuGet package used this pattern which prevented their customers from deploying the library in Android applications.
See dotnet/maui#6811
Testing
New CI test and manual testing
Risk
Low. The change affects validation done by the runtime while loading classes. A null pointer dereference is avoided. No existing checks are removed, no new checks are added. Code generation is unchanged. Any code that previously worked will continue to work.