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[metadata] Fields whose types are gparams with a reference type const…
…raint aren't blittlable. (mono#15761) * [metadata] Fields whose types are gparams with a reference type constraint aren't blittlable. Don't try to layout the field to find out if it's blittable. For gshared gparams, follow the blittability of the constraint. Fixes certain recursive examples. ``` using System; namespace TestRecursiveType { class Program { static void Main(string[] args) { SubClass subC = new SubClass(); Console.WriteLine(subC.GetTest()); } } public struct ValueTest<U> { // When U is instantiated with T, from BaseClass, we know it'll be a // reference field, so we know the instantiation ValueTest<T> won't // be blittable. public readonly U value; } public abstract class BaseClass<T> where T : BaseClass<T> { public ValueTest<T> valueTest = default(ValueTest<T>); } public class SubClass : BaseClass<SubClass> { private String test = "test"; public string GetTest() { return test; } } } ``` Fixes mono#15760 --- The failure is happening when we are doing mono_class_setup_fields ("BaseClass<T>") which needs to decide for each field whether it is blittable or not. So therefore we are trying to decide if ValueTest<T> (that is: the ValueTest<U> field inside BaseClass<T>) is blittable or not. So we instantiate U with T. Now to decide whether VaueTest<T> is blittable or not, we look at every field. So then we look at T value. To decide if T is blittable we first check if it's a reference type. That check is currently inadequate for generic parameters - what the PR adds is the ability to see if theres a T : class constraint or a T : C constraint - where C is some class. As soon as we know that T's constraint will force it to be a reference type we can definitely say that T won't be blittable without having to initialize C, at all. Previously, Mono would see that T is some kind of type for which it couldn't definitively decide that it's a reference type and it would call: mono_class_setup_fields (field_class) which would then try to setup the fields of the parent class BaseClass<T>. And that would hit the recursion check. Unity cherry-pick note: Needed to bring MONO_CLASS_IS_INTERFACE_INTERNAL and mono_class_get_valuetype_class forward
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,45 @@ | ||
| using System; | ||
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| class Program { | ||
| static void Main (string[] args) | ||
| { | ||
| // If this runs without a TLE, the test passed. A | ||
| // TypeLoadException due to recursion during type | ||
| // initialization is a failure. | ||
| var subC = new SubClass (); | ||
| Console.WriteLine (subC.GetTest ()); | ||
| // same as above, but try to land in generic sharing code. | ||
| var genSubC = new GenericSubClass<object> (); | ||
| Console.WriteLine (genSubC.GetTest ()); | ||
| } | ||
| } | ||
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| public struct ValueTest<U> { | ||
| // When U is instantiated with T, from BaseClass, we know it'll be a | ||
| // reference field without having to fully initialize its parent | ||
| // (namely BaseClass<T> itself), so we know the instantiation | ||
| // ValueTest<T> won't be blittable. | ||
| public readonly U value; | ||
| } | ||
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| public abstract class BaseClass<T> where T : BaseClass<T> { | ||
| public ValueTest<T> valueTest = default (ValueTest<T>); | ||
| } | ||
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| public class SubClass : BaseClass<SubClass> { | ||
| private string test = "test"; | ||
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| public string GetTest() | ||
| { | ||
| return test; | ||
| } | ||
| } | ||
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| public class GenericSubClass<T> : BaseClass<GenericSubClass<T>> { | ||
| private string test = "test"; | ||
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| public string GetTest() | ||
| { | ||
| return test; | ||
| } | ||
| } |
I had to add this null check here. Upstream the MonoGenericParamInfo is stored within the MonoGenericParam struct and is possibly why this is never null?