Warn about covariant mutable array usage.

[be5invis]

Γ ⊢ A <: B should NOT imply Γ ⊢ A[] <: B[], unless the array is immutable.
Reflect: dotnet/roslyn#4211

[jnm2]

What are you proposing, taking backwards compatibility into account?

[be5invis]

@JMM2 Maybe a warning about assigning a parent class to a subclass array?

[jnm2]

@be5invis Roslyn analyzer or C# compiler warning? And what about contravariance? One errors on read, the other errors on write.

[be5invis]

@jnm2 Both are okay.

[svick]

@be5invis

Are you talking about code like this?

class Base {}
class Derived : Base {}

…

Base[] b = new Base[1];
Derived[] d = b;

Because that currently reports an error ("CS0266 Cannot implicitly convert type 'Base[]' to 'Derived[]'. An explicit conversion exists (are you missing a cast?)"). It compiles if you add a cast (i.e. Derived[] d = (Derived[])b;), but presumably such cast means you know what you're doing, so a warning is not necessary.

[gafter]

The language specification generally does not mandate warnings. It sounds more like a compiler or analyzer request, which would be more appropriate in the Roslyn repo.

[hez2010]

This also prevents a bunch of possibilities of JIT optimization due to covariant arrays.
IMO arrays should be made invariant by default, no one writes

Base[] array = new Derived[10];
array[0] = new Base();

[hez2010]

But the JIT compiler can't assume arrays are invariant so related optimizations cannot be performed.

[CyrusNajmabadi]

Adding a warning won't change that. Even with a warning, the compiler wouldn't be able to make such an assumption.

[hez2010]

I think the compiler may emit an attribute like NoConvariantCastArray at assembly level to help JIT understand there's no covariance on arrays in an assembly.

[ufcpp]

dotnet/runtime#58696

[ufcpp]

dotnet/runtime#23393