Generic cast could be elided

[xtqqczze]

Generic casts such as the following pattern result in a redundant cast helper call:

runtime/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Dictionary.cs

Lines 117 to 119 in a693af3

	if (enumerable.GetType() == typeof(Dictionary<TKey, TValue>))
	{
	Dictionary<TKey, TValue> source = (Dictionary<TKey, TValue>)enumerable;

This necessitates using Unsafe.As for optimal codegen.

Diffs: MihuBot/runtime-utils#1382

Related: #118684

[MihuBot]

Possible related and/or duplicate issue:

• Type check in shared generic doesn't eliminate casts to/from confirmed exact type #49614

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[EgorBo]

Yeah, it's a dup of #49614

[xtqqczze]

I'm not certain this is a duplicate of #49614, actually. It's a different kind of type check.

[EgorBo]

I'm not certain this is a duplicate of #49614, actually. It's a different kind of type check.

I think it is? We have an optimization that handles such casts inside typechecks, it's just that it doesn't work for shared generics