Array index from end in ValueTuple assignment throws IndexOutOfRangeException

[zmj]

Description

Assignment from tuple destructuring into a FromEnd index of a generic array throws IndexOutOfRangeException. It looks like stelem is called with the Index's raw value, instead of a computed value from the array length.

Reproduction Steps

M(new int[1]);

void M<T>(T[] a)
{
    (a[0], a[^1]) = (default, default);
}

Expected behavior

Assignment succeeds

Actual behavior

IndexOutOfRangeException

Regression?

No response

Known Workarounds

No response

Configuration

.NET 6.0.101

Other information

No response

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[CyrusNajmabadi]

Yup. I repro this, and it's super odd. @jcouv ?

[reflectronic]

Seems like there are two issues at play. Roslyn is emitting invalid IL for the second assignment:

.maxstack 4
.locals init (
    [0] !!T[],
    [1] valuetype [System.Private.CoreLib]System.Index,
    [2] !!T
)

// ...
IL_001b: ldloc.0
IL_001c: ldloc.1 // <---- Suspicious
IL_001d: ldloca.s 2
IL_001f: initobj !!T
IL_0025: ldloc.2
IL_0026: stelem !!T

The stelem opcode requires that the value pushed for index (which I emphasized in the above example with a comment) must be "of type native int or int32" (§III.4.26). However, the value pushed by ldloc.1 is a System.Index. So, aside from Roslyn emitting invalid code, there is a secondary (and perhaps less critical) bug where the JIT isn't detecting this as an invalid program.

[CyrusNajmabadi]

repros for:

M(new int[1]);

void M<T>(T[] a)
{
    (int v, a[^1]) = (default, default);
} 

Does not repro for:

M(new int[1]);

void M(int[] a)
{
    (int v, a[^1]) = (default, default);
} 

[Youssef1313]

Doesn't reproduce in main branch. A test-only change to prevent regression can close this.