Ref assignment to ref ternary silently falls back to non-ref assignment

[jjonescz]

Version Used: cbe141c

Steps to Reproduce:

static void M(bool b, ref string x, ref string y, ref string z)
{
    (b ? ref x : ref y) = ref z;
}

SharpLab

Expected Behavior:

Emitted equivalently to

if (b) { x = ref z; } else { y = ref z; }

Or perhaps an error like

error CS8373: The left-hand side of a ref assignment must be a ref variable.

Actual Behavior:

Emitted like non-ref assignment (b ? ref x : ref y) = z.

Notes:

When emitting with a Debug roslyn build, this assert fails:

roslyn/src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_AssignmentOperator.cs

Line 258 in bd5c00e

Debug.Assert(!isRef);

Search keywords: ref conditional operator.

This issue is referenced in code (via #74498).

[jjonescz]

Similar thing happens also when the LHS is a ref assignment - these two currently emit the same IL:

static void M1(ref int x, ref int y, ref int z)
{
    (x = ref y) = ref z;
}
static void M2(ref int x, ref int y, ref int z)
{
    (x = ref y) = z;
}

[jjonescz]

Looks like both cases should be errors, only locals and parameters are allowed on the LHS per the spec:

The left operand shall be an expression that binds to a reference variable (§9.7), a reference parameter (other than this), an output parameter, or an input parameter.