Allow deconstruction without ValueTuple type

[jcouv]

From discussion with Neal and Mads today, it would be nice if (x, y) = (1, 2); didn't require the System.ValueTuple types.

Presently, the only deconstruction where ValueTuple is not referenced is when the right-hand-side is a non-tuple expression and the result of the deconstruction-assignment expression is not used (see #18629).

One difficulty with this is, what will the semantic model show for the right-hand-side?

Also, when we do this, we should think about safe-to-escape rules. We may want to do element-wise checks (including nested deconstructions and foreach-deconstructions). The scenario is (global1, local1) = (global2, local2);.

FYI @gafter

[jcouv]

Note: although ValueTuple is required to bind, not reference to it ends up emitted.
From discussion with Aleksey that makes sense: the language says there is an expression on the right-hand-side of the deconstruction, and it is a tuple.
I need to check with Neal and Mads whether that is sufficient/acceptable.

        [Fact]
        public void ValueTupleRequiredWhenRightHandSideIsTupleButNoReferenceEmitted()
        {
            string source = @"
class C
{
    public static void Main()
    {
        int x, y;
        (x, y) = (1, 2);
    }
}
";
            var comp = CreateStandardCompilation(source, parseOptions: TestOptions.Regular7, references: s_valueTupleRefs);
            comp.VerifyDiagnostics();

            Action<PEAssembly> assemblyValidator = assembly =>
            {
                var reader = assembly.GetMetadataReader();
                var names = reader.GetAssemblyRefNames().Select(name => reader.GetString(name));
                Assert.Empty(names.Where(name => name.Contains("ValueTuple")));
            };

            CompileAndVerify(comp, assemblyValidator: assemblyValidator);
        }

Note: this test should be checked in before closing this issue.

[jcouv]

Confirmed with Neal that the current behavior makes sense.

• (x, y) = (1, 2) requires SVT for binding, although it's not emitted.
• (x, y) = e doesn't require SVT for binding (or emitting)

Closing the issue.