Correct the evaluation order of conversions in deconstructions

[jcouv]

When deconstructing an object that has a Deconstruct method, the order of evaluation should be:

1. evaluate the left-hand side for side-effects
2. evaluate the right-hand side and evaluate the Deconstruct method (or methods in the case of nested deconstruction)
3. convert all the results
4. assign the results
5. construct a tuple representing the result of the assignment

[Youssef1313]

@jcouv This issue is still referenced in source.

roslyn/src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenDeconstructTests.cs

Lines 3004 to 3038 in 87bf0d0

	[Fact(Skip = "https://github.com/dotnet/roslyn/issues/12400")]
	[WorkItem(12400, "https://github.com/dotnet/roslyn/issues/12400")]
	public void AssignWithPostfixOperator()
	{
	string source = @"
	class C
	{
	int state = 1;
	static void Main()
	{
	long x;
	string y;
	C c = new C();
	(x, y) = c++;
	System.Console.WriteLine(x + "" "" + y);
	}
	public void Deconstruct(out int a, out string b)
	{
	a = state;
	b = ""hello"";
	}
	public static C operator ++(C c1)
	{
	return new C() { state = 2 };
	}
	}
	";
	// https://github.com/dotnet/roslyn/issues/12400
	// we expect "2 hello" instead, which means the evaluation order is wrong
	var comp = CompileAndVerify(source, expectedOutput: "1 hello");
	comp.VerifyDiagnostics();
	}

The test passes when unskipped, but there is a comment that says:

         // we expect "2 hello" instead, which means the evaluation order is wrong 

I don't quite get why "2 hello" is expected if we're using postfix operator?