[Bug] Inline Temporary Variable Refactoring produces invalid code (VS2017 RTM)

[axel-habermaier]

Version Used:
VS 2017 RTM

Steps to Reproduce:

1. Add the following code to a C# project:

class X
{
	void M<T>(T o) // where T : class does not make a difference
	{
		var x = o as X;
		if (x != null)
			;
	}
}

2. Invoke the "Inline Temporary Variable" refactoring on variable x.

Actual Behavior:
The refactoring preview shows that the generated code is going to be invalid. The refactoring can still be applied, yielding

class X
{
	void M<T>(T o)
	{
		if (o is X x)
			;
	}
}

The following compiler error results:
error CS8121: An expression of type T cannot be handled by a pattern of type X.

Expected Behavior:
Ideally, the compiler would support the code generated by the refactoring, as I understand the is pattern match in this case to be semantically equivalent to the original code.

At the very least, though, the refactoring should not be offered by the IDE.

[alrz]

This was reported before and is per spec as (X)o is not permitted. Perhaps the analyzer should check convertibility of the expression in the same sense of direct cast to cover corner-cases until this is fixed in the compiler (if possible). /cc @CyrusNajmabadi

[alrz]

Related: dotnet/csharplang#154

[axel-habermaier]

Thanks @alrz. A very unfortunate language limitation, especially given that the refactoring is offered (so even the IDE does not expect this limitation 😄).