Fix #5144: Avoid Expr of method type

[nicolasstucki]

Change the type of lifted method references to function types and
allow method references in Holes.

[smarter]

You could enforce this invariant by adding assert(resType.isValueTypeOrWildcard, resType.toString) in the constructor of ExprType, similarly to what is done in OrType and AndType.

[liufengyun]

Otherwise, LGTM

[liufengyun]

Just a curiosity: we do eta-expansion for methods when the expected type is a function, so that a method is always applied. In this case of quotes, it seems it breaks this invariant.

[nicolasstucki]

It is unrelated with the eta-expansion. This is about lifting a tree that represents a reference to a method that is outside the quote. It could not come from eta-expansion the code above could be def f(x: Int): Int = ~eval1('{ f(1); 2 }) where f also needs lifting.

[nicolasstucki]

@smarter I dont see the relation with ExprType, these where generating types such as scala.quoted.Expr[(x: Int): Int].

[smarter]

I see, so there's a typeapply where the argument is a methodtype? There could be an assertion to check for that too

[nicolasstucki]

Yes. I will add an assertion.

[nicolasstucki]

The conditions for the assertion in AppliedType are more complex and may add unwanted overhead. Let's explore the invariants of the types and how to check them in a separate PR.