relates to:

• SR-7405 Protocol compositions: improve composing diagnostics

Issue Description:

In SE-0156, the following rule is given:

To improve readability and reduce confusion, a class conforming to a typealias which contains a class type constraint does not implicitly inherit the class type: inheritance should stay explicit. Here are a few examples to remind what the current rules are and to make the previous sentence clearer:

The proposal does not change the rule which forbids using the protocol composition syntax in the inheritance clause:

protocol P1 {}
protocol P2 {}
class C {}
class D : P1 & P2 {} // Compiler error
class E : C & P1 {} // Compiler error

Class D in the previous example does not inherit a base class so it can be expressed using the inheritance/conformance syntax or through a typealias:

class D : P1, P2 {} // Valid
typealias P12 = P1 & P2
class D : P12 {} // Valid

Class E above inherits a base class. The inheritance must be explicitly declared in the inheritance clause and can't be implicitly derived from a typealias:

class E : C, P1 {} // Valid
typealias CP1 = C & P1
class E : CP1 {} // Compiler error: class 'E' does not inherit from class 'C'
class E : C, CP1 {} // Valid: the inheritance is explicitly declared

However this rule doesn't appear to match the current implementation. The following doesn't yield a compiler error, despite the proposal saying the inheritance must be spelled explicitly:

protocol P1 {}
protocol P2 {}
class C {}
typealias CP1 = C & P1
class E : CP1 {}

and the following doesn't compile, despite the proposal saying that it should:

protocol P1 {}
protocol P2 {}
class C {}
typealias CP1 = C & P1
class E : C, CP1 {} // error: Multiple inheritance from classes 'C' and 'C'