Slava Pestov 6f6974654a Sema: Fix type safety hole with convenience initializers which delegate to protocol extension initializers ...

A protocol extension initializer creates a new instance of the
static type of Self at the call site.

However a convenience initializer in a class is expected to
initialize an instance of the dynamic type of the 'self' value,
because convenience initializers can be inherited by subclasses.

This means that when a convenience initializer delegates to a
protocol extension initializer, we have to substitute the
'Self' type in the protocol extension generic signature with
the DynamicSelfType, and not the static type.

Since the substitution is formed from the type of the 'self'
parameter in the class convenience initializer, the solution is
to change the type of 'self' in a class convenience initializer
to DynamicSelfType, just like we do for methods that return
'Self'.

This fixes cases where we allowed code to type check that
should not type check (if the protocol extension initializer
has 'Self' in contravariant position, and we pass in an
instance of the static type).

It also fixes a miscompile with valid code -- if the protocol
extension initializer was implemented by calling 'Self()',
it would again use the static type and not the dynamic type.

Note that the SILGen change is necessary because Sema now creates
CovariantReturnExprs that convert a static class type to
DynamicSelfType, but the latter lowers to the former at the
SIL level, so we have to peephole away unnecessary unchecked_ref_cast
instructions in this case.

Because this change breaks source compatibility, it is guarded
by a '-swift-version 5' check.

2017-08-26 01:18:35 -07:00

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