Include parent contexts in search for protocol extension contexts.

The previous check excluded nested functions that took and returned
static Self.  e.g.

protocol P {}
extension P {
  func foo() -> Self {
    func bar() -> Self {
      return self
    }
    return bar()
  }
}

Instead, search up the decl context chain until we find a protocol
extension context.

Author: CodaFi

lib/Sema/TypeCheckDecl.cpp

@@ -4047,20 +4047,23 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
     if (simpleRepr->getIdentifier() != TC.Context.Id_Self)
       return false;
 
+    // 'Self' in protocol extensions is not dynamic 'Self'.
+    DeclContext *dc = func->getDeclContext();
+    for (auto parentDC = dc; !parentDC->isModuleScopeContext();
+         parentDC = parentDC->getParent()) {
+      if (parentDC->getAsProtocolExtensionContext()) {
+        return false;
+      }
+    }
+
     // Dynamic 'Self' is only permitted on methods.
-    auto dc = func->getDeclContext();
     if (!dc->isTypeContext()) {
       TC.diagnose(simpleRepr->getIdLoc(), diag::dynamic_self_non_method,
                   dc->isLocalContext());
       simpleRepr->setInvalid();
       return true;
     }
 
-    // 'Self' in protocol extensions is not dynamic 'Self'.
-    if (dc->getAsProtocolExtensionContext()) {
-      return false;
-    }
-
     // 'Self' is only a dynamic self on class methods and
     // protocol requirements.
     auto declaredType = dc->getDeclaredTypeOfContext();

test/decl/func/dynamic_self.swift

@@ -35,6 +35,30 @@ protocol P0 {
   func g(_ ds: Self) // okay
 }
 
+extension P0 {
+  func h() -> Self { // okay
+    func g(_ t : Self) -> Self { // okay
+      return t
+    }
+    return g(self)
+  }
+}
+
+protocol P1: class {
+  func f() -> Self // okay
+
+  func g(_ ds: Self) // okay
+}
+
+extension P1 {
+  func h() -> Self { // okay
+    func g(_ t : Self) -> Self { // okay
+      return t
+    }
+    return g(self)
+  }
+}
+
 // ----------------------------------------------------------------------------
 // The 'self' type of a Self method is based on Self
 class C1 {