Enable failable initializers for noncopyable types

Noncopyable types were prevented from having failable initializers
because `Optional` itself didn't support noncopyable types. Now
`Optional` does, so lift this restriction and add a test.

(cherry picked from commit bd696f9)

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -7727,8 +7727,6 @@ ERROR(noncopyable_enums_do_not_support_indirect,none,
       "noncopyable enum %0 cannot be marked indirect or have indirect cases yet", (Identifier))
 ERROR(noncopyable_cast,none,
       "noncopyable types cannot be conditionally cast", ())
-ERROR(noncopyable_failable_init,none,
-      "noncopyable types cannot have failable initializers yet", ())
 ERROR(noncopyable_objc_enum, none,
       "noncopyable enums cannot be marked '@objc'", ())
 ERROR(moveOnly_not_allowed_here,none,

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -4035,16 +4035,6 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       addDelayedFunction(CD);
     }
 
-    // a move-only / noncopyable type cannot have a failable initializer, since
-    // that would require the ability to wrap one inside an optional
-    if (CD->isFailable()) {
-      if (auto *nom = CD->getDeclContext()->getSelfNominalTypeDecl()) {
-        if (!nom->canBeCopyable()) {
-          CD->diagnose(diag::noncopyable_failable_init);
-        }
-      }
-    }
-
     checkDefaultArguments(CD->getParameters());
     checkVariadicParameters(CD->getParameters(), CD);
 

test/Generics/inverse_generics.swift

@@ -109,7 +109,7 @@ struct NeverCopyableDeinit<T: ~Copyable>: ~Copyable {
 }
 
 protocol Test: ~Copyable {
-  init?() // expected-error {{noncopyable types cannot have failable initializers yet}}
+  init?()
 }
 
 struct NoncopyableAndSendable: ~Copyable, Sendable {}

test/SILGen/moveonly_failable_init.swift

@@ -0,0 +1,29 @@
+// RUN: %target-swift-emit-sil -module-name test %s | %FileCheck %s --enable-var-scope
+
+struct MoveWithDeinit: ~Copyable {
+  deinit { }
+}
+
+struct MyType: ~Copyable {
+  var handle: MoveWithDeinit
+
+  // CHECK-LABEL: sil hidden @$s4test6MyTypeV6handleACSgAA14MoveWithDeinitVSg_tcfC : $@convention(method) (@owned Optional<MoveWithDeinit>, @thin MyType.Type) -> @owned Optional<MyType>
+  // CHECK: bb0(%0 : $Optional<MoveWithDeinit>, %1 : $@thin MyType.Type):
+  init?(handle: consuming MoveWithDeinit?) {
+    // CHECK: switch_enum [[SUBJECT:%.*]] : $Optional<MoveWithDeinit>, case #Optional.some!enumelt: bb2, case #Optional.none!enumelt: bb1
+    guard let handle = consume handle else {
+      // CHECK: bb1:
+      // CHECK: [[NONE:%.*]] = enum $Optional<MyType>, #Optional.none!enumelt
+      // CHECK: br bb3([[NONE]] : $Optional<MyType>)
+      return nil
+    }
+
+    // CHECK: bb2([[WRAPPED:%.*]] : $MoveWithDeinit):
+    // CHECK: br bb3
+    self.handle = handle
+  }
+}
+
+func test() -> MyType? {
+  return MyType(handle: MoveWithDeinit())
+}

test/Sema/moveonly_restrictions.swift

@@ -6,13 +6,13 @@ class CopyableKlass {}
 
 @_moveOnly
 class MoveOnlyKlass { // expected-note 6{{class 'MoveOnlyKlass' has '~Copyable' constraint preventing 'Copyable' conformance}}
-    init?() {} // expected-error {{noncopyable types cannot have failable initializers yet}}
+    init?() {}
 }
 
 @_moveOnly
 struct MoveOnlyStruct { // expected-note {{struct 'MoveOnlyStruct' has '~Copyable' constraint preventing 'Copyable' conformance}}
-    init?(one: Bool) {} // expected-error {{noncopyable types cannot have failable initializers yet}}
-    init!(two: Bool) {} // expected-error {{noncopyable types cannot have failable initializers yet}}
+    init?(one: Bool) {}
+    init!(two: Bool) {}
 }
 
 class C {
@@ -78,19 +78,19 @@ enum EMoveOnly {
     case lhs(CopyableKlass)
     case rhs(MoveOnlyKlass)
 
-    init?() {} // expected-error {{noncopyable types cannot have failable initializers yet}}
+    init?() {}
 }
 
 extension EMoveOnly {
-    init!(three: Bool) {} // expected-error {{noncopyable types cannot have failable initializers yet}}
+    init!(three: Bool) {}
 }
 
 extension MoveOnlyKlass {
-    convenience init?(three: Bool) {} // expected-error {{noncopyable types cannot have failable initializers yet}}
+    convenience init?(three: Bool) {}
 }
 
 extension MoveOnlyStruct {
-    init?(three: Bool) {} // expected-error {{noncopyable types cannot have failable initializers yet}}
+    init?(three: Bool) {}
 }
 
 func foo() {