[Sema] SPI requirements in SPI protocols don't need a default implementation

The check for SPI protocol requirements needing a default implementation
should ignore requirements in a protocol that is SPI too. The protocol
can’t be conformed to without knowing of the requirement.

rdar://65286171

Author: xymus

lib/Sema/TypeCheckProtocol.cpp

@@ -5758,8 +5758,9 @@ void TypeChecker::inferDefaultWitnesses(ProtocolDecl *proto) {
     ResolveWitnessResult result = checker.resolveWitnessViaLookup(valueDecl);
 
     if (result == ResolveWitnessResult::Missing &&
-        requirement->isSPI()) {
-      // SPI requirements need a default value.
+        requirement->isSPI() &&
+        !proto->isSPI()) {
+      // SPI requirements need a default value, unless the protocol is SPI too.
       valueDecl->diagnose(diag::spi_attribute_on_protocol_requirement,
                           valueDecl->getName());
     }

test/SPI/protocol_requirement.swift

@@ -75,3 +75,24 @@ extension PublicProto {
   @_spi(Private)
   public init() { }
 }
+
+@_spi(Private)
+public protocol SPIProtocol {
+  @_spi(Private)
+  func reqWithoutDefault()
+
+  @_spi(Private)
+  var property: Int { get set }
+
+  @_spi(Private)
+  var propertyWithoutSetter: Int { get set }
+
+  @_spi(Private)
+  subscript(index: Int) -> Int { get set }
+
+  @_spi(Private)
+  init()
+
+  @_spi(Private)
+  static var staticProperty: Int { get }
+}