[Sema] Move the check for SPI protocol requirements to existing logic

Asides from removing duplicated code this change also now checks implicit SPI
requirements and applies only in library evolution mode.

Author: xymus

lib/Sema/TypeCheckAttr.cpp

@@ -873,44 +873,6 @@ void AttributeChecker::visitSPIAccessControlAttr(SPIAccessControlAttr *attr) {
                             D->getDescriptiveKind());
     }
 
-    // If VD is a public protocol requirement it can be SPI only if there's
-    // a default implementation.
-    if (auto protocol = dyn_cast<ProtocolDecl>(D->getDeclContext())) {
-      auto implementations = TypeChecker::lookupMember(
-                                             D->getDeclContext(),
-                                             protocol->getDeclaredType(),
-                                             VD->createNameRef(),
-                                             NameLookupFlags::ProtocolMembers);
-      bool hasDefaultImplementation = llvm::any_of(implementations,
-        [&](const LookupResultEntry &entry) {
-          auto entryDecl = entry.getValueDecl();
-          auto DC = entryDecl->getDeclContext();
-          auto extension = dyn_cast<ExtensionDecl>(DC);
-
-          // The implementation must be defined in the same module in
-          // an unconstrained extension.
-          if (!extension ||
-              extension->getParentModule() != protocol->getParentModule() ||
-              extension->isConstrainedExtension())
-            return false;
-
-          // For computed properties and subscripts, check that the default
-          // implementation defines `set` if the protocol declares it.
-          if (auto protoStorage = dyn_cast<AbstractStorageDecl>(VD))
-            if (auto entryStorage = dyn_cast<AbstractStorageDecl>(entryDecl))
-              if (protoStorage->supportsMutation() &&
-                  !entryStorage->supportsMutation())
-                return false;
-
-          return true;
-        });
-
-      if (!hasDefaultImplementation)
-        diagnoseAndRemoveAttr(attr,
-                              diag::spi_attribute_on_protocol_requirement,
-                              VD->getName());
-    }
-
     // Forbid stored properties marked SPI in frozen types.
     if (auto property = dyn_cast<AbstractStorageDecl>(VD))
       if (auto DC = dyn_cast<NominalTypeDecl>(D->getDeclContext()))

lib/Sema/TypeCheckProtocol.cpp

@@ -5755,7 +5755,14 @@ void TypeChecker::inferDefaultWitnesses(ProtocolDecl *proto) {
     if (!valueDecl->isProtocolRequirement())
       continue;
 
-    checker.resolveWitnessViaLookup(valueDecl);
+    ResolveWitnessResult result = checker.resolveWitnessViaLookup(valueDecl);
+
+    if (result == ResolveWitnessResult::Missing &&
+        requirement->isSPI()) {
+      // SPI requirements need a default value.
+      valueDecl->diagnose(diag::spi_attribute_on_protocol_requirement,
+                          valueDecl->getName());
+    }
   }
 
   // Find defaults for any associated conformances rooted on defaulted

test/SPI/protocol_requirement.swift

@@ -1,23 +1,23 @@
 // Test limitations on SPI protocol requirements.
 
-// RUN: %target-typecheck-verify-swift
+// RUN: %target-typecheck-verify-swift -enable-library-evolution
 
 // Reject SPI protocol requirements without a default implementation.
 public protocol PublicProtoRejected {
-  @_spi(Private) // expected-error{{protocol requirement 'reqWithoutDefault()' cannot be declared '@_spi' without a default implementation in a protocol extension}}
-  func reqWithoutDefault()
+  @_spi(Private)
+  func reqWithoutDefault() // expected-error{{protocol requirement 'reqWithoutDefault()' cannot be declared '@_spi' without a default implementation in a protocol extension}}
 
-  @_spi(Private) // expected-error{{protocol requirement 'property' cannot be declared '@_spi' without a default implementation in a protocol extension}}
-  var property: Int { get set }
+  @_spi(Private)
+  var property: Int { get set } // expected-error{{protocol requirement 'property' cannot be declared '@_spi' without a default implementation in a protocol extension}}
 
-  @_spi(Private) // expected-error{{protocol requirement 'propertyWithoutSetter' cannot be declared '@_spi' without a default implementation in a protocol extension}}
-  var propertyWithoutSetter: Int { get set }
+  @_spi(Private)
+  var propertyWithoutSetter: Int { get set } // expected-error{{protocol requirement 'propertyWithoutSetter' cannot be declared '@_spi' without a default implementation in a protocol extension}}
 
-  @_spi(Private) // expected-error{{protocol requirement 'subscript(_:)' cannot be declared '@_spi' without a default implementation in a protocol extension}}
-  subscript(index: Int) -> Int { get set }
+  @_spi(Private)
+  subscript(index: Int) -> Int { get set } // expected-error{{protocol requirement 'subscript(_:)' cannot be declared '@_spi' without a default implementation in a protocol extension}}
 
-  @_spi(Private) // expected-error{{protocol requirement 'init()' cannot be declared '@_spi' without a default implementation in a protocol extension}}
-  init()
+  @_spi(Private)
+  init() // expected-error{{protocol requirement 'init()' cannot be declared '@_spi' without a default implementation in a protocol extension}}
 
   @_spi(Private) // expected-error{{'@_spi' attribute cannot be applied to this declaration}}
   associatedtype T