[Type checker] Warn about unavailable witnesses used to satisfy a requirement

include/swift/AST/DiagnosticsSema.def

@@ -1533,6 +1533,10 @@ NOTE(declared_protocol_conformance_here,none,
      "%0 implicitly conforms to protocol %2}1 here",
      (Type, unsigned, DeclName, DeclName))
 
+WARNING(witness_unavailable,none,
+        "unavailable %0 %1 was used to satisfy a requirement of protocol %2",
+        (DescriptiveDeclKind, DeclName, DeclName))
+
 ERROR(redundant_conformance,none,
       "redundant conformance of %0 to protocol %1", (Type, DeclName))
 

lib/Sema/TypeCheckProtocol.cpp

@@ -274,6 +274,9 @@ namespace {
     /// The witness is a constructor which is more failable than the
     /// requirement.
     ConstructorFailability,
+
+    /// The witness itself is inaccessible.
+    WitnessUnavailable,
   };
 
   /// Describes an optional adjustment made to a witness.
@@ -1539,6 +1542,11 @@ checkWitness(AccessScope requiredAccessScope,
     }
   }
 
+  if (match.Witness->getAttrs().isUnavailable(TC.Context) &&
+      !requirement->getAttrs().isUnavailable(TC.Context)) {
+    return CheckKind::WitnessUnavailable;
+  }
+
   return CheckKind::Success;
 }
 
@@ -2905,7 +2913,7 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
           auto ctor = cast<ConstructorDecl>(requirement);
           auto witnessCtor = cast<ConstructorDecl>(witness);
           auto &diags = witness->getASTContext().Diags;
-          diags.diagnose(witness->getLoc(),
+          diags.diagnose(witness,
                          diag::witness_initializer_failability,
                          ctor->getFullName(),
                          witnessCtor->getFailability()
@@ -2914,6 +2922,19 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
         });
 
       break;
+
+    case CheckKind::WitnessUnavailable:
+      diagnoseOrDefer(requirement, /*isError=*/false,
+        [witness, requirement](NormalProtocolConformance *conformance) {
+          auto &diags = witness->getASTContext().Diags;
+          diags.diagnose(witness, diag::witness_unavailable,
+                         witness->getDescriptiveKind(),
+                         witness->getFullName(),
+                         conformance->getProtocol()->getFullName());
+          diags.diagnose(requirement, diag::protocol_requirement_here,
+                         requirement->getFullName());
+        });
+      break;
     }
 
     ClassDecl *classDecl = Adoptee->getClassOrBoundGenericClass();

test/decl/protocol/req/unavailable.swift

@@ -23,3 +23,13 @@ class Bar : NonObjCProto { // expected-error {{type 'Bar' does not conform to pr
   func good() {}
 }
 
+
+// Warn about unavailable witnesses.
+protocol P {
+  func foo(bar: Foo) // expected-note{{requirement 'foo(bar:)' declared here}}
+}
+
+struct ConformsToP : P {
+  @available(*, unavailable)
+  func foo(bar: Foo) { } // expected-warning{{unavailable instance method 'foo(bar:)' was used to satisfy a requirement of protocol 'P'}}
+}