If a member is a witness to any @objc requirement, don't call it a near miss.

When @objc inference was extended to look at the conformances of
superclasses, the code that diagnosed near misses was not similarly
updated, so we could end up producing a near-miss diagnostic on a
declaration that was already a witness to another protocol. Use the
same witness-finding logic that we use for @objc inference so this
doesn't happen again.

Fixes the rest of rdar://problem/27348369

Author: DougGregor

lib/Sema/TypeCheckProtocol.cpp

@@ -4848,18 +4848,6 @@ void TypeChecker::checkConformancesInContext(DeclContext *dc,
   // If there were any unsatisfied requirements, check whether there
   // are any near-matches we should diagnose.
   if (!unsatisfiedReqs.empty() && !anyInvalid) {
-    // Collect the set of witnesses that came from this context.
-    llvm::SmallPtrSet<ValueDecl *, 16> knownWitnesses;
-    for (auto conformance : conformances) {
-      conformance->forEachValueWitness(
-        nullptr,
-        [&](ValueDecl *req, ConcreteDeclRef witness) {
-          // If there is a witness, record it if it's within this context.
-          if (witness.getDecl() && witness.getDecl()->getDeclContext() == dc)
-            knownWitnesses.insert(witness.getDecl());
-         });
-    }
-
     // Find all of the members that aren't used to satisfy
     // requirements, and check whether they are close to an
     // unsatisfied or defaulted requirement.
@@ -4869,8 +4857,12 @@ void TypeChecker::checkConformancesInContext(DeclContext *dc,
       auto value = dyn_cast<ValueDecl>(member);
       if (!value) continue;
       if (isa<TypeDecl>(value)) continue;
-      if (knownWitnesses.count(value) > 0) continue;
       if (!value->getFullName()) continue;
+      
+      // If this member is a witness to any @objc requirement, ignore it.
+      if (!findWitnessedObjCRequirements(value, /*anySingleRequirement=*/true)
+            .empty())
+        continue;
 
       // Find the unsatisfied requirements with the nearest-matching
       // names.

test/decl/protocol/conforms/near_miss_objc.swift

@@ -119,3 +119,18 @@ class C7a : P7 {
   // expected-note@-4{{make 'method(foo:)' private to silence this warning}}
 }
 
+// Don't complain about near-misses that satisfy other protocol
+// requirements.
+@objc protocol P8 {
+  @objc optional func foo(exactMatch: Int)
+}
+
+@objc protocol P9 : P8 {
+  @objc optional func foo(nearMatch: Int)
+}
+
+class C8Super : P8 { }
+
+class C9Sub : C8Super, P9 {
+  func foo(exactMatch: Int) { }
+}