Don't look for near-miss witnesses for unavailable requirements.

We use unavailable requirements as migration hints, but conformance
checking would try a little too hard to actually satisfy them, even
though they wouldn't actually be used. This led to errors about the
/new/ name not matching the /old/ name.

(We now get /two/ diagnostics for using the old name, but we can fix
that later. Filed rdar://problem/26358593.)

rdar://problem/26313044

Author: jrose-apple

lib/Sema/TypeCheckProtocol.cpp

@@ -2036,7 +2036,8 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
   bool doNotDiagnoseMatches = false;
   bool ignoringNames = false;
   bool considerRenames =
-    !canDerive && !requirement->getAttrs().hasAttribute<OptionalAttr>();
+    !canDerive && !requirement->getAttrs().hasAttribute<OptionalAttr>() &&
+    !requirement->getAttrs().isUnavailable(TC.Context);
 
   if (findBestWitness(requirement,
                       considerRenames ? &ignoringNames : nullptr,
@@ -2049,6 +2050,7 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
     // If the name didn't actually line up, complain.
     if (ignoringNames && 
         requirement->getFullName() != best.Witness->getFullName()) {
+
       diagnoseOrDefer(requirement, false,
         [witness, requirement](TypeChecker &tc,
                                NormalProtocolConformance *conformance) {
@@ -4570,6 +4572,9 @@ void TypeChecker::checkConformancesInContext(DeclContext *dc,
       SmallVector<ValueDecl *, 4> bestOptionalReqs;
       unsigned bestScore = UINT_MAX;
       for (auto req : unsatisfiedReqs) {
+        // Skip unavailable requirements.
+        if (req->getAttrs().isUnavailable(Context)) continue;
+
         // Score this particular optional requirement.
         auto score = scorePotentiallyMatching(req, value, bestScore);
 

test/ClangModules/Inputs/protocol-member-renaming.h

@@ -0,0 +1,13 @@
+@import ObjectiveC;
+
+@interface Foo : NSObject
+@end
+
+@protocol FooDelegate
+- (void)foo:(Foo *)foo willConsumeObject:(id)obj __attribute__((swift_name("foo(_:willConsume:)")));
+@end
+
+@protocol OptionalButUnavailable
+@optional
+- (void)doTheThing:(id)thingToDoItWith __attribute__((unavailable));
+@end

test/ClangModules/protocol-member-renaming.swift

@@ -0,0 +1,18 @@
+// RUN: %target-swift-frontend -parse -import-objc-header %S/Inputs/protocol-member-renaming.h -verify %s
+
+// REQUIRES: objc_interop
+
+class Modern : NSObject, FooDelegate {
+  func foo(_ foo: Foo, willConsume object: AnyObject) {}
+}
+
+class PreMigration : NSObject, FooDelegate {
+  func foo(_ foo: Foo, willConsumeObject object: AnyObject) {}
+  // expected-error@-1 {{'foo(_:willConsumeObject:)' has been renamed to 'foo(_:willConsume:)'}} {{24-41=willConsume}}
+  // expected-error@-2 {{method 'foo(_:willConsumeObject:)' has different argument names from those required by protocol 'FooDelegate' ('foo(_:willConsume:)')}} {{24-41=willConsume}}
+}
+
+class OptionalButUnavailableImpl : OptionalButUnavailable {
+  // Note the argument label that causes this not to match the requirement.
+  func doTheThing(object: AnyObject) {} // no-warning
+}