Don't allow silent failures in protocol conformances.

As of r31224, we tried to suppress redundant protocol-conformance
diagnostics for witnesses that look like they could have matched
something in an unconstrained protocol extension. However, this was
allowing ill-formed conformances to be accepted, causing a crash in
the AST verifier. Fixes rdar://problem/23033862.

Swift SVN r32671

lib/Sema/TypeCheckProtocol.cpp

@@ -2392,7 +2392,9 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
   // If there was an invalid witness that might have worked, just
   // suppress the diagnostic entirely. This stops the diagnostic cascade.
   // FIXME: We could do something crazy, like try to fix up the witness.
-  if (invalidWitness || (numViable == 0 && anyFromUnconstrainedExtension)) {
+  if (invalidWitness ||
+      (numViable == 0 && anyFromUnconstrainedExtension &&
+       Conformance->isInvalid())) {
     return ResolveWitnessResult::ExplicitFailed;
   }
 

test/decl/protocol/conforms/failure.swift

@@ -77,17 +77,25 @@ struct P5Conformer : P5 { // expected-error {{does not conform}}
 protocol P6Base {
   typealias Foo
   func foo()
-  func bar() -> Foo
+  func bar() -> Foo // expected-note{{protocol requires function 'bar()' }}
 }
 extension P6Base {
 }
 protocol P6 : P6Base {
   typealias Bar // expected-note {{protocol requires nested type 'Bar'}}
 }
 extension P6 {
-  func bar() -> Bar? { return nil }
+  func bar() -> Bar? { return nil } // expected-note{{candidate has non-matching type}}
 }
 
-struct P6Conformer : P6 { // expected-error {{does not conform}}
+struct P6Conformer : P6 { // expected-error 2 {{does not conform}}
   func foo() {}
 }
+
+// rdar://problem/23033862
+// expected-error@+2{{type 'A' does not conform to protocol 'OptionSetType'}}
+// expected-error@+1{{type 'A' does not conform to protocol 'RawRepresentable'}}
+struct A: OptionSetType {
+    let rawValue = 0
+    init() { } // expected-note 2{{candidate has non-matching type '()'}}
+}