[Sema] Record invalid protocol witnesses to avoid re-diagnosis. (swiftlang#6698)

This is a /little/ risky because previously a recorded witness would
only be empty if the requirement was optional, but it's a relatively
non-intrusive way to improve diagnostics. The conformance is still
marked invalid, which is the important part.

Groundwork for rdar://problem/28543037.

Author: jrose-apple

lib/Sema/TypeCheckProtocol.cpp

@@ -1709,6 +1709,9 @@ namespace {
     /// Record that the given optional requirement has no witness.
     void recordOptionalWitness(ValueDecl *requirement);
 
+    /// Record that the given requirement has no valid witness.
+    void recordInvalidWitness(ValueDecl *requirement);
+
     /// Record a type witness.
     ///
     /// \param assocType The associated type whose witness is being recorded.
@@ -2186,6 +2189,20 @@ void ConformanceChecker::recordOptionalWitness(ValueDecl *requirement) {
   Conformance->setWitness(requirement, Witness());
 }
 
+void ConformanceChecker::recordInvalidWitness(ValueDecl *requirement) {
+  assert(Conformance->isInvalid());
+
+  // If we already recorded this witness, don't do so again.
+  if (Conformance->hasWitness(requirement)) {
+    assert(!Conformance->getWitness(requirement, nullptr).getDecl() &&
+           "Already have a non-optional witness?");
+    return;
+  }
+
+  // Record that there is no witness.
+  Conformance->setWitness(requirement, Witness());
+}
+
 void ConformanceChecker::recordTypeWitness(AssociatedTypeDecl *assocType,
                                            Type type,
                                            TypeDecl *typeDecl,
@@ -4019,6 +4036,7 @@ void ConformanceChecker::resolveSingleWitness(ValueDecl *requirement) {
 
   case ResolveWitnessResult::ExplicitFailed:
     Conformance->setInvalid();
+    recordInvalidWitness(requirement);
     return;
 
   case ResolveWitnessResult::Missing:
@@ -4033,6 +4051,7 @@ void ConformanceChecker::resolveSingleWitness(ValueDecl *requirement) {
 
   case ResolveWitnessResult::ExplicitFailed:
     Conformance->setInvalid();
+    recordInvalidWitness(requirement);
     return;
 
   case ResolveWitnessResult::Missing:
@@ -4047,6 +4066,7 @@ void ConformanceChecker::resolveSingleWitness(ValueDecl *requirement) {
 
   case ResolveWitnessResult::ExplicitFailed:
     Conformance->setInvalid();
+    recordInvalidWitness(requirement);
     return;
 
   case ResolveWitnessResult::Missing:

test/ClangImporter/objc_parse.swift

@@ -356,14 +356,14 @@ class ProtocolAdopter2 : FooProto {
     set { /* do nothing! */ }
   }
 }
-class ProtocolAdopterBad1 : FooProto { // expected-error 2{{type 'ProtocolAdopterBad1' does not conform to protocol 'FooProto'}}
-  @objc var bar: Int = 0 // expected-note 2{{candidate has non-matching type 'Int'}}
+class ProtocolAdopterBad1 : FooProto { // expected-error {{type 'ProtocolAdopterBad1' does not conform to protocol 'FooProto'}}
+  @objc var bar: Int = 0 // expected-note {{candidate has non-matching type 'Int'}}
 }
-class ProtocolAdopterBad2 : FooProto { // expected-error 2{{type 'ProtocolAdopterBad2' does not conform to protocol 'FooProto'}}
-  let bar: CInt = 0 // expected-note 2{{candidate is not settable, but protocol requires it}}
+class ProtocolAdopterBad2 : FooProto { // expected-error {{type 'ProtocolAdopterBad2' does not conform to protocol 'FooProto'}}
+  let bar: CInt = 0 // expected-note {{candidate is not settable, but protocol requires it}}
 }
-class ProtocolAdopterBad3 : FooProto { // expected-error 2{{type 'ProtocolAdopterBad3' does not conform to protocol 'FooProto'}}
-  var bar: CInt { // expected-note 2{{candidate is not settable, but protocol requires it}}
+class ProtocolAdopterBad3 : FooProto { // expected-error {{type 'ProtocolAdopterBad3' does not conform to protocol 'FooProto'}}
+  var bar: CInt { // expected-note {{candidate is not settable, but protocol requires it}}
     return 42
   }
 }