Make LookupConformanceFn callbacks return Optional<ProtocolConformanceRef>.

NFC yet, but this is a step toward centralizing error handling policy for failed conformance lookups instead of leaving it ad-hoc.

include/swift/AST/GenericSignature.h

@@ -129,9 +129,10 @@ public:
                           SubstitutionMap &subMap) const;
 
   using LookupConformanceFn =
-      llvm::function_ref<ProtocolConformanceRef(CanType dependentType,
+      llvm::function_ref<auto(CanType dependentType,
                               Type conformingReplacementType,
-                                              ProtocolType *conformedProtocol)>;
+                              ProtocolType *conformedProtocol)
+                         -> Optional<ProtocolConformanceRef>>;
 
   /// Build an array of substitutions from an interface type substitution map,
   /// using the given function to look up conformances.

lib/AST/GenericEnvironment.cpp

@@ -277,7 +277,7 @@ ArrayRef<Substitution>
 GenericEnvironment::getForwardingSubstitutions(ModuleDecl *M) const {
   auto lookupConformanceFn =
       [&](CanType original, Type replacement, ProtocolType *protoType)
-          -> ProtocolConformanceRef {
+      -> Optional<ProtocolConformanceRef> {
         return ProtocolConformanceRef(protoType->getDecl());
       };
 

lib/AST/GenericSignature.cpp

@@ -315,8 +315,9 @@ getSubstitutions(ModuleDecl &mod,
     for (auto req: reqs) {
       assert(req.getKind() == RequirementKind::Conformance);
       auto protoType = req.getSecondType()->castTo<ProtocolType>();
+      // TODO: Error handling for failed conformance lookup.
       currentConformances.push_back(
-        lookupConformance(depTy->getCanonicalType(), currentReplacement,
+        *lookupConformance(depTy->getCanonicalType(), currentReplacement,
                            protoType));
     }
 
@@ -336,8 +337,8 @@ getSubstitutions(ModuleDecl &mod,
                  SmallVectorImpl<Substitution> &result) const {
   auto lookupConformanceFn =
       [&](CanType original, Type replacement, ProtocolType *protoType)
-          -> ProtocolConformanceRef {
-    return *subMap.lookupConformance(original, protoType->getDecl());
+      -> Optional<ProtocolConformanceRef> {
+        return subMap.lookupConformance(original, protoType->getDecl());
       };
 
   getSubstitutions(mod, subMap.getMap(), lookupConformanceFn, result);

lib/AST/Module.cpp

@@ -628,8 +628,7 @@ TypeBase::gatherAllSubstitutions(Module *module,
 
   auto lookupConformanceFn =
       [&](CanType original, Type replacement, ProtocolType *protoType)
-          -> ProtocolConformanceRef {
-
+      -> Optional<ProtocolConformanceRef> {
         auto *proto = protoType->getDecl();
         
         // If the type is a type variable or is dependent, just fill in empty
@@ -638,12 +637,14 @@ TypeBase::gatherAllSubstitutions(Module *module,
             replacement->isTypeParameter())
           return ProtocolConformanceRef(proto);
         
-    // Otherwise, find the conformance.
+        // Otherwise, try to find the conformance.
         auto conforms = module->lookupConformance(replacement, proto, resolver);
         if (conforms)
           return *conforms;
         
         // FIXME: Should we ever end up here?
+        // We should return None and let getSubstitutions handle the error
+        // if we do.
         return ProtocolConformanceRef(proto);
       };
 

lib/Sema/CSApply.cpp

@@ -108,20 +108,18 @@ Type Solution::computeSubstitutions(
 
   auto lookupConformanceFn =
       [&](CanType original, Type replacement, ProtocolType *protoType)
-          -> ProtocolConformanceRef {
+          -> Optional<ProtocolConformanceRef> {
     if (replacement->hasError() ||
         isOpenedAnyObject(replacement) ||
         replacement->is<GenericTypeParamType>()) {
       return ProtocolConformanceRef(protoType->getDecl());
     }
 
-    auto conformance = tc.conformsToProtocol(
-                         replacement,
+    return tc.conformsToProtocol(replacement,
                                  protoType->getDecl(),
                                  getConstraintSystem().DC,
                                  (ConformanceCheckFlags::InExpression|
                                   ConformanceCheckFlags::Used));
-    return *conformance;
   };
 
   sig->getSubstitutions(*mod, subs, lookupConformanceFn, result);