Sema: TypeChecker::conformsToProtocol() should only compute conditional requirements if checking them

This fixes a crash with the next commit.

Author: slavapestov

lib/Sema/TypeCheckNameLookup.cpp

@@ -555,7 +555,9 @@ LookupTypeResult TypeChecker::lookupMemberType(DeclContext *dc,
       auto typeDecl =
         concrete->getTypeWitnessAndDecl(assocType, lazyResolver).second;
 
-      assert(typeDecl && "Missing type witness?");
+      // Circularity.
+      if (!typeDecl)
+        continue;
 
       auto memberType =
           substMemberTypeWithBase(dc->getParentModule(), typeDecl, type);

lib/Sema/TypeCheckProtocol.cpp

@@ -4113,18 +4113,17 @@ Optional<ProtocolConformanceRef> TypeChecker::conformsToProtocol(
     recordDependency();
   }
 
+  if (options.contains(ConformanceCheckFlags::SkipConditionalRequirements))
+    return lookupResult;
+
   auto condReqs = lookupResult->getConditionalRequirementsIfAvailable();
+  assert(condReqs &&
+         "unhandled recursion: missing conditional requirements when they're "
+         "required");
+
   // If we have a conditional requirements that
   // we need to check, do so now.
-  if (!condReqs) {
-    assert(
-        options.contains(
-            ConformanceCheckFlags::AllowUnavailableConditionalRequirements) &&
-        "unhandled recursion: missing conditional requirements when they're "
-        "required");
-  } else if (!condReqs->empty() &&
-             !options.contains(
-                 ConformanceCheckFlags::SkipConditionalRequirements)) {
+  if (!condReqs->empty()) {
     // Figure out the location of the conditional conformance.
     auto conformanceDC = lookupResult->getConcrete()->getDeclContext();
     SourceLoc noteLoc;

lib/Sema/TypeCheckType.cpp

@@ -937,7 +937,7 @@ static void maybeDiagnoseBadConformanceRef(DeclContext *dc,
           parentTy, protocol, dc,
           (ConformanceCheckFlags::InExpression |
            ConformanceCheckFlags::SuppressDependencyTracking |
-           ConformanceCheckFlags::AllowUnavailableConditionalRequirements))) {
+           ConformanceCheckFlags::SkipConditionalRequirements))) {
     if (conformanceRef->isConcrete())
       conformance = conformanceRef->getConcrete();
   }

lib/Sema/TypeChecker.h

@@ -502,13 +502,6 @@ enum class ConformanceCheckFlags {
   /// correctly used. Otherwise (the default), all of the conditional
   /// requirements will be checked.
   SkipConditionalRequirements = 0x04,
-
-  /// Whether to require that the conditional requirements have been computed.
-  ///
-  /// When set, if the conditional requirements aren't available, they are just
-  /// skipped, and the caller is responsible for detecting and handling this
-  /// case (likely via another call to getConditionalRequirementsIfAvailable).
-  AllowUnavailableConditionalRequirements = 0x08,
 };
 
 /// Options that control protocol conformance checking.