[Associated type inference] Allow _Default_Foo typealias guides for inference.

lib/Sema/TypeCheckGeneric.cpp

@@ -1307,6 +1307,13 @@ RequirementCheckResult TypeChecker::checkGenericArguments(
         secondType = req.getSecondType();
       }
 
+      // Don't do further checking on error types.
+      if (firstType->hasError() || (secondType && secondType->hasError())) {
+        // Another requirement will fail later; just continue.
+        valid = false;
+        continue;
+      }
+
       bool requirementFailure = false;
       if (listener && !listener->shouldCheck(kind, firstType, secondType))
         continue;

lib/Sema/TypeCheckProtocol.h

@@ -139,7 +139,7 @@ struct InferredTypeWitnessesSolution {
 #ifndef NDEBUG
   LLVM_ATTRIBUTE_USED
 #endif
-  void dump();
+  void dump() const;
 };
 
 class RequirementEnvironment;
@@ -707,6 +707,12 @@ class AssociatedTypeInference {
                    const llvm::SetVector<AssociatedTypeDecl *> &allUnresolved,
                    ValueDecl *req);
 
+  /// Infer associated type witnesses for the given associated type.
+  InferredAssociatedTypesByWitnesses inferTypeWitnessesViaAssociatedType(
+                   ConformanceChecker &checker,
+                   const llvm::SetVector<AssociatedTypeDecl *> &allUnresolved,
+                   AssociatedTypeDecl *assocType);
+
   /// Infer associated type witnesses for all relevant value requirements.
   ///
   /// \param assocTypes The set of associated types we're interested in.
@@ -728,12 +734,30 @@ class AssociatedTypeInference {
   /// known to the compiler.
   Type computeDerivedTypeWitness(AssociatedTypeDecl *assocType);
 
+  /// Compute a type witness without using a specific potential witness,
+  /// e.g., using a fixed type (from a refined protocol), default type
+  /// on an associated type, or deriving the type.
+  ///
+  /// \param allowDerived Whether to allow "derived" type witnesses.
+  Type computeAbstractTypeWitness(AssociatedTypeDecl *assocType,
+                                  bool allowDerived);
+
   /// Substitute the current type witnesses into the given interface type.
   Type substCurrentTypeWitnesses(Type type);
 
+  /// Retrieve substitution options with a tentative type witness
+  /// operation that queries the current set of type witnesses.
+  SubstOptions getSubstOptionsWithCurrentTypeWitnesses();
+
   /// Check whether the current set of type witnesses meets the
   /// requirements of the protocol.
-  bool checkCurrentTypeWitnesses();
+  bool checkCurrentTypeWitnesses(
+         const SmallVectorImpl<std::pair<ValueDecl *, ValueDecl *>>
+           &valueWitnesses);
+
+  /// Check the current type witnesses against the
+  /// requirements of the given constrained extension.
+  bool checkConstrainedExtension(ExtensionDecl *ext);
 
   /// Top-level operation to find solutions for the given unresolved
   /// associated types.