AST: Add SubstFlags::DesugarMemberTypes

For code completion to use Type::subst() without undoing
previously-added heuristics, we need a form of member lookup
that strips the outermost typealias type. For example, if we
have

protocol P {
  associatedtype Fruit
}

struct Apple {}
typealias Pear = Apple

class Bowl : P {
  typealias Fruit = Pear
}

With DesugarMemberTypes OFF, the substitution 'T := Bowl'
applied to 'T.Fruit' yields 'Bowl.Fruit'.

With DesugarMemberTypes ON, the same substitution instead
produces 'Pear' (note: not 'Apple'; we only desugar one
level here, to match the current code completion behavior).

Also, add another tweak that will become important shortly;
if a type witness cannot be derived, set the type witness
to an alias type pointing to an error type, instead of
directly storing an error type in there.

Author: slavapestov

include/swift/AST/Type.h

@@ -62,6 +62,8 @@ enum class SubstFlags {
   /// is just changing between contextual and interface type
   /// representations, using Type::subst() is allowed.
   AllowLoweredTypes = 0x02,
+  /// Map member types to their desugared witness type.
+  DesugarMemberTypes = 0x04,
 };
 
 /// Options for performing substitutions into a type.

lib/AST/Type.cpp

@@ -2876,8 +2876,17 @@ static Type getMemberForBaseType(ConformanceSource conformances,
         !conformance->getConcrete()->hasTypeWitness(assocType, nullptr))
       return Type();
 
-    return conformance->getConcrete()->getTypeWitness(assocType, resolver)
-             .getReplacement();
+    auto witness = conformance->getConcrete()
+        ->getTypeWitness(assocType, resolver).getReplacement();
+
+    // This is a hacky feature allowing code completion to migrate to
+    // using Type::subst() without changing output.
+    if (options & SubstFlags::DesugarMemberTypes)
+      if (auto *aliasType = dyn_cast<NameAliasType>(witness.getPointer()))
+        if (!aliasType->is<ErrorType>())
+          witness = aliasType->getSinglyDesugaredType();
+
+    return witness;
   }
 
   // FIXME: This is a fallback. We want the above, conformance-based

lib/Sema/TypeCheckProtocol.cpp

@@ -1976,9 +1976,13 @@ void ConformanceChecker::recordTypeWitness(AssociatedTypeDecl *assocType,
                                                     DC);
     aliasDecl->computeType();
     aliasDecl->setImplicit();
-    if (type->is<ErrorType>())
+    if (type->is<ErrorType>()) {
       aliasDecl->setInvalid();
-    if (type->hasArchetype()) {
+
+      // If we're recording a failed type witness, keep the sugar around for
+      // code completion.
+      type = aliasDecl->getDeclaredType();
+    } else if (type->hasArchetype()) {
       Type metaType = MetatypeType::get(type);
       aliasDecl->setInterfaceType(
         ArchetypeBuilder::mapTypeOutOfContext(DC, metaType));