[Completion] NFC: Factor out getMemberBaseType()

Author: hamishknight

include/swift/IDE/CompletionLookup.h

@@ -185,6 +185,11 @@ class CompletionLookup final : public swift::VisibleDeclConsumer {
   /// \p selfTy must be a \c Self type of the context.
   static bool canBeUsedAsRequirementFirstType(Type selfTy, TypeAliasDecl *TAD);
 
+  /// Retrieve the type to use as the base for a member completion.
+  Type getMemberBaseType() const {
+    return BaseType ? BaseType : ExprType;
+  }
+
 public:
   struct RequestedResultsTy {
     const ModuleDecl *TheModule;

lib/IDE/CompletionLookup.cpp

@@ -763,9 +763,7 @@ Type CompletionLookup::getTypeOfMember(const ValueDecl *VD, Type ExprType) {
 }
 
 Type CompletionLookup::getAssociatedTypeType(const AssociatedTypeDecl *ATD) {
-  Type BaseTy = BaseType;
-  if (!BaseTy)
-    BaseTy = ExprType;
+  Type BaseTy = getMemberBaseType();
   if (!BaseTy && CurrDeclContext)
     BaseTy =
         CurrDeclContext->getInnermostTypeContext()->getDeclaredTypeInContext();