[AST Printer] Swap the order of "Self == Self.A" requirements associated with A

It looks better this way.

Author: DougGregor

lib/AST/ASTPrinter.cpp

@@ -932,6 +932,7 @@ class PrintAST : public ASTVisitor<PrintAST> {
     PrintRequirements = 2,
     InnermostOnly = 4,
     SkipSelfRequirement = 8,
+    SwapSelfAndDependentMemberType = 16,
   };
 
   void printWhereClauseFromRequirementSignature(ProtocolDecl *proto,
@@ -1323,8 +1324,11 @@ bestRequirementPrintLocation(ProtocolDecl *proto, const Requirement &req) {
 void PrintAST::printWhereClauseFromRequirementSignature(ProtocolDecl *proto,
                                                         Decl *attachingTo) {
   assert(proto->isRequirementSignatureComputed());
+  unsigned flags = PrintRequirements;
+  if (isa<AssociatedTypeDecl>(attachingTo))
+    flags |= SwapSelfAndDependentMemberType;
   printGenericSignature(
-      proto->getRequirementSignature(), PrintRequirements,
+      proto->getRequirementSignature(), flags,
       [&](const Requirement &req) {
         auto location = bestRequirementPrintLocation(proto, req);
         return location.AttachedTo == attachingTo && location.InWhereClause;
@@ -1441,6 +1445,8 @@ void PrintAST::printSingleDepthOfGenericSignature(
     llvm::function_ref<bool(const Requirement &)> filter) {
   bool printParams = (flags & PrintParams);
   bool printRequirements = (flags & PrintRequirements);
+  bool swapSelfAndDependentMemberType =
+    (flags & SwapSelfAndDependentMemberType);
 
   SubstitutionMap subMap;
   if (CurrentType) {
@@ -1517,6 +1523,13 @@ void PrintAST::printSingleDepthOfGenericSignature(
         Printer << ", ";
       }
 
+      // Swap the order of Self == Self.A requirements if requested.
+      if (swapSelfAndDependentMemberType &&
+          req.getKind() == RequirementKind::SameType &&
+          first->is<GenericTypeParamType>() &&
+          second->is<DependentMemberType>())
+        std::swap(first, second);
+
       Printer.callPrintStructurePre(PrintStructureKind::GenericRequirement);
       if (second) {
         Requirement substReq(req.getKind(), first, second);

test/IDE/print_ast_tc_decls.swift

@@ -1345,7 +1345,7 @@ protocol ProtocolWithWhereClauseAndAssoc : QuxProtocol where Qux == Int, Self :
 
   // FIXME: this same type requirement with Self should be printed here
   associatedtype A2 : QuxProtocol where A2.Qux == Self
-// PREFER_TYPE_REPR_PRINTING-DAG: {{^}}  associatedtype A2 : QuxProtocol where Self == Self.A2.Qux{{$}}
+// PREFER_TYPE_REPR_PRINTING-DAG: {{^}}  associatedtype A2 : QuxProtocol where Self.A2.Qux == Self{{$}}
 }
 
 #if true