[Mangling] Generalize the mangling of associated witness table accessors.

Use a general ‘type’ production for the conforming type of an associated
witness table accessor mangling, so that we can mangle base protocol
witness table accessors. These entities are always internal symbols, so the
mangling itself doesn’t affect the ABI.

(cherry picked from commit 22b20cc)

Author: DougGregor

docs/ABI/Mangling.rst

@@ -146,7 +146,8 @@ Globals
   global ::= type protocol-conformance 'WL'   // lazy protocol witness table cache variable
 
   global ::= protocol-conformance identifier 'Wt' // associated type metadata accessor (HISTORICAL)
-  global ::= protocol-conformance assoc-type-list nominal-type 'WT' // associated type witness table accessor
+  global ::= protocol-conformance assoc-type-list protocol 'WT' // associated type witness table accessor
+  global ::= protocol-conformance type protocol 'WT' // inherited protocol witness table accessor
   global ::= type protocol-conformance 'Wl' // lazy protocol witness table accessor
 
   global ::= type 'WV'                   // value witness table

lib/Demangling/Demangler.cpp

@@ -2423,11 +2423,22 @@ NodePointer Demangler::demangleWitness() {
     }
     case 'T': {
       NodePointer ProtoTy = popNode(Node::Kind::Type);
-      auto AssocTypePath = popAssocTypePath();
+      NodePointer ConformingType = nullptr;
+      if (auto Type = popNode(Node::Kind::Type)) {
+        if (Type->getChild(0) &&
+            Type->getChild(0)->getKind() ==
+            Node::Kind::DependentGenericParamType) {
+          ConformingType = Type;
+        } else {
+          pushNode(Type);
+        }
+      }
 
+      if (!ConformingType)
+        ConformingType = popAssocTypePath();
       NodePointer Conf = popProtocolConformance();
       return createWithChildren(Node::Kind::AssociatedTypeWitnessTableAccessor,
-                                Conf, AssocTypePath, ProtoTy);
+                                Conf, ConformingType, ProtoTy);
     }
     case 'O': {
       switch (nextChar()) {

lib/Demangling/OldRemangler.cpp

@@ -955,7 +955,7 @@ void Remangler::mangleAssociatedTypeWitnessTableAccessor(Node *node) {
   Out << "WT";
   assert(node->getNumChildren() == 3);
   mangleChildNode(node, 0); // protocol conformance
-  mangleChildNode(node, 1); // identifier
+  mangleChildNode(node, 1); // type
   mangleProtocolWithoutPrefix(node->begin()[2]); // type
 }
 

lib/Demangling/Remangler.cpp

@@ -596,7 +596,7 @@ void Remangler::mangleDefaultAssociatedTypeMetadataAccessor(Node *node) {
 }
 
 void Remangler::mangleAssociatedTypeWitnessTableAccessor(Node *node) {
-  mangleChildNodes(node); // protocol conformance, identifier, type
+  mangleChildNodes(node); // protocol conformance, type, protocol
   Buffer << "WT";
 }
 

lib/IRGen/IRGenMangler.h

@@ -266,8 +266,12 @@ class IRGenMangler : public Mangle::ASTMangler {
                                       const ProtocolDecl *Proto) {
     beginMangling();
     appendProtocolConformance(Conformance);
-    bool isFirstAssociatedTypeIdentifier = true;
-    appendAssociatedTypePath(AssociatedType, isFirstAssociatedTypeIdentifier);
+    if (isa<GenericTypeParamType>(AssociatedType)) {
+      appendType(AssociatedType);
+    } else {
+      bool isFirstAssociatedTypeIdentifier = true;
+      appendAssociatedTypePath(AssociatedType, isFirstAssociatedTypeIdentifier);
+    }
     appendAnyGenericType(Proto);
     appendOperator("WT");
     return finalize();