AST: Don't build useless inherited conformances

Author: slavapestov

include/swift/AST/ASTContext.h

@@ -1255,7 +1255,7 @@ class ASTContext final {
   /// \param type The type for which we are retrieving the conformance.
   ///
   /// \param inherited The inherited conformance.
-  InheritedProtocolConformance *
+  ProtocolConformance *
   getInheritedConformance(Type type, ProtocolConformance *inherited);
 
   /// Get the lazy data for the given declaration.

lib/AST/ASTContext.cpp

@@ -2448,30 +2448,39 @@ ASTContext::getSpecializedConformance(Type type,
   return result;
 }
 
-InheritedProtocolConformance *
+ProtocolConformance *
 ASTContext::getInheritedConformance(Type type, ProtocolConformance *inherited) {
   // Collapse multiple levels of inherited conformance.
-  while (auto *otherInherited = dyn_cast<InheritedProtocolConformance>(inherited))
+  if (auto *otherInherited = dyn_cast<InheritedProtocolConformance>(inherited))
     inherited = otherInherited->getInheritedConformance();
 
   assert(isa<SpecializedProtocolConformance>(inherited) ||
          isa<NormalProtocolConformance>(inherited) ||
          isa<BuiltinProtocolConformance>(inherited));
 
+  // Collapse useless inherited conformances. Conformance lookup with aa
+  // archetype T that has a superclass bound C will return a concrete
+  // conformance if C conforms to the protocol P. This is wrapped in an
+  // inherited conformance with the archetype type T. If you then substitute
+  // T := C, you don't want to form an inherited conformance with a type of
+  // C, because the underlying conformance already has a type of C.
+  if (inherited->getType()->isEqual(type))
+    return inherited;
+
   llvm::FoldingSetNodeID id;
   InheritedProtocolConformance::Profile(id, type, inherited);
 
   // Figure out which arena this conformance should go into.
   AllocationArena arena = getArena(type->getRecursiveProperties());
 
-  // Did we already record the normal protocol conformance?
+  // Did we already record the inherited protocol conformance?
   void *insertPos;
   auto &inheritedConformances = getImpl().getArena(arena).InheritedConformances;
   if (auto result
         = inheritedConformances.FindNodeOrInsertPos(id, insertPos))
     return result;
 
-  // Build a new normal protocol conformance.
+  // Build a new inherited protocol conformance.
   auto result = new (*this, arena) InheritedProtocolConformance(type, inherited);
   inheritedConformances.InsertNode(result, insertPos);
   return result;