[6.0] Runtime: Fix runtime type resolution when mangled names refer to protocol extensions with Self same type constraints.

stdlib/public/runtime/MetadataLookup.cpp

@@ -400,6 +400,38 @@ _findExtendedTypeContextDescriptor(const ContextDescriptor *maybeExtension,
   Demangle::NodePointer &node = demangledNode ? *demangledNode : localNode;
 
   auto mangledName = extension->getMangledExtendedContext();
+
+  // A extension of the form `extension Protocol where Self == ConcreteType`
+  // is formally a protocol extension, so the formal generic parameter list
+  // is `<Self>`, but because of the same type constraint, the extended context
+  // looks like a reference to that nominal type. We want to match the
+  // extension's formal generic environment rather than the nominal type's
+  // in this case, so we should skip out on this case.
+  //
+  // We can detect this by looking at whether the generic context of the
+  // extension has a first generic parameter, which would be the Self parameter,
+  // with a same type constraint matching the extended type.
+  for (auto &reqt : extension->getGenericRequirements()) {
+    if (reqt.getKind() != GenericRequirementKind::SameType) {
+      continue;
+    }
+    // 'x' is the mangling of the first generic parameter
+    if (!reqt.getParam().equals("x")) {
+      continue;
+    }
+    // Is the generic parameter same-type-constrained to the same type
+    // we're extending? Then this is a `Self == ExtendedType` constraint.
+    // This is impossible for normal generic nominal type extensions because
+    // that would mean that you had:
+    //   struct Foo<T> {...}
+    //   extension Foo where T == Foo<T> {...}
+    // which would mean that the extended type is the infinite expansion
+    // Foo<Foo<Foo<Foo<...>>>>, which we don't allow.
+    if (reqt.getMangledTypeName().data() == mangledName.data()) {
+      return nullptr;
+    }
+  }
+
   node = demangler.demangleType(mangledName,
                                 ResolveAsSymbolicReference(demangler));
   if (!node)
@@ -1252,7 +1284,8 @@ _gatherGenericParameters(const ContextDescriptor *context,
   (void)_gatherGenericParameterCounts(context,
                                       genericParamCounts, demangler);
   unsigned numTotalGenericParams =
-    genericParamCounts.empty() ? 0 : genericParamCounts.back();
+    genericParamCounts.empty() ? context->getNumGenericParams()
+                               : genericParamCounts.back();
 
   // Check whether we have the right number of generic arguments.
   if (genericArgs.size() == getLocalGenericParams(context).size()) {

test/Runtime/protocol_self_same_type_extension.swift

@@ -0,0 +1,27 @@
+// RUN: %target-run-simple-swift | %FileCheck %s
+// REQUIRES: executable_test
+// UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: back_deployment_runtime
+
+// rdar://130168101: Make sure that we correctly resolve types in
+// the generic context of a protocol extension with a `Self` same
+// type constraint.
+
+
+protocol P { }
+
+struct P2: P { }
+
+extension P where Self == P2 {
+    @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
+    dynamic func p2() -> some P {
+        return self
+    }
+}
+
+// CHECK: P2()
+if #available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *) {
+    print(P2().p2())
+} else {
+    print(P2())
+}