GSB: Fix runaway recursion through EquivalenceClass::lookupNestedType()

We shouldn't generate NestedTypeNameMatch same-type constraints
between associated types we haven't realized yet.

Otherwise, since maybeResolveEquivalenceClass() can call
lookupNestedType() before looking up a PotentialArchetype, it
is possible that maybeResolveEquivalenceClass() will return
the newly-realized type even when resolutionKind is AlreadyKnown.

This can cause an infinite recursion in expandConformanceRequirement().

However, we don't really have to do this here at all, because if
a PotentialArchetype is registered with the same name later, we
will introduce the same-type constraint in addedNestedType().

It suffices for lookupNestedType() to return the best associated
type anchor and ignore the rest.

Fixes https://bugs.swift.org/browse/SR-14289 / rdar://problem/74876047.

Author: slavapestov

lib/AST/GenericSignatureBuilder.cpp

@@ -2092,7 +2092,6 @@ TypeDecl *EquivalenceClass::lookupNestedType(
 
   // Look for types with the given name in protocols that we know about.
   AssociatedTypeDecl *bestAssocType = nullptr;
-  llvm::SmallSetVector<AssociatedTypeDecl *, 4> assocTypeAnchors;
   SmallVector<TypeDecl *, 4> concreteDecls;
   for (const auto &conforms : conformsTo) {
     ProtocolDecl *proto = conforms.first;
@@ -2104,7 +2103,6 @@ TypeDecl *EquivalenceClass::lookupNestedType(
       if (auto assocType = dyn_cast<AssociatedTypeDecl>(member)) {
         // Retrieve the associated type anchor.
         assocType = assocType->getAssociatedTypeAnchor();
-        assocTypeAnchors.insert(assocType);
 
         if (!bestAssocType ||
              compareAssociatedTypes(assocType, bestAssocType) < 0)
@@ -2131,23 +2129,6 @@ TypeDecl *EquivalenceClass::lookupNestedType(
         lookupConcreteNestedType(decl, name, concreteDecls);
   }
 
-  // Infer same-type constraints among same-named associated type anchors.
-  if (assocTypeAnchors.size() > 1) {
-    auto anchorType = getAnchor(builder, builder.getGenericParams());
-    auto inferredSource =
-      FloatingRequirementSource::forNestedTypeNameMatch(
-        assocTypeAnchors.front()->getName());
-    for (auto assocType : assocTypeAnchors) {
-      if (assocType == bestAssocType) continue;
-
-      builder.addRequirement(
-        Requirement(RequirementKind::SameType,
-                    DependentMemberType::get(anchorType, bestAssocType),
-                    DependentMemberType::get(anchorType, assocType)),
-        nullptr, inferredSource, nullptr, nullptr);
-    }
-  }
-
   // Form the new cache entry.
   CachedNestedType entry;
   entry.numConformancesPresent = conformsTo.size();

test/Generics/sr14289.swift

@@ -0,0 +1,15 @@
+// RUN: %target-swift-frontend -emit-ir %s
+
+public protocol One {
+    associatedtype MyType
+}
+
+public protocol Two {
+    associatedtype MyType
+}
+
+public protocol Three: One, Two where MyType: Three {}
+
+extension Three {
+  public func doStuff(_: MyType.MyType.MyType) {}
+}