RequirementMachine: Don't consider protocol typealiases with UnboundGenericType

These are stealth generic typealiases and should not participate in the
rewrite system.

Author: slavapestov

lib/AST/RequirementMachine/RequirementLowering.cpp

@@ -705,14 +705,16 @@ StructuralRequirementsRequest::evaluate(Evaluator &evaluator,
     // underlying type of the typealias.
     if (auto *typeAliasDecl = dyn_cast<TypeAliasDecl>(decl)) {
       if (!typeAliasDecl->isGeneric()) {
-        // Ignore the typealias if we have an associated type with the same anme
+        // Ignore the typealias if we have an associated type with the same name
         // in the same protocol. This is invalid anyway, but it's just here to
         // ensure that we produce the same requirement signature on some tests
         // with -requirement-machine-protocol-signatures=verify.
         if (assocTypes.contains(typeAliasDecl->getName()))
           continue;
 
         auto underlyingType = typeAliasDecl->getStructuralType();
+        if (underlyingType->is<UnboundGenericType>())
+          continue;
 
         auto subjectType = DependentMemberType::get(
             selfTy, typeAliasDecl->getName());
@@ -752,6 +754,20 @@ TypeAliasRequirementsRequest::evaluate(Evaluator &evaluator,
     (ctx.LangOpts.RequirementMachineProtocolSignatures ==
      RequirementMachineMode::Enabled);
 
+  auto getStructuralType = [](TypeDecl *typeDecl) -> Type {
+    if (auto typealias = dyn_cast<TypeAliasDecl>(typeDecl)) {
+      if (typealias->getUnderlyingTypeRepr() != nullptr) {
+        auto type = typealias->getStructuralType();
+        if (auto *aliasTy = cast<TypeAliasType>(type.getPointer()))
+          return aliasTy->getSinglyDesugaredType();
+        return type;
+      }
+      return typealias->getUnderlyingType();
+    }
+
+    return typeDecl->getDeclaredInterfaceType();
+  };
+
   // Collect all typealiases from inherited protocols recursively.
   llvm::MapVector<Identifier, TinyPtrVector<TypeDecl *>> inheritedTypeDecls;
   for (auto *inheritedProto : ctx.getRewriteContext().getInheritedProtocols(proto)) {
@@ -762,25 +778,17 @@ TypeAliasRequirementsRequest::evaluate(Evaluator &evaluator,
           if (genReq->getGenericParams())
             continue;
 
+        // Ignore typealiases with UnboundGenericType, since they
+        // are like generic typealiases.
+        if (auto *typeAlias = dyn_cast<TypeAliasDecl>(req))
+          if (getStructuralType(typeAlias)->is<UnboundGenericType>())
+            continue;
+
         inheritedTypeDecls[typeReq->getName()].push_back(typeReq);
       }
     }
   }
 
-  auto getStructuralType = [](TypeDecl *typeDecl) -> Type {
-    if (auto typealias = dyn_cast<TypeAliasDecl>(typeDecl)) {
-      if (typealias->getUnderlyingTypeRepr() != nullptr) {
-        auto type = typealias->getStructuralType();
-        if (auto *aliasTy = cast<TypeAliasType>(type.getPointer()))
-          return aliasTy->getSinglyDesugaredType();
-        return type;
-      }
-      return typealias->getUnderlyingType();
-    }
-
-    return typeDecl->getDeclaredInterfaceType();
-  };
-
   // An inferred same-type requirement between the two type declarations
   // within this protocol or a protocol it inherits.
   auto recordInheritedTypeRequirement = [&](TypeDecl *first, TypeDecl *second) {

lib/AST/RequirementMachine/Symbol.cpp

@@ -78,6 +78,7 @@ struct Symbol::Storage final
   Storage(Symbol::Kind kind, CanType type, ArrayRef<Term> substitutions) {
     assert(kind == Symbol::Kind::Superclass ||
            kind == Symbol::Kind::ConcreteType);
+    assert(!type->hasUnboundGenericType());
     assert(!type->hasTypeVariable());
     assert(type->hasTypeParameter() != substitutions.empty());
 

test/Generics/protocol_typealias_unbound_generic.swift

@@ -0,0 +1,7 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol P {
+  typealias A = Array
+
+  associatedtype X where X == A // expected-error {{reference to generic type 'Self.A' (aka 'Array') requires arguments in <...>}}
+}