[Sema] SR-15807: Associated Type Inference fails across module boundaries

include/swift/AST/NameLookup.h

@@ -545,6 +545,11 @@ SmallVector<InheritedNominalEntry, 4> getDirectlyInheritedNominalTypeDecls(
 SelfBounds getSelfBoundsFromWhereClause(
     llvm::PointerUnion<const TypeDecl *, const ExtensionDecl *> decl);
 
+/// Retrieve the set of nominal type declarations that appear as the
+/// constraint type of any "Self" constraints in the generic signature of the
+/// given protocol or protocol extension.
+SelfBounds getSelfBoundsFromGenericSignature(const ExtensionDecl *extDecl);
+
 /// Retrieve the TypeLoc at the given \c index from among the set of
 /// type declarations that are directly "inherited" by the given declaration.
 inline const TypeLoc &getInheritedTypeLocAtIndex(

include/swift/AST/NameLookupRequests.h

@@ -270,6 +270,22 @@ class SelfBoundsFromWhereClauseRequest
            llvm::PointerUnion<const TypeDecl *, const ExtensionDecl *>) const;
 };
 
+/// Request the nominal types that occur as the right-hand side of "Self: Foo"
+/// constraints in the generic signature of a protocol extension.
+class SelfBoundsFromGenericSignatureRequest
+    : public SimpleRequest<SelfBoundsFromGenericSignatureRequest,
+                           SelfBounds(const ExtensionDecl *),
+                           RequestFlags::Uncached> {
+public:
+  using SimpleRequest::SimpleRequest;
+
+private:
+  friend SimpleRequest;
+
+  // Evaluation.
+  SelfBounds evaluate(Evaluator &evaluator, const ExtensionDecl *extDecl) const;
+};
+
 /// Request all type aliases and nominal types that appear in the "where"
 /// clause of an extension.
 class TypeDeclsFromWhereClauseRequest :

include/swift/AST/NameLookupTypeIDZone.def

@@ -68,6 +68,9 @@ SWIFT_REQUEST(NameLookup, SelfBoundsFromWhereClauseRequest,
               SelfBounds(llvm::PointerUnion<const TypeDecl *,
                                             const ExtensionDecl *>),
               Uncached, NoLocationInfo)
+SWIFT_REQUEST(NameLookup, SelfBoundsFromGenericSignatureRequest,
+              SelfBounds(const ExtensionDecl * extDecl),
+              Uncached, NoLocationInfo)
 SWIFT_REQUEST(NameLookup, SuperclassDeclRequest, ClassDecl *(NominalTypeDecl *),
               SeparatelyCached, NoLocationInfo)
 SWIFT_REQUEST(NameLookup, HasMissingDesignatedInitializersRequest,

lib/AST/NameLookup.cpp

@@ -980,6 +980,39 @@ SelfBounds swift::getSelfBoundsFromWhereClause(
                            SelfBoundsFromWhereClauseRequest{decl}, {});
 }
 
+SelfBounds SelfBoundsFromGenericSignatureRequest::evaluate(
+    Evaluator &evaluator, const ExtensionDecl *extDecl) const {
+  SelfBounds result;
+  ASTContext &ctx = extDecl->getASTContext();
+  auto selfType = extDecl->getSelfInterfaceType();
+  for (const auto &req : extDecl->getGenericRequirements()) {
+    auto kind = req.getKind();
+    if (kind != RequirementKind::Conformance &&
+        kind != RequirementKind::Superclass)
+      continue;
+    // The left-hand side of the type constraint must be 'Self'.
+    bool isSelfLHS = selfType->isEqual(req.getFirstType());
+    if (!isSelfLHS)
+      continue;
+
+    auto rhsDecls = directReferencesForType(req.getSecondType());
+    SmallVector<ModuleDecl *, 2> modulesFound;
+    auto rhsNominals = resolveTypeDeclsToNominal(
+        evaluator, ctx, rhsDecls, modulesFound, result.anyObject);
+    result.decls.insert(result.decls.end(), rhsNominals.begin(),
+                        rhsNominals.end());
+  }
+
+  return result;
+}
+
+SelfBounds
+swift::getSelfBoundsFromGenericSignature(const ExtensionDecl *extDecl) {
+  auto &ctx = extDecl->getASTContext();
+  return evaluateOrDefault(ctx.evaluator,
+                           SelfBoundsFromGenericSignatureRequest{extDecl}, {});
+}
+
 TinyPtrVector<TypeDecl *>
 TypeDeclsFromWhereClauseRequest::evaluate(Evaluator &evaluator,
                                           ExtensionDecl *ext) const {

lib/Sema/TypeCheckProtocolInference.cpp

@@ -220,8 +220,16 @@ AssociatedTypeInference::inferTypeWitnessesViaValueWitnesses(
     if (!checkConformance(proto))
       return false;
 
-    // Now check any additional bounds on 'Self' from the where clause.
-    auto bounds = getSelfBoundsFromWhereClause(extension);
+    // Source file and module file have different ways to get self bounds.
+    // Source file extension will have trailing where clause which can avoid
+    // computing a generic signature. Module file will not have
+    // trailing where clause, so it will compute generic signature to get
+    // self bounds which might result in slow performance.
+    SelfBounds bounds;
+    if (extension->getParentSourceFile() != nullptr)
+      bounds = getSelfBoundsFromWhereClause(extension);
+    else
+      bounds = getSelfBoundsFromGenericSignature(extension);
     for (auto *decl : bounds.decls) {
       if (auto *proto = dyn_cast<ProtocolDecl>(decl)) {
         if (!checkConformance(proto))

test/Sema/Inputs/where_clause_across_module_boundaries_module.swift

@@ -0,0 +1,25 @@
+public protocol DefaultsBridge {
+  associatedtype T
+}
+
+public struct BridgeStructSerializable<T: Codable>: DefaultsBridge {
+}
+
+public struct BridgeStructRawRepresentable<T: RawRepresentable>: DefaultsBridge {
+}
+
+public protocol DefaultsSerializable {
+  associatedtype Bridge: DefaultsBridge
+  static var _defaults: Bridge { get }
+}
+
+public extension DefaultsSerializable where Self: Codable {
+  static var _defaults: BridgeStructSerializable<Self> { return BridgeStructSerializable() }
+}
+
+public extension DefaultsSerializable where Self: RawRepresentable {
+  static var _defaults: BridgeStructRawRepresentable<Self> { return BridgeStructRawRepresentable() }
+}
+
+public struct ModuleAFoo: Codable, DefaultsSerializable {
+}

test/Sema/where_clause_across_module_boundaries.swift

@@ -0,0 +1,16 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -o %t/ModuleA.swiftmodule %S/Inputs/where_clause_across_module_boundaries_module.swift
+// RUN: %target-typecheck-verify-swift -I %t
+
+// SR-15807:
+// Associated Type Inference fails across module boundaries
+// Self bounds from where clause cannot be accessed across modules.
+// This test is intended to test whether it can use generic signature to get self bounds.
+import ModuleA
+
+struct ModuleBFoo: Codable, DefaultsSerializable {
+}
+
+enum ModuleBBar: Int, Codable, DefaultsSerializable { // expected-error {{type 'ModuleBBar' does not conform to protocol 'DefaultsSerializable'}}
+  case foo, bar
+}