[AST] Introduce the notion of a protocol requirement signature.

include/swift/AST/ArchetypeBuilder.h

@@ -83,6 +83,10 @@ class RequirementSource {
     ///
     /// These are dropped when building the GenericSignature.
     Inherited,
+
+    /// The requirement is the Self: Protocol requirement, when computing a
+    /// protocol's requirement signature.
+    ProtocolRequirementSignatureSelf,
   };
 
   RequirementSource(Kind kind, SourceLoc loc) : StoredKind(kind), Loc(loc) { }

include/swift/AST/Decl.h

@@ -3575,6 +3575,15 @@ class ProtocolDecl : public NominalTypeDecl {
   /// created yet.
   void createGenericParamsIfMissing();
 
+  /// Retrieve the generic signature representing the requirements introduced by
+  /// this protocol.
+  ///
+  /// These are the requirements like any inherited protocols and conformances
+  /// for associated types that are mentioned literally in this
+  /// decl. Requirements implied via inheritance are not mentioned, nor is the
+  /// conformance of Self to this protocol.
+  GenericSignature *getRequirementSignature();
+
   // Implement isa/cast/dyncast/etc.
   static bool classof(const Decl *D) {
     return D->getKind() == DeclKind::Protocol;

lib/AST/ArchetypeBuilder.cpp

@@ -63,6 +63,10 @@ void RequirementSource::dump(llvm::raw_ostream &out,
   case Inherited:
     out << "inherited";
     break;
+
+  case ProtocolRequirementSignatureSelf:
+    out << "protocol_requirement_signature_self";
+    break;
   }
 
   if (srcMgr && getLoc().isValid()) {
@@ -753,6 +757,7 @@ Type ArchetypeBuilder::PotentialArchetype::getTypeInContext(
     case RequirementSource::Explicit:
     case RequirementSource::Inferred:
     case RequirementSource::Protocol:
+    case RequirementSource::ProtocolRequirementSignatureSelf:
     case RequirementSource::Redundant:
       Protos.push_back(conforms.first);
       break;
@@ -1002,8 +1007,14 @@ bool ArchetypeBuilder::addConformanceRequirement(PotentialArchetype *PAT,
   if (!T->addConformance(Proto, /*updateExistingSource=*/true, Source, *this))
     return false;
 
-  RequirementSource InnerSource(RequirementSource::Redundant, Source.getLoc());
-
+  // Conformances to inherit protocols are explicit in a protocol requirement
+  // signature, but inferred from this conformance otherwise.
+  auto InnerKind =
+      Source.getKind() == RequirementSource::ProtocolRequirementSignatureSelf
+          ? RequirementSource::Explicit
+          : RequirementSource::Redundant;
+  RequirementSource InnerSource(InnerKind, Source.getLoc());
+
   bool inserted = Visited.insert(Proto).second;
   assert(inserted);
   (void) inserted;
@@ -1024,10 +1035,15 @@ bool ArchetypeBuilder::addConformanceRequirement(PotentialArchetype *PAT,
     if (auto AssocType = dyn_cast<AssociatedTypeDecl>(Member)) {
       // Add requirements placed directly on this associated type.
       auto AssocPA = T->getNestedType(AssocType, *this);
+      // Requirements introduced by the main protocol are explicit in a protocol
+      // requirement signature, but inferred from this conformance otherwise.
+      auto Kind = Source.getKind() ==
+                          RequirementSource::ProtocolRequirementSignatureSelf
+                      ? RequirementSource::Explicit
+                      : RequirementSource::Protocol;
+
       if (AssocPA != T) {
-        if (addAbstractTypeParamRequirements(AssocType, AssocPA,
-                                             RequirementSource::Protocol,
-                                             Visited))
+        if (addAbstractTypeParamRequirements(AssocType, AssocPA, Kind, Visited))
           return true;
       }
 
@@ -2060,7 +2076,7 @@ void ArchetypeBuilder::dump(llvm::raw_ostream &out) {
 
 void ArchetypeBuilder::addGenericSignature(GenericSignature *sig) {
   if (!sig) return;
-  
+
   RequirementSource::Kind sourceKind = RequirementSource::Explicit;
   for (auto param : sig->getGenericParams())
     addGenericParameter(param);
@@ -2090,6 +2106,7 @@ static void collectRequirements(ArchetypeBuilder &builder,
     case RequirementSource::Protocol:
     case RequirementSource::Redundant:
     case RequirementSource::Inherited:
+    case RequirementSource::ProtocolRequirementSignatureSelf:
       // The requirement was redundant, drop it.
       return;
     }

lib/AST/Decl.cpp

@@ -16,6 +16,7 @@
 
 #include "swift/AST/Decl.h"
 #include "swift/AST/AccessScope.h"
+#include "swift/AST/ArchetypeBuilder.h"
 #include "swift/AST/AST.h"
 #include "swift/AST/ASTContext.h"
 #include "swift/AST/ASTWalker.h"
@@ -2999,6 +3000,30 @@ void ProtocolDecl::createGenericParamsIfMissing() {
     setGenericParams(createGenericParams(this));
 }
 
+GenericSignature *ProtocolDecl::getRequirementSignature() {
+  auto module = getParentModule();
+
+  auto genericSig = getGenericSignature();
+  // The signature should look like <Self where Self : ThisProtocol>, and we
+  // reuse the two parts of it because the parameter and the requirement are
+  // exactly what we need.
+  assert(genericSig->getGenericParams().size() == 1 &&
+         genericSig->getRequirements().size() == 1 &&
+         "getRequirementSignature with unexpected generic signature");
+
+  auto selfType = genericSig->getGenericParams()[0];
+  auto requirement = genericSig->getRequirements()[0];
+
+  RequirementSource source(RequirementSource::ProtocolRequirementSignatureSelf,
+                           getLoc());
+
+  ArchetypeBuilder builder(getASTContext(), LookUpConformanceInModule(module));
+  builder.addGenericParameter(selfType);
+  builder.addRequirement(requirement, source);
+
+  return builder.getGenericSignature();
+}
+
 /// Returns the default witness for a requirement, or nullptr if there is
 /// no default.
 Witness ProtocolDecl::getDefaultWitness(ValueDecl *requirement) const {

lib/Sema/TypeCheckDecl.cpp

@@ -4371,6 +4371,20 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       visit(Member);
 
     TC.checkDeclAttributes(PD);
+
+    if (TC.Context.LangOpts.DebugGenericSignatures) {
+      auto requirementsSig = PD->getRequirementSignature();
+
+      llvm::errs() << "Protocol requirement signature:\n";
+      PD->dumpRef(llvm::errs());
+      llvm::errs() << "\n";
+      llvm::errs() << "Requirement signature: ";
+      requirementsSig->print(llvm::errs());
+      llvm::errs() << "\n";
+      llvm::errs() << "Canonical requirement signature: ";
+      requirementsSig->getCanonicalSignature()->print(llvm::errs());
+      llvm::errs() << "\n";
+    }
   }
 
   void visitVarDecl(VarDecl *VD) {

test/Generics/protocol_requirement_signatures.swift

@@ -0,0 +1,70 @@
+// RUN: %target-typecheck-verify-swift -typecheck %s -verify
+// RUN: %target-typecheck-verify-swift -typecheck -debug-generic-signatures %s > %t.dump 2>&1
+// RUN: %FileCheck %s < %t.dump
+
+// CHECK-LABEL: .P1@
+// CHECK-NEXT: Requirement signature: <Self>
+// CHECK-NEXT: Canonical requirement signature: <τ_0_0>
+protocol P1 {}
+
+// CHECK-LABEL: .P2@
+// CHECK-NEXT: Requirement signature: <Self>
+// CHECK-NEXT: Canonical requirement signature: <τ_0_0>
+protocol P2 {}
+
+// CHECK-LABEL: .P3@
+// CHECK-NEXT: Requirement signature: <Self>
+// CHECK-NEXT: Canonical requirement signature: <τ_0_0>
+protocol P3 {}
+
+// basic protocol
+// CHECK-LABEL: .Q1@
+// CHECK-NEXT: Requirement signature: <Self where Self.X : P1>
+// CHECK-NEXT: Canonical requirement signature: <τ_0_0 where τ_0_0.X : P1>
+protocol Q1 {
+    associatedtype X: P1
+}
+
+// inheritance
+// CHECK-LABEL: .Q2@
+// CHECK-NEXT: Requirement signature: <Self where Self : Q1>
+// CHECK-NEXT: Canonical requirement signature: <τ_0_0 where τ_0_0 : Q1>
+protocol Q2: Q1 {}
+
+// inheritance without any new requirements
+// CHECK-LABEL: .Q3@
+// CHECK-NEXT: Requirement signature: <Self where Self : Q1>
+// CHECK-NEXT: Canonical requirement signature: <τ_0_0 where τ_0_0 : Q1>
+protocol Q3: Q1 {
+    associatedtype X
+}
+
+// inheritance adding a new conformance
+// CHECK-LABEL: .Q4@
+// CHECK-NEXT: Requirement signature: <Self where Self : Q1, Self.X : P2>
+// CHECK-NEXT: Canonical requirement signature: <τ_0_0 where τ_0_0 : Q1, τ_0_0.X : P2>
+protocol Q4: Q1 {
+    associatedtype X: P2
+}
+
+// multiple inheritance
+// CHECK-LABEL: .Q5@
+// CHECK-NEXT: Requirement signature: <Self where Self : Q2, Self : Q3, Self : Q4>
+// CHECK-NEXT: Canonical requirement signature: <τ_0_0 where τ_0_0 : Q2, τ_0_0 : Q3, τ_0_0 : Q4>
+protocol Q5: Q2, Q3, Q4 {}
+
+// multiple inheritance without any new requirements
+// CHECK-LABEL: .Q6@
+// CHECK-NEXT: Requirement signature: <Self where Self : Q2, Self : Q3, Self : Q4>
+// CHECK-NEXT: Canonical requirement signature: <τ_0_0 where τ_0_0 : Q2, τ_0_0 : Q3, τ_0_0 : Q4>
+protocol Q6: Q2, Q3, Q4 {
+    associatedtype X: P1
+}
+
+// multiple inheritance with a new conformance
+// CHECK-LABEL: .Q7@
+// CHECK-NEXT: Requirement signature: <Self where Self : Q2, Self : Q3, Self : Q4, Self.X : P3>
+// CHECK-NEXT: Canonical requirement signature: <τ_0_0 where τ_0_0 : Q2, τ_0_0 : Q3, τ_0_0 : Q4, τ_0_0.X : P3>
+protocol Q7: Q2, Q3, Q4 {
+    associatedtype X: P3
+}