Sema: Generalize the 'invariant Self requirement' check to handle a related case

There are three kinds of invariant 'Self' uses here:

- 'Self' appears as the left hand side of a same-type requirement between type parameters
- 'Self' appears as a structural component of the right hand side of a concrete type requirement
- 'Self' appears as a structural component of the right hand side of a superclass requirement

My previous fix only handled the first case. Generalize it to handle all three.

Fixes rdar://problem/74944514.

Author: slavapestov

include/swift/AST/DiagnosticsSema.def

@@ -1935,8 +1935,8 @@ WARNING(type_does_not_conform_swiftui_warning,none,
 
 ERROR(non_final_class_cannot_conform_to_self_same_type,none,
       "non-final class %0 cannot safely conform to protocol %1, "
-      "which requires that 'Self' is exactly equal to %2",
-      (Type, Type, Type))
+      "which requires that %2 %select{is exactly equal to|inherit from}3 %4",
+      (Type, Type, Type, unsigned, Type))
 
 ERROR(cannot_use_nil_with_this_type,none,
       "'nil' cannot be used in context expecting type %0", (Type))

lib/Sema/TypeCheckProtocol.cpp

@@ -4840,6 +4840,102 @@ ResolveWitnessResult ConformanceChecker::resolveTypeWitnessViaLookup(
   return ResolveWitnessResult::ExplicitFailed;
 }
 
+/// FIXME: It feels like this could be part of findExistentialSelfReferences().
+static Optional<Requirement>
+hasInvariantSelfRequirement(const ProtocolDecl *proto,
+                            ArrayRef<Requirement> reqSig) {
+  auto selfTy = proto->getSelfInterfaceType();
+
+  auto containsInvariantSelf = [&](Type t) -> bool {
+    struct Walker : public TypeWalker {
+      Type SelfTy;
+      bool Found = false;
+
+      Walker(Type selfTy) : SelfTy(selfTy) {}
+
+      Action walkToTypePre(Type ty) override {
+        // Check for 'Self'.
+        if (ty->isEqual(SelfTy)) {
+          Found = true;
+          return Action::Stop;
+        }
+
+        // 'Self.A' is OK.
+        if (ty->is<DependentMemberType>())
+          return Action::SkipChildren;
+
+        return Action::Continue;
+      }
+    };
+
+    Walker walker(selfTy);
+    t.walk(walker);
+
+    return walker.Found;
+  };
+
+  for (auto req : reqSig) {
+    switch (req.getKind()) {
+    case RequirementKind::SameType:
+      if (req.getSecondType()->isTypeParameter()) {
+        if (req.getFirstType()->isEqual(selfTy))
+          return req;
+      } else {
+        if (containsInvariantSelf(req.getSecondType()))
+          return req;
+      }
+      continue;
+    case RequirementKind::Superclass:
+      if (containsInvariantSelf(req.getSecondType()))
+        return req;
+      continue;
+    case RequirementKind::Conformance:
+    case RequirementKind::Layout:
+      continue;
+    }
+
+    llvm_unreachable("Bad requirement kind");
+  }
+
+  return None;
+}
+
+static void diagnoseInvariantSelfRequirement(
+    SourceLoc loc, Type adoptee, const ProtocolDecl *proto,
+    Requirement req, DiagnosticEngine &diags) {
+  Type firstType, secondType;
+  unsigned kind = 0;
+
+  switch (req.getKind()) {
+  case RequirementKind::SameType:
+  if (req.getSecondType()->isTypeParameter()) {
+      // eg, 'Self == Self.A.B'
+      firstType = req.getSecondType();
+      secondType = req.getFirstType();
+    } else {
+      // eg, 'Self.A.B == G<Self>'
+      firstType = req.getFirstType();
+      secondType = req.getSecondType();
+    }
+    kind = 0;
+    break;
+  case RequirementKind::Superclass:
+    // eg, 'Self.A.B : G<Self>'
+    firstType = req.getFirstType();
+    secondType = req.getSecondType();
+    kind = 1;
+    break;
+  case RequirementKind::Conformance:
+  case RequirementKind::Layout:
+    llvm_unreachable("Invalid requirement kind");
+  }
+
+  diags.diagnose(loc, diag::non_final_class_cannot_conform_to_self_same_type,
+                 adoptee, proto->getDeclaredInterfaceType(),
+                 firstType, kind, secondType)
+      .warnUntilSwiftVersion(6);
+}
+
 void ConformanceChecker::ensureRequirementsAreSatisfied() {
   Conformance->finishSignatureConformances();
   auto proto = Conformance->getProtocol();
@@ -4867,15 +4963,8 @@ void ConformanceChecker::ensureRequirementsAreSatisfied() {
   // instead of the concrete class type itself.
   if (auto *classDecl = Adoptee->getClassOrBoundGenericClass()) {
     if (!classDecl->isSemanticallyFinal()) {
-      for (auto req : reqSig) {
-        if (req.getKind() == RequirementKind::SameType &&
-            req.getFirstType()->isEqual(proto->getSelfInterfaceType())) {
-          diags.diagnose(Loc, diag::non_final_class_cannot_conform_to_self_same_type,
-                         Adoptee, proto->getDeclaredInterfaceType(),
-                         req.getSecondType())
-              .warnUntilSwiftVersion(6);
-          break;
-        }
+      if (auto req = hasInvariantSelfRequirement(proto, reqSig)) {
+        diagnoseInvariantSelfRequirement(Loc, Adoptee, proto, *req, diags);
       }
     }
   }

test/Generics/non_final_class_conforms_same_type_requirement_on_self.swift

@@ -14,7 +14,7 @@ public protocol Q {
 // exactly equal C; since C is not final, this means the conformance
 // is not covariant.
 public class C : P {
-// expected-warning@-1 {{non-final class 'C' cannot safely conform to protocol 'P', which requires that 'Self' is exactly equal to 'Self.A.B'; this is an error in Swift 6}}
+// expected-warning@-1 {{non-final class 'C' cannot safely conform to protocol 'P', which requires that 'Self.A.B' is exactly equal to 'Self'; this is an error in Swift 6}}
   public typealias A = D
 }
 
@@ -45,3 +45,32 @@ public func takesBoth1<T>(_: T) where T : P, T : C {}
 
 // CHECK-LABEL: Generic signature: <U where U : C>
 public func takesBoth2<U>(_: U) where U : C, U : P {}
+
+// 'Self' can also occur inside of a concrete type or superclass requirement.
+public class G<T> {}
+
+public protocol ConcreteExampleP {
+  associatedtype A : Q where A.B == G<Self>
+}
+
+public class ConcreteExampleC : ConcreteExampleP {
+// expected-warning@-1 {{non-final class 'ConcreteExampleC' cannot safely conform to protocol 'ConcreteExampleP', which requires that 'Self.A.B' is exactly equal to 'G<Self>'; this is an error in Swift 6}}
+  public typealias A = ConcreteExampleD
+}
+
+public class ConcreteExampleD : Q {
+  public typealias B = G<ConcreteExampleC>
+}
+
+public protocol SuperclassExampleP {
+  associatedtype A : Q where A.B : G<Self>
+}
+
+public class SuperclassExampleC : SuperclassExampleP {
+// expected-warning@-1 {{non-final class 'SuperclassExampleC' cannot safely conform to protocol 'SuperclassExampleP', which requires that 'Self.A.B' inherit from 'G<Self>'; this is an error in Swift 6}}
+  public typealias A = SuperclassExampleD
+}
+
+public class SuperclassExampleD : Q {
+  public typealias B = G<SuperclassExampleC>
+}

test/Generics/rdar80503090.swift

@@ -20,7 +20,7 @@ extension P where T : Q {
 }
 
 class C : P {}
-// expected-warning@-1 {{non-final class 'C' cannot safely conform to protocol 'P', which requires that 'Self' is exactly equal to 'Self.T'; this is an error in Swift 6}}
+// expected-warning@-1 {{non-final class 'C' cannot safely conform to protocol 'P', which requires that 'Self.T' is exactly equal to 'Self'; this is an error in Swift 6}}
 
 extension P where T : C {
   // CHECK-LABEL: Generic signature: <Self where Self : C>

validation-test/compiler_crashers_2_fixed/0189-sr10033.swift

@@ -15,9 +15,9 @@ extension P2 {
 }
 
 class C1 : P1 {
-// expected-warning@-1 {{non-final class 'C1' cannot safely conform to protocol 'P1', which requires that 'Self' is exactly equal to 'Self.A2.A1'; this is an error in Swift 6}}
+// expected-warning@-1 {{non-final class 'C1' cannot safely conform to protocol 'P1', which requires that 'Self.A2.A1' is exactly equal to 'Self'; this is an error in Swift 6}}
   class A2 : P2 {
-  // expected-warning@-1 {{non-final class 'C1.A2' cannot safely conform to protocol 'P2', which requires that 'Self' is exactly equal to 'Self.A1.A2'; this is an error in Swift 6}}
+  // expected-warning@-1 {{non-final class 'C1.A2' cannot safely conform to protocol 'P2', which requires that 'Self.A1.A2' is exactly equal to 'Self'; this is an error in Swift 6}}
     typealias A1 = C1
   }
 }