TypeCheckType: Warn, not error, about missing existential any until Swift 7

The conditions for when and how (error or warning) a missing `any` is
diagnosed are a mouthful even if we set aside source compatibility
hacks. This change builds on
#78347 by downgrading all missing
`any` diagnostics to warnings until Swift 7, making the behavior less
confusing for developers and giving us an opportunity to patch up
remaining holes in the diagnosis gracefully.

Author: AnthonyLatsis

lib/Sema/TypeCheckType.cpp

@@ -904,11 +904,15 @@ static Type applyGenericArguments(Type type,
     auto parameterized =
         ParameterizedProtocolType::get(ctx, protoType, argTys);
 
+    // FIXME: Can this not be done in ExistentialTypeSyntaxChecker?
     if (resolution.getOptions().isConstraintImplicitExistential() &&
         !ctx.LangOpts.hasFeature(Feature::ImplicitSome)) {
-      diags.diagnose(loc, diag::existential_requires_any, parameterized,
-                     ExistentialType::get(parameterized),
-                     /*isAlias=*/isa<TypeAliasType>(type.getPointer()));
+      diags
+          .diagnose(loc, diag::existential_requires_any, parameterized,
+                    ExistentialType::get(parameterized),
+                    /*isAlias=*/isa<TypeAliasType>(type.getPointer()))
+          .warnUntilSwiftVersion(7);
+
       return ErrorType::get(ctx);
     }
 
@@ -6161,16 +6165,13 @@ class ExistentialTypeSyntaxChecker : public ASTWalker {
   ASTContext &Ctx;
   const bool checkStatements;
   bool hitTopStmt;
-  const bool downgradeErrorsToWarnings;
 
   unsigned exprCount = 0;
   llvm::SmallVector<TypeRepr *, 4> reprStack;
 
 public:
-  ExistentialTypeSyntaxChecker(ASTContext &ctx, bool checkStatements,
-                               bool downgradeErrorsToWarnings = false)
-      : Ctx(ctx), checkStatements(checkStatements), hitTopStmt(false),
-        downgradeErrorsToWarnings(downgradeErrorsToWarnings) {}
+  ExistentialTypeSyntaxChecker(ASTContext &ctx, bool checkStatements)
+      : Ctx(ctx), checkStatements(checkStatements), hitTopStmt(false) {}
 
   MacroWalking getMacroWalkingBehavior() const override {
     return MacroWalking::ArgumentsAndExpansion;
@@ -6361,42 +6362,42 @@ class ExistentialTypeSyntaxChecker : public ASTWalker {
   /// \param constraintTy The constraint type that is missing the keyword.
   /// \param isInverted Whether the constraint type is an object of an `~`
   ///  inversion.
-  static DiagnosticBehavior getDiagnosticBehaviorForMissingAnyOrSomeKeyword(
-      Type constraintTy, bool isInverted, ASTContext &ctx) {
+  static bool shouldDiagnoseMissingAnyOrSomeKeyword(Type constraintTy,
+                                                    bool isInverted,
+                                                    ASTContext &ctx) {
     // `Any` and `AnyObject` are always exempt from `any` syntax.
     if (constraintTy->isAny() || constraintTy->isAnyObject()) {
-      return DiagnosticBehavior::Ignore;
+      return false;
+    }
+
+    // A missing `any` or `some` is always diagnosed if this feature is enabled.
+    if (ctx.LangOpts.hasFeature(Feature::ExistentialAny)) {
+      return true;
     }
 
-    // If the type is inverted, a missing `any` or `some` is an error.
+    // If the type is inverted, a missing `any` or `some` is always diagnosed.
     if (isInverted) {
-      return DiagnosticBehavior::Error;
+      return true;
     }
 
     // If one of the protocols is inverted, a missing `any` or `some` is
-    // an error.
+    // always diagnosed.
     if (auto *PCT = constraintTy->getAs<ProtocolCompositionType>()) {
       if (!PCT->getInverses().empty()) {
-        return DiagnosticBehavior::Error;
+        return true;
       }
     }
 
     // If one of the protocols has "Self or associated type" requirements,
-    // a missing `any` or `some` is an error.
+    // a missing `any` or `some` is always diagnosed.
     auto layout = constraintTy->getExistentialLayout();
     for (auto *protoDecl : layout.getProtocols()) {
       if (protoDecl->hasSelfOrAssociatedTypeRequirements()) {
-        return DiagnosticBehavior::Error;
+        return true;
       }
     }
 
-    if (ctx.LangOpts.hasFeature(Feature::ExistentialAny)) {
-      // For anything else, a missing `any` or `some` is a warning if this
-      // feature is enabled.
-      return DiagnosticBehavior::Warning;
-    }
-
-    return DiagnosticBehavior::Ignore;
+    return false;
   }
 
   void checkDeclRefTypeRepr(DeclRefTypeRepr *T) const {
@@ -6447,20 +6448,11 @@ class ExistentialTypeSyntaxChecker : public ASTWalker {
       return dyn_cast<InverseTypeRepr>(*it);
     }();
 
-    DiagnosticBehavior behavior =
-        this->getDiagnosticBehaviorForMissingAnyOrSomeKeyword(
-            type, /*isInverted=*/outerInversion, this->Ctx);
-
-    if (behavior == DiagnosticBehavior::Ignore) {
+    if (!shouldDiagnoseMissingAnyOrSomeKeyword(
+            type, /*isInverted=*/outerInversion, this->Ctx)) {
       return;
     }
 
-    // If we were asked to downgrade errors, respect it.
-    if (behavior == DiagnosticBehavior::Error &&
-        this->downgradeErrorsToWarnings) {
-      behavior = DiagnosticBehavior::Warning;
-    }
-
     std::optional<InFlightDiagnostic> diag;
     if (outerInversion) {
       diag.emplace(Ctx.Diags.diagnose(outerInversion->getTildeLoc(),
@@ -6472,7 +6464,7 @@ class ExistentialTypeSyntaxChecker : public ASTWalker {
                                       /*isAlias=*/isa<TypeAliasDecl>(decl)));
     }
 
-    diag->limitBehaviorUntilSwiftVersion(behavior, 7);
+    diag->warnUntilSwiftVersion(7);
     emitInsertAnyFixit(*diag, T);
   }
 
@@ -6549,14 +6541,7 @@ void TypeChecker::checkExistentialTypes(ASTContext &ctx, Stmt *stmt,
   if (sourceFile && sourceFile->Kind == SourceFileKind::Interface)
     return;
 
-  // Previously we missed this diagnostic on 'catch' statements, downgrade
-  // to a warning until Swift 7.
-  auto downgradeErrorsToWarnings = false;
-  if (auto *CS = dyn_cast<CaseStmt>(stmt))
-    downgradeErrorsToWarnings = CS->getParentKind() == CaseParentKind::DoCatch;
-
-  ExistentialTypeSyntaxChecker checker(ctx, /*checkStatements=*/true,
-                                       downgradeErrorsToWarnings);
+  ExistentialTypeSyntaxChecker checker(ctx, /*checkStatements=*/true);
   stmt->walk(checker);
 }
 

test/Generics/inverse_generics.swift

@@ -471,8 +471,8 @@ func checkExistentials() {
 
 typealias NotCopyable = ~Copyable
 typealias EmptyComposition = ~Copyable & ~Escapable
-func test(_ t: borrowing NotCopyable) {} // expected-error {{use of 'NotCopyable' (aka '~Copyable') as a type must be written 'any NotCopyable'}}
-func test(_ t: borrowing EmptyComposition) {} // expected-error {{use of 'EmptyComposition' (aka '~Copyable & ~Escapable') as a type must be written 'any EmptyComposition' (aka 'any ~Copyable & ~Escapable')}}
+func test(_ t: borrowing NotCopyable) {} // expected-warning {{use of 'NotCopyable' (aka '~Copyable') as a type must be written 'any NotCopyable'}}
+func test(_ t: borrowing EmptyComposition) {} // expected-warning {{use of 'EmptyComposition' (aka '~Copyable & ~Escapable') as a type must be written 'any EmptyComposition' (aka 'any ~Copyable & ~Escapable')}}
 
 typealias Copy = Copyable
 func test(_ z1: Copy, _ z2: Copyable) {}

test/Macros/macros_diagnostics.swift

@@ -196,8 +196,8 @@ struct MyStruct<T: MyProto> {
 
 struct SomeType {
   #genericUnary<Equatable>(0 as Hashable)
-  // expected-error@-1{{use of protocol 'Equatable' as a type must be written 'any Equatable'}}
-  // expected-error@-2{{use of protocol 'Hashable' as a type must be written 'any Hashable'}}
+  // expected-warning@-1{{use of protocol 'Equatable' as a type must be written 'any Equatable'}}
+  // expected-warning@-2{{use of protocol 'Hashable' as a type must be written 'any Hashable'}}
   // expected-error@-3{{external macro implementation type}}
 }
 

test/Macros/top_level_freestanding.swift

@@ -107,12 +107,12 @@ func testGlobalVariable() {
 
 // expected-note @+1 6 {{in expansion of macro 'anonymousTypes' here}}
 #anonymousTypes(causeErrors: true) { "foo" }
-// DIAG_BUFFERS-DAG: @__swiftmacro_9MacroUser0033top_level_freestandingswift_DbGHjfMX108_0_33_082AE7CFEFA6960C804A9FE7366EB5A0Ll14anonymousTypesfMf0_{{.*}}: error: use of protocol 'Equatable' as a type must be written 'any Equatable'
-// DIAG_BUFFERS-DAG: @__swiftmacro_9MacroUser00142___swiftmacro_9MacroUser0033top_level_freestandingswift_DbGHjfMX108_0_33_082AE7CFEFA6960C804A9FE7366EB5A0Ll14anonymousTypesfMf0_swift_DAIABdjIbfMX23_2_33_082AE7CFEFA6960C804A9FE7366EB5A0Ll27introduceTypeCheckingErrorsfMf_{{.*}}: error: use of protocol 'Hashable' as a type must be written 'any Hashable'
+// DIAG_BUFFERS-DAG: @__swiftmacro_9MacroUser0033top_level_freestandingswift_DbGHjfMX108_0_33_082AE7CFEFA6960C804A9FE7366EB5A0Ll14anonymousTypesfMf0_{{.*}}: warning: use of protocol 'Equatable' as a type must be written 'any Equatable'
+// DIAG_BUFFERS-DAG: @__swiftmacro_9MacroUser00142___swiftmacro_9MacroUser0033top_level_freestandingswift_DbGHjfMX108_0_33_082AE7CFEFA6960C804A9FE7366EB5A0Ll14anonymousTypesfMf0_swift_DAIABdjIbfMX23_2_33_082AE7CFEFA6960C804A9FE7366EB5A0Ll27introduceTypeCheckingErrorsfMf_{{.*}}: warning: use of protocol 'Hashable' as a type must be written 'any Hashable'
 
 // expected-note @+1 2 {{in expansion of macro 'anonymousTypes' here}}
 #anonymousTypes { () -> String in
-  // expected-error @+1 {{use of protocol 'Equatable' as a type must be written 'any Equatable'}}
+  // expected-warning @+1 {{use of protocol 'Equatable' as a type must be written 'any Equatable'}}
   _ = 0 as Equatable
   return "foo"
 }

test/decl/nested/protocol.swift

@@ -237,7 +237,7 @@ extension OuterProtocol {
 
 struct ConformsToOuterProtocol : OuterProtocol {
   typealias Hen = Int
-  func f() { let _ = InnerProtocol.self } // expected-error {{use of protocol 'InnerProtocol' as a type must be written 'any InnerProtocol'}}
+  func f() { let _ = InnerProtocol.self } // expected-warning {{use of protocol 'InnerProtocol' as a type must be written 'any InnerProtocol'}}
 }
 
 extension OuterProtocol {

test/decl/protocol/existential_member_access/generic_fixit.swift

@@ -12,12 +12,12 @@ protocol P {
 protocol Q {}
 
 do {
-  func test(p: P) { // expected-error {{use of protocol 'P' as a type must be written 'any P'}}
+  func test(p: P) { // expected-warning {{use of protocol 'P' as a type must be written 'any P'}}
     p.method(false) // expected-error {{member 'method' cannot be used on value of type 'any P'; consider using a generic constraint instead}} {{-1:16--1:17=some P}} {{none}}
   }
 }
 do {
-  func test(p: ((P))) { // expected-error {{use of protocol 'P' as a type must be written 'any P'}}
+  func test(p: ((P))) { // expected-warning {{use of protocol 'P' as a type must be written 'any P'}}
     p.method(false) // expected-error {{member 'method' cannot be used on value of type 'any P'; consider using a generic constraint instead}} {{-1:18--1:19=some P}} {{none}}
   }
 }
@@ -57,12 +57,12 @@ do {
   }
 }
 do {
-  func test(p: P.Type) { // expected-error {{use of protocol 'P' as a type must be written 'any P'}}
+  func test(p: P.Type) { // expected-warning {{use of protocol 'P' as a type must be written 'any P'}}
     p.staticMethod(false) // expected-error {{member 'staticMethod' cannot be used on value of type 'any P.Type'; consider using a generic constraint instead}} {{-1:16--1:17=(some P)}} {{none}}
   }
 }
 do {
-  func test(p: (P).Type) { // expected-error {{use of protocol 'P' as a type must be written 'any P'}}
+  func test(p: (P).Type) { // expected-warning {{use of protocol 'P' as a type must be written 'any P'}}
     p.staticMethod(false) // expected-error {{member 'staticMethod' cannot be used on value of type 'any P.Type'; consider using a generic constraint instead}} {{-1:17--1:18=some P}} {{none}}
   }
 }
@@ -78,12 +78,12 @@ do {
 }
 
 do {
-  func test(p: P & Q) { // expected-error {{use of protocol 'P' as a type must be written 'any P'}}
+  func test(p: P & Q) { // expected-warning {{use of protocol 'P' as a type must be written 'any P'}}
     p.method(false) // expected-error {{member 'method' cannot be used on value of type 'any P & Q'; consider using a generic constraint instead}} {{-1:16--1:21=some P & Q}} {{none}}
   }
 }
 do {
-  func test(p: ((P & Q))) { // expected-error {{use of protocol 'P' as a type must be written 'any P'}}
+  func test(p: ((P & Q))) { // expected-warning {{use of protocol 'P' as a type must be written 'any P'}}
     p.method(false) // expected-error {{member 'method' cannot be used on value of type 'any P & Q'; consider using a generic constraint instead}} {{-1:18--1:23=some P & Q}} {{none}}
   }
 }
@@ -123,12 +123,12 @@ do {
   }
 }
 do {
-  func test(p: (P & Q).Type) { // expected-error {{use of protocol 'P' as a type must be written 'any P'}}
+  func test(p: (P & Q).Type) { // expected-warning {{use of protocol 'P' as a type must be written 'any P'}}
     p.staticMethod(false) // expected-error {{member 'staticMethod' cannot be used on value of type 'any (P & Q).Type'; consider using a generic constraint instead}} {{-1:17--1:22=some P & Q}} {{none}}
   }
 }
 do {
-  func test(p: ((P & Q)).Type) { // expected-error {{use of protocol 'P' as a type must be written 'any P'}}
+  func test(p: ((P & Q)).Type) { // expected-warning {{use of protocol 'P' as a type must be written 'any P'}}
     p.staticMethod(false) // expected-error {{member 'staticMethod' cannot be used on value of type 'any (P & Q).Type'; consider using a generic constraint instead}} {{-1:18--1:23=some P & Q}} {{none}}
   }
 }

test/decl/protocol/protocols.swift

@@ -475,7 +475,7 @@ func i<T : C3>(_ x : T?) -> Bool {
   // expected-warning@-1 {{checking a value with optional type 'T?' against type 'any P1' succeeds whenever the value is non-nil; did you mean to use '!= nil'?}}
 }
 func j(_ x : C1) -> Bool {
-  return x is P1 // expected-error {{use of protocol 'P1' as a type must be written 'any P1'}}
+  return x is P1 // expected-warning {{use of protocol 'P1' as a type must be written 'any P1'}}
 }
 func k(_ x : C1?) -> Bool {
   return x is any P1

test/decl/protocol/recursive_requirement.swift

@@ -1,5 +1,5 @@
 // RUN: %target-typecheck-verify-swift
-// RUN: not %target-swift-frontend -typecheck %s -debug-generic-signatures 2>&1 | %FileCheck %s
+// RUN: %target-swift-frontend -typecheck %s -debug-generic-signatures 2>&1 | %FileCheck %s
 
 // -----
 
@@ -95,7 +95,7 @@ protocol AsExistentialB {
 }
 
 protocol AsExistentialAssocTypeA {
-  var delegate : AsExistentialAssocTypeB? { get } // expected-error {{use of protocol 'AsExistentialAssocTypeB' as a type must be written 'any AsExistentialAssocTypeB'}}
+  var delegate : (any AsExistentialAssocTypeB)? { get }
 }
 protocol AsExistentialAssocTypeB {
   func aMethod(_ object : AsExistentialAssocTypeA)
@@ -107,7 +107,7 @@ protocol AsExistentialAssocTypeAgainA {
   associatedtype Bar
 }
 protocol AsExistentialAssocTypeAgainB {
-  func aMethod(_ object : AsExistentialAssocTypeAgainA) // expected-error {{use of protocol 'AsExistentialAssocTypeAgainA' as a type must be written 'any AsExistentialAssocTypeAgainA'}}
+  func aMethod(_ object : any AsExistentialAssocTypeAgainA)
 }
 
 // https://github.com/apple/swift/issues/43164