TypeCheckType: Rework IUO diagnostics using behavior limitation

include/swift/AST/DiagnosticsSema.def

@@ -6145,14 +6145,12 @@ ERROR(closure_unlabeled_parameter_following_variadic_parameter,none,
 ERROR(parameter_vararg_default,none,
       "variadic parameter cannot have a default value", ())
 
-WARNING(implicitly_unwrapped_optional_in_illegal_position_interpreted_as_optional,none,
-        "using '!' is not allowed here; treating this as '?' instead", ())
-
-WARNING(implicitly_unwrapped_optional_deprecated_in_this_position,Deprecation,
-        "using '!' here is deprecated and will be removed in a future release", ())
-
-ERROR(implicitly_unwrapped_optional_in_illegal_position,none,
-        "using '!' is not allowed here; perhaps '?' was intended?", ())
+ERROR(iuo_invalid_here, none,
+      "using '!' is not allowed here", ())
+ERROR(iuo_deprecated_here,Deprecation,
+      "using '!' here is deprecated", ())
+NOTE(iuo_use_optional_instead,none,
+     "use '?' instead", ())
 
 // Ownership
 ERROR(invalid_ownership_type,none,

lib/Sema/TypeCheckType.cpp

@@ -5405,32 +5405,40 @@ NeverNullType TypeResolver::resolveImplicitlyUnwrappedOptionalType(
   }
 
   if (doDiag && !options.contains(TypeResolutionFlags::SilenceErrors)) {
-    // Prior to Swift 5, we allow 'as T!' and turn it into a disjunction.
-    if (ctx.isSwiftVersionAtLeast(5)) {
-      // Mark this repr as invalid. This is the only way to indicate that
-      // something went wrong without supressing checking other reprs in
-      // the same type. For example:
-      //
-      // struct S<T, U> { ... }
-      //
-      // _ = S<Int!, String!>(...)
-      //
-      // Compiler should diagnose both `Int!` and `String!` as invalid,
-      // but returning `ErrorType` from here would stop type resolution
-      // after `Int!`.
+    // In language modes up to Swift 5, we allow `T!` in invalid position for
+    // compatibility and downgrade the error to a warning.
+    const unsigned swiftLangModeForError = 5;
+
+    // If we are about to error, mark this node as invalid.
+    // This is the only way to indicate that something went wrong without
+    // supressing checking of sibling nodes.
+    // For example:
+    //
+    // struct S<T, U> { ... }
+    //
+    // _ = S<Int!, String!>(...)
+    //
+    // Compiler should diagnose both `Int!` and `String!` as invalid,
+    // but returning `ErrorType` from here would stop type resolution
+    // after `Int!`.
+    if (ctx.isSwiftVersionAtLeast(swiftLangModeForError)) {
       repr->setInvalid();
+    }
 
-      diagnose(repr->getStartLoc(),
-               diag::implicitly_unwrapped_optional_in_illegal_position)
-          .fixItReplace(repr->getExclamationLoc(), "?");
-    } else if (options.is(TypeResolverContext::ExplicitCastExpr)) {
-      diagnose(
-          repr->getStartLoc(),
-          diag::implicitly_unwrapped_optional_deprecated_in_this_position);
-    } else {
-      diagnose(
-          repr->getStartLoc(),
-          diag::implicitly_unwrapped_optional_in_illegal_position_interpreted_as_optional)
+    Diag<> diagID = diag::iuo_deprecated_here;
+    if (ctx.isSwiftVersionAtLeast(swiftLangModeForError)) {
+      diagID = diag::iuo_invalid_here;
+    }
+
+    diagnose(repr->getExclamationLoc(), diagID)
+        .warnUntilSwiftVersion(swiftLangModeForError);
+
+    // Suggest a regular optional, but not when `T!` is the right-hand side of
+    // a cast expression.
+    // In this case, the user is likely trying to cast an expression of optional
+    // type, and this fix-it is not generally helpful.
+    if (!options.is(TypeResolverContext::ExplicitCastExpr)) {
+      diagnose(repr->getExclamationLoc(), diag::iuo_use_optional_instead)
           .fixItReplace(repr->getExclamationLoc(), "?");
     }
   }

test/Constraints/iuo.swift

@@ -179,43 +179,6 @@ class rdar37241550 {
   }
 }
 
-class B {}
-class D : B {
-  var i: Int!
-}
-
-func coerceToIUO(d: D?) -> B {
-  return d as B! // expected-warning {{using '!' here is deprecated and will be removed in a future release}}
-}
-
-func forcedDowncastToOptional(b: B?) -> D? {
-  return b as! D! // expected-warning {{using '!' here is deprecated and will be removed in a future release}}
-}
-
-func forcedDowncastToObject(b: B?) -> D {
-  return b as! D! // expected-warning {{using '!' here is deprecated and will be removed in a future release}}
-}
-
-func forcedDowncastToObjectIUOMember(b: B?) -> Int {
-  return (b as! D!).i // expected-warning {{using '!' here is deprecated and will be removed in a future release}}
-}
-
-func forcedUnwrapViaForcedCast(b: B?) -> B {
-  return b as! B! // expected-warning {{forced cast from 'B?' to 'B' only unwraps optionals; did you mean to use '!'?}}
-  // expected-warning@-1 {{using '!' here is deprecated and will be removed in a future release}}
-}
-
-func conditionalDowncastToOptional(b: B?) -> D? {
-  return b as? D! // expected-warning {{using '!' here is deprecated and will be removed in a future release}}
-}
-
-func conditionalDowncastToObject(b: B?) -> D {
-  return b as? D! // expected-error {{value of optional type 'D?' must be unwrapped to a value of type 'D'}}
-  // expected-note@-1 {{coalesce using '??' to provide a default when the optional value contains 'nil'}}
-  // expected-note@-2 {{force-unwrap using '!' to abort execution if the optional value contains 'nil'}}
-  // expected-warning@-3 {{using '!' here is deprecated and will be removed in a future release}}
-}
-
 // https://github.com/apple/swift/issues/49536
 // Ensure that we select the overload that does *not* involve forcing an IUO.
 do {