AST: Add a diagnostic group for any syntax diagnostics

Allow users to escalate `any` syntax warnings to errors with
`-Werror ExistentialAny`.

Author: AnthonyLatsis

include/swift/AST/DiagnosticGroups.def

@@ -25,6 +25,7 @@ GROUP(no_group, "")
 GROUP(DeprecatedDeclaration, "DeprecatedDeclaration.md")
 GROUP(Unsafe, "Unsafe.md")
 GROUP(UnknownWarningGroup, "UnknownWarningGroup.md")
+GROUP(ExistentialAny, "ExistentialAny.md")
 
 #define UNDEFINE_DIAGNOSTIC_GROUPS_MACROS
 #include "swift/AST/DefineDiagnosticGroupsMacros.h"

include/swift/AST/DiagnosticsSema.def

@@ -5817,11 +5817,12 @@ ERROR(any_not_existential,none,
 ERROR(incorrect_optional_any,none,
       "optional 'any' type must be written %0",
       (Type))
-ERROR(existential_requires_any,none,
-      "use of %select{protocol |}2%0 as a type must be written %1",
-      (Type, Type, bool))
-ERROR(inverse_requires_any,none,
-      "constraint that suppresses conformance requires 'any'", ())
+
+GROUPED_ERROR(existential_requires_any,ExistentialAny,none,
+              "use of %select{protocol |}2%0 as a type must be written %1",
+              (Type, Type, bool))
+GROUPED_ERROR(inverse_requires_any,ExistentialAny,none,
+              "constraint that suppresses conformance requires 'any'", ())
 
 ERROR(nonisolated_mutable_storage,none,
       "'nonisolated' cannot be applied to mutable stored properties",

test/type/explicit_existential_diagnostic_group.swift

@@ -0,0 +1,12 @@
+// RUN: %target-typecheck-verify-swift -enable-upcoming-feature ExistentialAny -Werror ExistentialAny -swift-version 5
+
+// REQUIRES: swift_feature_ExistentialAny
+
+do {
+  protocol P {}
+
+  let _: P
+  // expected-error@-1{{use of protocol 'P' as a type must be written 'any P'; this will be an error in a future Swift language mode}}
+  let _: ~Copyable
+  // expected-error@-1 {{constraint that suppresses conformance requires 'any'; this will be an error in a future Swift language mode}}
+}

userdocs/diagnostic_groups/ExistentialAny.md

@@ -0,0 +1,24 @@
+# `ExistentialAny`
+
+This diagnostic group includes errors and warnings pertaining to the `any` type
+syntax proposed in [SE-0335].
+`any` syntax draws a line between constraint types and existential or boxed
+types.
+
+For example, `any Collection` is a boxed type abstracting over a value of a
+dynamic type that conforms to the protocol `Collection`, whereas the
+`Collection` part is the conformance constraint imposed on the type of the
+underlying value *as well as* a constraint type.
+The distinction between a conformance constraint and a constraint type can be
+clearly seen in classic generic syntax: `<T: Collection>`.
+
+Constraint types exist to express conformances and have no meaning in relation
+to values.
+
+```swift
+func sillyFunction(collection: Collection) { // error
+  // ...
+}
+```
+
+[SE-0335]: https://github.com/swiftlang/swift-evolution/blob/main/proposals/0335-existential-any.md