[CS] Don't apply compatibility logic in Swift 6 mode

This was only intended to preserve compatibility
for cases where the constraint system may have
previously accepted invalid code. Drop it in Swift 6
mode such that invalid collection coercions become
errors.

Author: hamishknight

lib/Sema/CSSimplify.cpp

@@ -9555,17 +9555,17 @@ ConstraintSystem::simplifyBridgingConstraint(Type type1,
   // we generated somewhat reasonable code that performed a force cast. To
   // maintain compatibility with that behavior, allow the coercion between
   // two collections, but add a warning fix telling the user to use as! or as?
-  // instead.
+  // instead. In Swift 6 mode, this becomes an error.
   //
   // We only need to perform this compatibility logic if this is a coercion of
   // something that isn't a collection expr (as collection exprs would have
   // crashed in codegen due to CSApply peepholing them). Additionally, the LHS
   // type must be a (potentially optional) type variable, as only such a
   // constraint could have been previously been left unsolved.
-  //
-  // FIXME: Once we get a new language version, change this condition to only
-  // preserve compatibility for Swift 5.x mode.
   auto canUseCompatFix = [&]() {
+    if (Context.isSwiftVersionAtLeast(6))
+      return false;
+
     if (!rawType1->lookThroughAllOptionalTypes()->isTypeVariableOrMember())
       return false;
 

test/Constraints/casts_swift6.swift

@@ -0,0 +1,78 @@
+// RUN: %target-typecheck-verify-swift -enable-objc-interop -swift-version 6
+
+// -swift-version 6 is currently asserts-only
+// REQUIRES: asserts
+
+func id<T>(_ x: T) -> T { x }
+func ohno<T>(_ x: T) -> T? { nil }
+
+// Swift 6 version of the test in casts.swift
+func test_compatibility_coercions(_ arr: [Int], _ optArr: [Int]?, _ dict: [String: Int], _ set: Set<Int>, _ i: Int, _ stringAnyDict: [String: Any]) {
+  // These have always been fine.
+  _ = arr as [Any]?
+  _ = dict as [String: Int]?
+  _ = set as Set<Int>
+
+  // These have always been errors.
+  _ = arr as [String] // expected-error {{cannot convert value of type '[Int]' to type '[String]' in coercion}}
+  // expected-note@-1 {{arguments to generic parameter 'Element' ('Int' and 'String') are expected to be equal}}
+  _ = dict as [String: String] // expected-error {{cannot convert value of type '[String : Int]' to type '[String : String]' in coercion}}
+  // expected-note@-1 {{arguments to generic parameter 'Value' ('Int' and 'String') are expected to be equal}}
+  _ = dict as [String: String]? // expected-error {{'[String : Int]' is not convertible to '[String : String]?'}}
+  // expected-note@-1 {{did you mean to use 'as!' to force downcast?}} {{12-14=as!}}
+  _ = (dict as [String: Int]?) as [String: Int] // expected-error {{value of optional type '[String : Int]?' must be unwrapped to a value of type '[String : Int]'}}
+  // 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'}}
+  _ = set as Set<String>  // expected-error {{cannot convert value of type 'Set<Int>' to type 'Set<String>' in coercion}}
+   // expected-note@-1 {{arguments to generic parameter 'Element' ('Int' and 'String') are expected to be equal}}
+
+  // Make sure we error on the following in Swift 6 mode.
+
+  // FIXME: Bad diagnostics (SR-15843)
+  _ = id(arr) as [String] // expected-error {{type of expression is ambiguous without more context}}
+  _ = (arr ?? []) as [String] // expected-error {{type of expression is ambiguous without more context}}
+  _ = (arr ?? [] ?? []) as [String] // expected-error {{type of expression is ambiguous without more context}}
+  _ = (optArr ?? []) as [String] // expected-error {{type of expression is ambiguous without more context}}
+
+  _ = (arr ?? []) as [String]? // expected-error {{'[Int]' is not convertible to '[String]?'}}
+  // expected-note@-1 {{did you mean to use 'as!' to force downcast?}}
+  _ = (arr ?? []) as [String?]? // expected-error {{'[Int]' is not convertible to '[String?]?'}}
+  // expected-note@-1 {{did you mean to use 'as!' to force downcast?}}
+  _ = (arr ?? []) as [String??]?? // expected-error {{'[Int]' is not convertible to '[String??]??'}}
+  // expected-note@-1 {{did you mean to use 'as!' to force downcast?}}
+  _ = (dict ?? [:]) as [String: String?]? // expected-error {{'[String : Int]' is not convertible to '[String : String?]?'}}
+  // expected-note@-1 {{did you mean to use 'as!' to force downcast?}}
+  _ = (set ?? []) as Set<String>?? // expected-error {{'Set<Int>' is not convertible to 'Set<String>??'}}
+  // expected-note@-1 {{did you mean to use 'as!' to force downcast?}}
+
+  _ = ohno(ohno(ohno(arr))) as [String] // expected-error {{cannot convert value of type '[Int]???' to type '[String]' in coercion}}
+  _ = ohno(ohno(ohno(arr))) as [Int] // expected-error {{cannot convert value of type '[Int]???' to type '[Int]' in coercion}}
+  _ = ohno(ohno(ohno(Set<Int>()))) as Set<String> // expected-error {{cannot convert value of type 'Set<Int>???' to type 'Set<String>' in coercion}}
+  _ = ohno(ohno(ohno(["": ""]))) as [Int: String] // expected-error {{cannot convert value of type '[String : String]???' to type '[Int : String]' in coercion}}
+  _ = ohno(ohno(ohno(dict))) as [String: Int] // expected-error {{cannot convert value of type '[String : Int]???' to type '[String : Int]' in coercion}}
+
+  // In this case the array literal can be inferred to be [String], so totally
+  // valid.
+  _ = ([] ?? []) as [String] // expected-warning {{left side of nil coalescing operator '??' has non-optional type '[String]', so the right side is never used}}
+  _ = (([] as Optional) ?? []) as [String]
+
+  // The array can also be inferred to be [Any].
+  _ = ([] ?? []) as Array // expected-warning {{left side of nil coalescing operator '??' has non-optional type '[Any]', so the right side is never used}}
+
+  // Cases from rdar://88334481
+  typealias Magic<T> = T
+  _ = [i] as [String] // expected-error {{cannot convert value of type 'Int' to expected element type 'String'}}
+  _ = [i] as Magic as [String] // expected-error {{cannot convert value of type 'Int' to expected element type 'String'}}
+  _ = ([i]) as Magic as [String] // expected-error {{cannot convert value of type 'Int' to expected element type 'String'}}
+  _  = [i: i] as [String: Any] // expected-error {{cannot convert value of type 'Int' to expected dictionary key type 'String'}}
+  _  = ([i: i]) as [String: Any] // expected-error {{cannot convert value of type 'Int' to expected dictionary key type 'String'}}
+  _  = [i: stringAnyDict] as [String: Any] // expected-error {{cannot convert value of type 'Int' to expected dictionary key type 'String'}}
+
+  _ = [i].self as Magic as [String] // expected-error {{cannot convert value of type 'Int' to expected element type 'String'}}
+  _ = (try [i]) as Magic as [String] // expected-error {{cannot convert value of type 'Int' to expected element type 'String'}}
+  // expected-warning@-1 {{no calls to throwing functions occur within 'try' expression}}
+
+  // These are wrong, but make sure we don't warn about the value cast always succeeding.
+  _  = [i: i] as! [String: Any]
+  _  = [i: stringAnyDict] as! [String: Any]
+}