[CS] Don't apply compatibility logic to collection literals

We currently permit (but warn on) coercions of
collections with unrelated element types in
certain cases. This is done in an effort to preserve
compatibility with pre-5.3 compilers that may have
allowed such code to compile due to dropping
constraints while solving.

This is limited to collection coercions as we
emit somewhat reasonable code for them that
performs a force cast of the elements at runtime.
However, it turns out that in the case of collection
literals, we peephole the element conversions in
CSApply, leading to codegen crashes. As such,
narrow down the condition of this compatibility
logic to not apply to collection literals, as this
never would have compiled properly.

rdar://88334481

Author: hamishknight

lib/Sema/CSSimplify.cpp

@@ -9557,14 +9557,33 @@ ConstraintSystem::simplifyBridgingConstraint(Type type1,
   // two collections, but add a warning fix telling the user to use as! or as?
   // instead.
   //
-  // We only need to perform this compatibility logic if the LHS type is a
-  // (potentially optional) type variable, as only such a constraint could have
-  // been previously been left unsolved.
+  // 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 =
-      rawType1->lookThroughAllOptionalTypes()->isTypeVariableOrMember();
+  auto canUseCompatFix = [&]() {
+    if (!rawType1->lookThroughAllOptionalTypes()->isTypeVariableOrMember())
+      return false;
+
+    SmallVector<LocatorPathElt, 4> elts;
+    auto anchor = locator.getLocatorParts(elts);
+    if (!elts.empty())
+      return false;
+
+    auto *coercion = getAsExpr<CoerceExpr>(anchor);
+    if (!coercion)
+      return false;
+
+    auto *subject = coercion->getSubExpr();
+    while (auto *paren = dyn_cast<ParenExpr>(subject))
+      subject = paren->getSubExpr();
+
+    return !isa<CollectionExpr>(subject);
+  }();
 
   // Unless we're allowing the collection compatibility fix, the source cannot
   // be more optional than the destination.

test/Constraints/casts.swift

@@ -280,7 +280,7 @@ func test_coercions_with_overloaded_operator(str: String, optStr: String?, veryO
 
 func id<T>(_ x: T) -> T { x }
 
-func test_compatibility_coercions(_ arr: [Int], _ optArr: [Int]?, _ dict: [String: Int], _ set: Set<Int>) {
+func test_compatibility_coercions(_ arr: [Int], _ optArr: [Int]?, _ dict: [String: Int], _ set: Set<Int>, _ i: Int, _ stringAnyDict: [String: Any]) {
   // Successful coercions don't raise a warning.
   _ = arr as [Any]?
   _ = dict as [String: Int]?
@@ -337,6 +337,24 @@ func test_compatibility_coercions(_ arr: [Int], _ optArr: [Int]?, _ dict: [Strin
 
   // 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}}
+
+  // rdar://88334481 – Don't apply the compatibility logic for collection literals.
+  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'}}
+
+  // These are currently not peepholed.
+  _ = [i].self as Magic as [String] // expected-warning {{coercion from '[Int]' to '[String]' may fail; use 'as?' or 'as!' instead}}
+  _ = (try [i]) as Magic as [String] // expected-warning {{coercion from '[Int]' to '[String]' may fail; use 'as?' or 'as!' instead}}
+  // 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]
 }
 
 // SR-13088

test/SILGen/collection_downcast.swift

@@ -242,8 +242,8 @@ func testSetDowncastBridgedConditional(_ dict: Set<NSObject>)
 
 func promote<T>(_ x: T) -> T? { nil }
 
-// CHECK-LABEL: sil hidden [ossa] @$s19collection_downcast36testCollectionCompatibilityCoercionsyySaySiG_SayypGSgShySSGSDySSSiGtF
-func testCollectionCompatibilityCoercions(_ arr: [Int], _ optArr: [Any]?, _ set: Set<String>, _ dict: [String: Int]) {
+// CHECK-LABEL: sil hidden [ossa] @$s19collection_downcast36testCollectionCompatibilityCoercionsyySaySiG_SayypGSgShySSGSDySSSiGSitF
+func testCollectionCompatibilityCoercions(_ arr: [Int], _ optArr: [Any]?, _ set: Set<String>, _ dict: [String: Int], _ i: Int) {
   // Make sure we generate reasonable code for all of the below examples.
 
   // CHECK: [[CAST_FN:%.+]] = function_ref @$ss15_arrayForceCastySayq_GSayxGr0_lF : $@convention(thin) <τ_0_0, τ_0_1> (@guaranteed Array<τ_0_0>) -> @owned Array<τ_0_1>
@@ -297,4 +297,15 @@ func testCollectionCompatibilityCoercions(_ arr: [Int], _ optArr: [Any]?, _ set:
   // CHECK: [[CAST_FN:%.+]] = function_ref @$ss17_dictionaryUpCastySDyq0_q1_GSDyxq_GSHRzSHR0_r2_lF : $@convention(thin) <τ_0_0, τ_0_1, τ_0_2, τ_0_3 where τ_0_0 : Hashable, τ_0_2 : Hashable> (@guaranteed Dictionary<τ_0_0, τ_0_1>) -> @owned Dictionary<τ_0_2, τ_0_3>
   // CHECK: apply [[CAST_FN]]<String, Int, Int, String>([[DICT]]) : $@convention(thin) <τ_0_0, τ_0_1, τ_0_2, τ_0_3 where τ_0_0 : Hashable, τ_0_2 : Hashable> (@guaranteed Dictionary<τ_0_0, τ_0_1>) -> @owned Dictionary<τ_0_2, τ_0_3>
   _ = promote(promote(promote(dict))) as [Int: String]
+
+  typealias Magic<T> = T
+
+  // These are currently not peepholed.
+  // CHECK: [[CAST_FN:%.+]] = function_ref @$ss15_arrayForceCastySayq_GSayxGr0_lF : $@convention(thin) <τ_0_0, τ_0_1> (@guaranteed Array<τ_0_0>) -> @owned Array<τ_0_1>
+  // CHECK: apply [[CAST_FN]]<Int, String>
+  [i].self as Magic as [String]
+
+  // CHECK: [[CAST_FN:%.+]] = function_ref @$ss15_arrayForceCastySayq_GSayxGr0_lF : $@convention(thin) <τ_0_0, τ_0_1> (@guaranteed Array<τ_0_0>) -> @owned Array<τ_0_1>
+  // CHECK: apply [[CAST_FN]]<Int, String>
+  (try [i]) as Magic as [String]
 }