Sema: Emit diagnostics when walking into collection literals with defaulted types

Resolves https://github.com/apple/swift/issues/60011

Gardening: clean up `checkTypeDefaultedCollectionExpr` function

lib/Sema/MiscDiagnostics.cpp

@@ -109,13 +109,15 @@ static void diagSyntacticUseRestrictions(const Expr *E, const DeclContext *DC,
     SmallPtrSet<DeclRefExpr*, 4> AlreadyDiagnosedBitCasts;
 
     bool IsExprStmt;
+    bool HasReachedSemanticsProvidingExpr;
 
-  public:
     ASTContext &Ctx;
     const DeclContext *DC;
 
+  public:
     DiagnoseWalker(const DeclContext *DC, bool isExprStmt)
-      : IsExprStmt(isExprStmt), Ctx(DC->getASTContext()), DC(DC) {}
+        : IsExprStmt(isExprStmt), HasReachedSemanticsProvidingExpr(false),
+          Ctx(DC->getASTContext()), DC(DC) {}
 
     std::pair<bool, Pattern*> walkToPatternPre(Pattern *P) override {
       return { false, P };
@@ -128,6 +130,17 @@ static void diagSyntacticUseRestrictions(const Expr *E, const DeclContext *DC,
     bool shouldWalkIntoTapExpression() override { return false; }
 
     std::pair<bool, Expr *> walkToExprPre(Expr *E) override {
+
+      if (auto collection = dyn_cast<CollectionExpr>(E)) {
+        if (collection->isTypeDefaulted()) {
+          // Diagnose type defaulted collection literals in subexpressions as
+          // warnings to preserve source compatibility.
+          diagnoseTypeDefaultedCollectionExpr(
+              collection, Ctx,
+              /*downgradeToWarning=*/HasReachedSemanticsProvidingExpr);
+        }
+      }
+
       // See through implicit conversions of the expression.  We want to be able
       // to associate the parent of this expression with the ultimate callee.
       auto Base = E;
@@ -328,6 +341,11 @@ static void diagSyntacticUseRestrictions(const Expr *E, const DeclContext *DC,
         checkMoveExpr(moveExpr);
       }
 
+      if (!HasReachedSemanticsProvidingExpr &&
+          E == E->getSemanticsProvidingExpr()) {
+        HasReachedSemanticsProvidingExpr = true;
+      }
+
       return { true, E };
     }
 
@@ -444,23 +462,34 @@ static void diagSyntacticUseRestrictions(const Expr *E, const DeclContext *DC,
       return false;
     }
 
-    /// We have a collection literal with a defaulted type, e.g. of [Any].  Emit
-    /// an error if it was inferred to this type in an invalid context, which is
-    /// one in which the parent expression is not itself a collection literal.
-    void checkTypeDefaultedCollectionExpr(CollectionExpr *c) {
-      // If the parent is a non-expression, or is not itself a literal, then
-      // produce an error with a fixit to add the type as an explicit
-      // annotation.
-      if (c->getNumElements() == 0)
-        Ctx.Diags.diagnose(c->getLoc(), diag::collection_literal_empty)
-          .highlight(c->getSourceRange());
-      else {
+    /// Diagnose a collection literal with a defaulted type such as \c [Any].
+    static void diagnoseTypeDefaultedCollectionExpr(CollectionExpr *c,
+                                                    ASTContext &ctx,
+                                                    bool downgradeToWarning) {
+      // Produce a diagnostic with a fixit to add the defaulted type as an
+      // explicit annotation.
+      auto &diags = ctx.Diags;
+
+      if (c->getNumElements() == 0) {
+        InFlightDiagnostic inFlight =
+            diags.diagnose(c->getLoc(), diag::collection_literal_empty);
+        inFlight.highlight(c->getSourceRange());
+
+        if (downgradeToWarning) {
+          inFlight.limitBehavior(DiagnosticBehavior::Warning);
+        }
+      } else {
         assert(c->getType()->hasTypeRepr() &&
                "a defaulted type should always be printable");
-        Ctx.Diags.diagnose(c->getLoc(), diag::collection_literal_heterogeneous,
-                           c->getType())
-          .highlight(c->getSourceRange())
-          .fixItInsertAfter(c->getEndLoc(), " as " + c->getType()->getString());
+        InFlightDiagnostic inFlight = diags.diagnose(
+            c->getLoc(), diag::collection_literal_heterogeneous, c->getType());
+        inFlight.highlight(c->getSourceRange());
+        inFlight.fixItInsertAfter(c->getEndLoc(),
+                                  " as " + c->getType()->getString());
+
+        if (downgradeToWarning) {
+          inFlight.limitBehavior(DiagnosticBehavior::Warning);
+        }
       }
     }
 
@@ -1334,16 +1363,6 @@ static void diagSyntacticUseRestrictions(const Expr *E, const DeclContext *DC,
 
   DiagnoseWalker Walker(DC, isExprStmt);
   const_cast<Expr *>(E)->walk(Walker);
-
-  // Diagnose uses of collection literals with defaulted types at the top
-  // level.
-  if (auto collection
-        = dyn_cast<CollectionExpr>(E->getSemanticsProvidingExpr())) {
-    if (collection->isTypeDefaulted()) {
-      Walker.checkTypeDefaultedCollectionExpr(
-        const_cast<CollectionExpr *>(collection));
-    }
-  }
 }
 
 

test/Constraints/array_literal.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift
+// RUN: %target-typecheck-verify-swift -disable-availability-checking
 
 struct IntList : ExpressibleByArrayLiteral {
   typealias Element = Int
@@ -142,19 +142,56 @@ func defaultToAny(i: Int, s: String) {
   // expected-error@-1{{empty collection literal requires an explicit type}}
   let _: Int = a5 // expected-error{{value of type '[Any]'}}
 
+  let _: [Any] = []
   let _: [Any] = [1, "a", 3.5]
   let _: [Any] = [1, "a", [3.5, 3.7, 3.9]]
   let _: [Any] = [1, "a", [3.5, "b", 3]]
+  // expected-warning@-1{{heterogeneous collection literal could only be inferred to '[Any]'; add explicit type annotation if this is intentional}}
   let _: [Any] = [1, [2, [3]]]
+  // expected-warning@-1{{heterogeneous collection literal could only be inferred to '[Any]'; add explicit type annotation if this is intentional}}
+
+  func f1() -> [Any] {
+    []
+  }
   
   let _: [Any?] = [1, "a", nil, 3.5]
   let _: [Any?] = [1, "a", nil, [3.5, 3.7, 3.9]]
   let _: [Any?] = [1, "a", nil, [3.5, "b", nil]]
+  // expected-warning@-1{{heterogeneous collection literal could only be inferred to '[Any?]'; add explicit type annotation if this is intentional}}
   let _: [Any?] = [1, [2, [3]]]
+  // expected-warning@-1{{heterogeneous collection literal could only be inferred to '[Any]'; add explicit type annotation if this is intentional}}
   let _: [Any?] = [1, nil, [2, nil, [3]]]
+  // expected-warning@-1{{heterogeneous collection literal could only be inferred to '[Any?]'; add explicit type annotation if this is intentional}}
 
   let a6 = [B(), C()]
   let _: Int = a6 // expected-error{{value of type '[A]'}}
+  
+  let a7: some Collection = [1, "Swift"]
+  // expected-warning@-1{{heterogeneous collection literal could only be inferred to '[Any]'; add explicit type annotation if this is intentional}} {{41-41= as [Any]}}
+  let _: (any Sequence)? = [1, "Swift"]
+  // expected-warning@-1{{heterogeneous collection literal could only be inferred to '[Any]'; add explicit type annotation if this is intentional}}
+  let _: any Sequence = [1, nil, "Swift"]
+  // expected-warning@-1{{heterogeneous collection literal could only be inferred to '[Any?]'; add explicit type annotation if this is intentional}}
+  let _ = [1, true, ([], 1)]
+  // expected-error@-1 {{heterogeneous collection literal could only be inferred to '[Any]'; add explicit type annotation if this is intentional}}
+  // expected-warning@-2 {{empty collection literal requires an explicit type}}
+  let _ = true ? [] : []
+  // expected-warning@-1{{empty collection literal requires an explicit type}}
+  let _ = (true, ([1, "Swift"]))
+  //expected-warning@-1{{heterogeneous collection literal could only be inferred to '[Any]'; add explicit type annotation if this is intentional}}
+  let _ = ([1, true])
+  //expected-error@-1{{heterogeneous collection literal could only be inferred to '[Any]'; add explicit type annotation if this is intentional}}
+
+  func f2<T>(_: [T]) {}
+
+  func f3<T>() -> [T]? {}
+
+  f2([])
+  // expected-warning@-1{{empty collection literal requires an explicit type}}
+  f2([1, nil, ""])
+  // expected-warning@-1{{heterogeneous collection literal could only be inferred to '[Any?]'; add explicit type annotation if this is intentional}}
+  _ = f3() ?? []
+  // expected-warning@-1{{empty collection literal requires an explicit type}}
 }
 
 func noInferAny(iob: inout B, ioc: inout C) {

test/Constraints/casts.swift

@@ -337,6 +337,7 @@ 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}}
+  // expected-warning@-1{{empty collection literal requires an explicit type}}
 
   // rdar://88334481 – Don't apply the compatibility logic for collection literals.
   typealias Magic<T> = T

test/Constraints/casts_swift6.swift

@@ -57,6 +57,7 @@ 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}}
+  // expected-warning@-1 {{empty collection literal requires an explicit type}}
 
   // Cases from rdar://88334481
   typealias Magic<T> = T

test/Constraints/construction.swift

@@ -172,6 +172,7 @@ SR_5245(s: SR_5245.S(f: [.e1, .e2]))
 
 // rdar://problem/34670592 - Compiler crash on heterogeneous collection literal
 _ = Array([1, "hello"]) // Ok
+// expected-warning@-1 {{heterogeneous collection literal could only be inferred to '[Any]'; add explicit type annotation if this is intentional}}
 
 func init_via_non_const_metatype(_ s1: S1.Type) {
   _ = s1(i: 42) // expected-error {{initializing from a metatype value must reference 'init' explicitly}} {{9-9=.init}}

test/Constraints/dictionary_literal.swift

@@ -139,6 +139,7 @@ func testDefaultExistentials() {
            "b": ["a", 2, 3.14159],
            "c": ["a": 2, "b": 3.5]]
   // expected-error@-3{{heterogeneous collection literal could only be inferred to '[String : Any]'; add explicit type annotation if this is intentional}}
+  // expected-warning@-3{{heterogeneous collection literal could only be inferred to '[Any]'; add explicit type annotation if this is intentional}}
 
   let d3 = ["b" : B(), "c" : C()]
   let _: Int = d3 // expected-error{{value of type '[String : A]'}}
@@ -193,4 +194,4 @@ f59215(["", ""]) //expected-error{{dictionary of type '[String : String]' cannot
 f59215(["", "", "", ""]) //expected-error{{dictionary of type '[String : String]' cannot be used with array literal}}
 // expected-note@-1{{did you mean to use a dictionary literal instead?}} {{11-12=:}} {{19-20=:}}
 f59215(["", "", "", ""]) //expected-error{{dictionary of type '[String : String]' cannot be used with array literal}}
-// expected-note@-1{{did you mean to use a dictionary literal instead?}} {{11-12=:}} {{19-20=:}}
+// expected-note@-1{{did you mean to use a dictionary literal instead?}} {{11-12=:}} {{19-20=:}}

test/Constraints/subscript.swift

@@ -104,6 +104,7 @@ extension Int {
 let _ = 1["1"]  // expected-error {{ambiguous use of 'subscript(_:)'}}
 
 let squares = [ 1, 2, 3 ].reduce([:]) { (dict, n) in
+  // expected-warning@-1 {{empty collection literal requires an explicit type}}
   var dict = dict
   dict[n] = n * n
   return dict

test/IDE/print_usrs_opaque_types.swift

@@ -9,17 +9,20 @@
 func testUnifyingGenericParams<T, U>(x: T) -> some Collection where T == U {
   // expected-warning@-1 {{same-type requirement makes generic parameters 'U' and 'T' equivalent}}
   return []
+  // expected-warning@-1 {{empty collection literal requires an explicit type}}
 }
 
 // CHECK: [[@LINE+1]]:{{[0-9]+}} s:14swift_ide_test0C22UnifyingGenericParams21xQrx_tSlRz7ElementQzRs_r0_lF
 func testUnifyingGenericParams2<T, U>(x: T) -> some Collection where T: Collection, U == T.Element {
   return []
+  // expected-warning@-1 {{empty collection literal requires an explicit type}}
 }
 
 // CHECK: [[@LINE+1]]:{{[0-9]+}} s:14swift_ide_test0C24ConcretizingGenericParam1xQrSi_tSiRszlF
 func testConcretizingGenericParam<T>(x: T) -> some Collection where T == Int {
   // expected-warning@-1 {{same-type requirement makes generic parameter 'T' non-generic}}
   return []
+  // expected-warning@-1 {{empty collection literal requires an explicit type}}
 }
 
 struct GenericContext<T> {
@@ -27,11 +30,13 @@ struct GenericContext<T> {
   func testUnifyingGenericParams<U>(x: T) -> some Collection where T == U {
     // expected-warning@-1 {{same-type requirement makes generic parameters 'U' and 'T' equivalent}}
     return []
+    // expected-warning@-1 {{empty collection literal requires an explicit type}}
   }
 
   // CHECK: [[@LINE+1]]:{{[0-9]+}} s:14swift_ide_test14GenericContextV0c8UnifyingD7Params21xQrx_tSlRz7ElementQzRsd__lF
   func testUnifyingGenericParams2<U>(x: T) -> some Collection where T: Collection, U == T.Element {
     return []
+    // expected-warning@-1 {{empty collection literal requires an explicit type}}
   }
 
   // CHECK: [[@LINE+1]]:{{[0-9]+}} s:14swift_ide_test14GenericContextVyQrxcqd__Rszluip
@@ -40,6 +45,7 @@ struct GenericContext<T> {
     // CHECK: [[@LINE+1]]:{{[0-9]+}} s:14swift_ide_test14GenericContextVyQrxcqd__Rszluig
     get {
       return []
+      // expected-warning@-1 {{empty collection literal requires an explicit type}}
     }
   }
 }
@@ -48,6 +54,7 @@ extension GenericContext where T == Int {
   // CHECK: [[@LINE+1]]:{{[0-9]+}} s:14swift_ide_test14GenericContextVAASiRszlE0c12ConcretizingD5Param1xQrSi_tF
   func testConcretizingGenericParam(x: T) -> some Collection {
     return []
+    // expected-warning@-1 {{empty collection literal requires an explicit type}}
   }
 }
 
@@ -58,6 +65,7 @@ extension TooGenericTooContext where T == U {
   // CHECK: [[@LINE+1]]:{{[0-9]+}} s:14swift_ide_test010TooGenericD7ContextVAAq_RszrlE0c8UnifyingE6Params1xQrx_tF
   func testUnifyingGenericParams(x: T) -> some Collection {
     return []
+    // expected-warning@-1 {{empty collection literal requires an explicit type}}
   }
 }
 
@@ -65,12 +73,14 @@ extension TooGenericTooContext where T: Collection, U == T.Element {
   // CHECK: [[@LINE+1]]:{{[0-9]+}} s:14swift_ide_test010TooGenericD7ContextVAASlRz7ElementQzRs_rlE0c8UnifyingE7Params21xQrx_tF
   func testUnifyingGenericParams2(x: T) -> some Collection {
     return []
+    // expected-warning@-1 {{empty collection literal requires an explicit type}}
   }
 }
 extension TooGenericTooContext where T == Int {
   // CHECK: [[@LINE+1]]:{{[0-9]+}} s:14swift_ide_test010TooGenericD7ContextVAASiRszrlE0c12ConcretizingE5Param1xQrSi_tF
   func testConcretizingGenericParam(x: T) -> some Collection {
     return []
+    // expected-warning@-1 {{empty collection literal requires an explicit type}}
   }
 }
 

test/expr/cast/objc_coerce_array.swift

@@ -7,9 +7,13 @@ var x = 1
 _ = [x] as [NSNumber]
 
 _ = ["x":["y":"z","a":1]] as [String : [String : AnyObject]]
+// expected-warning@-1{{heterogeneous collection literal could only be inferred to '[String : AnyObject]'; add explicit type annotation if this is intentiona}}
 _ = ["x":["z",1]] as [String : [AnyObject]]
+// expected-warning@-1{{heterogeneous collection literal could only be inferred to '[AnyObject]'; add explicit type annotation if this is intentional}}
 _ = [["y":"z","a":1]] as [[String : AnyObject]]
+// expected-warning@-1{{heterogeneous collection literal could only be inferred to '[String : AnyObject]'; add explicit type annotation if this is intentional}}
 _ = [["z",1]] as [[AnyObject]]
+// expected-warning@-1{{heterogeneous collection literal could only be inferred to '[AnyObject]'; add explicit type annotation if this is intentional}}
 
 var y: Any = 1