[Typechecker] Warn when casting a function type to an existential or archetype type

include/swift/AST/DiagnosticsSema.def

@@ -901,6 +901,8 @@ WARNING(is_expr_same_type,none,
         "'!= nil'?", (Type, Type))
 WARNING(downcast_to_unrelated,none,
         "cast from %0 to unrelated type %1 always fails", (Type, Type))
+NOTE(downcast_to_unrelated_fixit,none,
+     "did you mean to call %0 with '()'?", (Identifier))
 ERROR(downcast_to_more_optional,none,
       "cannot downcast from %0 to a more optional type %1",
       (Type, Type))

lib/Sema/TypeCheckConstraints.cpp

@@ -3896,6 +3896,53 @@ CheckedCastKind TypeChecker::typeCheckCheckedCast(Type fromType,
     }
   }
 
+  assert(!toType->isAny() && "casts to 'Any' should've been handled above");
+  assert(!toType->isAnyObject() &&
+         "casts to 'AnyObject' should've been handled above");
+
+  // A cast from a function type to an existential type (except `Any`)
+  // or an archetype type (with constraints) cannot succeed
+  auto toArchetypeType = toType->is<ArchetypeType>();
+  auto fromFunctionType = fromType->is<FunctionType>();
+  auto toExistentialType = toType->isAnyExistentialType();
+
+  auto toConstrainedArchetype = false;
+  if (toArchetypeType) {
+    auto archetype = toType->castTo<ArchetypeType>();
+    toConstrainedArchetype = archetype && !archetype->getConformsTo().empty();
+  }
+
+  if (fromFunctionType &&
+      (toExistentialType || (toArchetypeType && toConstrainedArchetype))) {
+    switch (contextKind) {
+    case CheckedCastContextKind::ConditionalCast:
+    case CheckedCastContextKind::ForcedCast:
+      diagnose(diagLoc, diag::downcast_to_unrelated, origFromType, origToType)
+          .highlight(diagFromRange)
+          .highlight(diagToRange);
+
+      // If we're referring to a function with a return value (not Void) then
+      // emit a fix-it suggesting to add `()` to call the function
+      if (auto DRE = dyn_cast<DeclRefExpr>(fromExpr)) {
+        if (auto FD = dyn_cast<FuncDecl>(DRE->getDecl())) {
+          if (!FD->getResultInterfaceType()->isVoid()) {
+            diagnose(diagLoc, diag::downcast_to_unrelated_fixit, FD->getName())
+                .fixItInsertAfter(fromExpr->getEndLoc(), "()");
+          }
+        }
+      }
+
+      return CheckedCastKind::ValueCast;
+      break;
+
+    case CheckedCastContextKind::IsPattern:
+    case CheckedCastContextKind::EnumElementPattern:
+    case CheckedCastContextKind::IsExpr:
+    case CheckedCastContextKind::None:
+      break;
+    }
+  }
+
   // If we can bridge through an Objective-C class, do so.
   if (Type bridgedToClass = getDynamicBridgedThroughObjCClass(dc, fromType,
                                                               toType)) {

test/expr/cast/as_coerce.swift

@@ -103,3 +103,32 @@ f(1 as String) // expected-error{{cannot convert value of type 'Int' to type 'St
 // <rdar://problem/19650402> Swift compiler segfaults while running the annotation tests
 let s : AnyObject = C3()
 s as C3 // expected-error{{'AnyObject' is not convertible to 'C3'; did you mean to use 'as!' to force downcast?}} {{3-5=as!}}
+
+// SR-6022
+func sr6022() -> Any { return 0 }
+func sr6022_1() { return; }
+protocol SR6022_P {}
+
+_ = sr6022 as! SR6022_P // expected-warning {{cast from '() -> Any' to unrelated type 'SR6022_P' always fails}} // expected-note {{did you mean to call 'sr6022' with '()'?}}{{11-11=()}}
+_ = sr6022 as? SR6022_P // expected-warning {{cast from '() -> Any' to unrelated type 'SR6022_P' always fails}} // expected-note {{did you mean to call 'sr6022' with '()'}}{{11-11=()}}
+_ = sr6022_1 as! SR6022_P // expected-warning {{cast from '() -> ()' to unrelated type 'SR6022_P' always fails}}
+_ = sr6022_1 as? SR6022_P // expected-warning {{cast from '() -> ()' to unrelated type 'SR6022_P' always fails}}
+
+func testSR6022_P<T: SR6022_P>(_: T.Type) {
+  _ = sr6022 as! T // expected-warning {{cast from '() -> Any' to unrelated type 'T' always fails}} // expected-note {{did you mean to call 'sr6022' with '()'?}}{{13-13=()}}
+  _ = sr6022 as? T // expected-warning {{cast from '() -> Any' to unrelated type 'T' always fails}} // expected-note {{did you mean to call 'sr6022' with '()'?}}{{13-13=()}}
+  _ = sr6022_1 as! T // expected-warning {{cast from '() -> ()' to unrelated type 'T' always fails}}
+  _ = sr6022_1 as? T // expected-warning {{cast from '() -> ()' to unrelated type 'T' always fails}}
+}
+
+func testSR6022_P_1<U>(_: U.Type) {
+  _ = sr6022 as! U // Okay
+  _ = sr6022 as? U // Okay
+  _ = sr6022_1 as! U // Okay
+  _ = sr6022_1 as? U // Okay
+}
+
+_ = sr6022 as! AnyObject // expected-warning {{forced cast from '() -> Any' to 'AnyObject' always succeeds; did you mean to use 'as'?}}
+_ = sr6022 as? AnyObject // expected-warning {{conditional cast from '() -> Any' to 'AnyObject' always succeeds}}
+_ = sr6022_1 as! Any // expected-warning {{forced cast from '() -> ()' to 'Any' always succeeds; did you mean to use 'as'?}}
+_ = sr6022_1 as? Any // expected-warning {{conditional cast from '() -> ()' to 'Any' always succeeds}}