Sema: Fix conditional downcasts from Swift types to CF types

Conditional and forced downcasts enter a constraint that almost
always succeeds; only when applying the solution do we evaluate
the feasability of the cast and determine if it always succeeds,
always fails, or conditionally succeeds. This changes how the
resulting AST is represented and can also emit diagnostics.

If the conditional cast is at this stage determined to always
succeed, we treat it as an unconditional cast, going through
ExprRewriter::coerceToType() to build the AST for the coercion.

However conditional cast constraints don't enter the same
restrictions into the solution as unconditional casts do, so
coerceToType() would fall over if casting a Swift type to a CF
type by first bridging the Swift type to Objective-C.

Get around this by checking for this case explicitly when
lowering a CoerceExpr.

It feels like there's a more fundamental issue here with how
casts are modeled in the constraint solver, but I'm not going
to try understanding that now.

Fixes <rdar://problem/32227571>.

Author: slavapestov

lib/Sema/CSApply.cpp

@@ -5816,6 +5816,8 @@ Expr *ExprRewriter::buildCollectionUpcastExpr(Expr *expr, Type toType,
 
 Expr *ExprRewriter::buildObjCBridgeExpr(Expr *expr, Type toType,
                                         ConstraintLocatorBuilder locator) {
+  auto &tc = cs.getTypeChecker();
+
   Type fromType = cs.getType(expr);
 
   // Bridged collection casts always succeed, so we treat them as
@@ -5838,6 +5840,20 @@ Expr *ExprRewriter::buildObjCBridgeExpr(Expr *expr, Type toType,
     if (!objcExpr)
       return nullptr;
 
+    // We might have a coercion of a Swift type to a CF type toll-free
+    // bridged to Objective-C.
+    //
+    // FIXME: Ideally we would instead have already recorded a restriction
+    // when solving the constraint, and we wouldn't need to duplicate this
+    // part of coerceToType() here.
+    if (auto foreignClass = toType->getClassOrBoundGenericClass()) {
+      if (foreignClass->getForeignClassKind() ==
+            ClassDecl::ForeignKind::CFType) {
+        return cs.cacheType(
+          new (tc.Context) ForeignObjectConversionExpr(objcExpr, toType));
+      }
+    }
+
     return coerceToType(objcExpr, toType, locator);
   }
 

test/Inputs/clang-importer-sdk/usr/include/CoreFoundation.h

@@ -14,6 +14,10 @@ typedef struct __attribute__((objc_bridge(NSString))) __CFString const *CFString
 typedef struct __CFTree *CFTreeRef;
 typedef const struct __attribute__((objc_bridge(CFURL))) __CFURL * CFURLRef;
 
+typedef struct __attribute__((objc_bridge(NSDictionary))) __CFDictionary const *CFDictionaryRef;
+typedef struct __attribute__((objc_bridge(NSArray))) __CFArray const *CFArrayRef;
+typedef struct __attribute__((objc_bridge(NSSet))) __CFSet const *CFSetRef;
+
 typedef CFTypeRef CFAliasForTypeRef;
 
 

test/expr/cast/cf.swift

@@ -80,3 +80,25 @@ func testCFConvWithIUO(_ x: CFString!, y: NSString!) {
   acceptNSString(x)
   acceptCFString(y)
 }
+
+func testBridgedCFDowncast(array: [Any], dictionary: [AnyHashable : Any], set: Set<AnyHashable>) {
+  let cfArray = array as CFArray
+  let cfDictionary = dictionary as CFDictionary
+  let cfSet = set as CFSet
+
+  _ = array as? CFArray // expected-warning {{conditional cast from '[Any]' to 'CFArray' always succeeds}}
+  _ = dictionary as? CFDictionary // expected-warning {{conditional cast from '[AnyHashable : Any]' to 'CFDictionary' always succeeds}}
+  _ = set as? CFSet // expected-warning {{conditional cast from 'Set<AnyHashable>' to 'CFSet' always succeeds}}
+
+  _ = array as! CFArray // expected-warning {{forced cast from '[Any]' to 'CFArray' always succeeds}}
+  _ = dictionary as! CFDictionary // expected-warning {{forced cast from '[AnyHashable : Any]' to 'CFDictionary' always succeeds}}
+  _ = set as! CFSet // expected-warning {{forced cast from 'Set<AnyHashable>' to 'CFSet' always succeeds}}
+
+  _ = cfArray as! [Any]
+  _ = cfDictionary as! [AnyHashable : Any]
+  _ = cfSet as! Set<AnyHashable>
+
+  _ = cfArray as? [Any]
+  _ = cfDictionary as? [AnyHashable : Any]
+  _ = cfSet as? Set<AnyHashable>
+}