[Import as Member] Error on convenience inits in extensions of CFTypes

Swift does not currently support user-defined factory inits. With
import as member, we're seeing many C functions now imported as
initializers, which gives users the false hope that they can define
their own factory inits as conveinence inits in extensions of CF
types. We issue an explicit error now, rather than crashing later.

Author: milseman

include/swift/AST/DiagnosticsSema.def

@@ -2034,6 +2034,9 @@ ERROR(enumstruct_convenience_init,none,
 ERROR(nonclass_convenience_init,none,
       "convenience initializer not allowed in non-class type %0",
       (Type))
+ERROR(cfclass_convenience_init,none,
+      "convenience initializers are not supported in extensions of CF types",
+      ())
 
 ERROR(dynamic_construct_class,none,
       "constructing an object of class type %0 with a metatype value must use "

lib/Sema/TypeCheckDecl.cpp

@@ -6337,8 +6337,16 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
     // extensions thereof.
     if (CD->isConvenienceInit()) {
       if (auto extType = CD->getExtensionType()) {
-        if (!extType->getClassOrBoundGenericClass() &&
-            !extType->is<ErrorType>()) {
+        auto extClass = extType->getClassOrBoundGenericClass();
+
+        // Forbid convenience inits on Foreign CF types, as Swift does not yet
+        // support user-defined factory inits.
+        if (extClass &&
+            extClass->getForeignClassKind() == ClassDecl::ForeignKind::CFType) {
+          TC.diagnose(CD->getLoc(), diag::cfclass_convenience_init);
+        }
+
+        if (!extClass && !extType->is<ErrorType>()) {
           auto ConvenienceLoc =
             CD->getAttrs().getAttribute<ConvenienceAttr>()->getLocation();
 

test/decl/init/cf-types.swift

@@ -0,0 +1,23 @@
+// RUN: %target-parse-verify-swift
+// REQUIRES: OS=macosx
+
+import CoreGraphics
+
+extension CGMutablePath {
+  public convenience init(p: Bool) { // expected-error{{convenience initializers are not supported in extensions of CF types}}
+    self.init()
+  }
+  public convenience init?(maybe: Bool) { // expected-error{{convenience initializers are not supported in extensions of CF types}}
+    self.init()
+  }
+
+  public convenience init(toss: Bool) throws { // expected-error{{convenience initializers are not supported in extensions of CF types}}
+    self.init()
+  }
+}
+
+public func useInit() {
+  let _ = CGMutablePath(p: true)
+  let _ = CGMutablePath(maybe: true)
+  let _ = try! CGMutablePath(toss: true)
+}