Merge pull request #6767 from rudkx/fix-rdar29907555

Disallow conversions from IUOs to extistentials.

Author: DougGregor

lib/Sema/CSSimplify.cpp

@@ -2009,9 +2009,22 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
   // Instance type check for the above. We are going to check conformance once
   // we hit commit_to_conversions below, but we have to add a token restriction
   // to ensure we wrap the metatype value in a metatype erasure.
-  if (concrete && type2->isExistentialType() &&
-      kind >= ConstraintKind::Subtype) {
-    conversionsOrFixes.push_back(ConversionRestrictionKind::Existential);
+
+  // Disallow direct IUO-to-Any conversions. This will allow us to
+  // force-unwrap the IUO before attempting to convert, which makes it
+  // possible to disambiguate certain cases where we would otherwise
+  // consider an IUO or plain optional to be equally desirable choices
+  // where we really want the IUO to decay to a plain optional.
+  {
+    bool disallowExistentialConversion = false;
+    if (type2->isAny() &&
+        type1->getRValueType()->getImplicitlyUnwrappedOptionalObjectType())
+      disallowExistentialConversion = true;
+
+    if (concrete && !disallowExistentialConversion &&
+        type2->isExistentialType() && kind >= ConstraintKind::Subtype) {
+      conversionsOrFixes.push_back(ConversionRestrictionKind::Existential);
+    }
   }
 
   // A value of type T! can be converted to type U if T is convertible

test/Constraints/diag_ambiguities.swift

@@ -35,3 +35,9 @@ func rdar29691909_callee(_ o: AnyObject) -> Any { return o } // expected-note {{
 func rdar29691909(o: AnyObject) -> Any? {
   return rdar29691909_callee(o) // expected-error{{ambiguous use of 'rdar29691909_callee'}}
 }
+
+// Ensure that we decay Any! to Any? rather than allowing Any!-to-Any
+// conversion directly and ending up with an ambiguity here.
+func rdar29907555(_ value: Any!) -> String {
+  return "\(value)" // no error
+}