Sema: Don't allow properties with inferred types to infer opaque result types.

This prevents opaque result types from propagating nontrivially into other declarations' types,
which may be confusing and create implementation complexities.

Author: jckarter

include/swift/AST/DiagnosticsSema.def

@@ -1501,6 +1501,9 @@ WARNING(function_type_usable_from_inline_warn,none,
 ERROR(opaque_type_invalid_constraint,none,
       "an 'opaque' type must specify only 'Any', 'AnyObject', protocols, "
       "and/or a base class", ())
+ERROR(inferred_opaque_type,none,
+      "property definition has inferred type %0, involving the '__opaque' "
+      "return type of another declaration", (Type))
 
 // Extensions
 ERROR(non_nominal_extension,none,

lib/Sema/TypeCheckDecl.cpp

@@ -979,6 +979,15 @@ static void validatePatternBindingEntry(TypeChecker &tc,
   if (!pattern->hasType() || pattern->getType()->hasUnboundGenericType()) {
     if (tc.typeCheckPatternBinding(binding, entryNumber))
       return;
+    
+    // A pattern binding at top level is not allowed to pick up another decl's
+    // opaque result type as its type by type inference.
+    if (!binding->getDeclContext()->isLocalContext()
+        && binding->getInit(entryNumber)->getType()->hasOpaqueArchetype()) {
+      // TODO: Check whether the type is the pattern binding's own opaque type.
+      tc.diagnose(binding, diag::inferred_opaque_type,
+                  binding->getInit(entryNumber)->getType());
+    }
   }
 
   // If the pattern binding appears in a type or library file context, then

test/type/opaque.swift

@@ -56,6 +56,14 @@ func zlop() -> __opaque C & AnyObject & P {
   return D()
 }
 
+// Don't allow opaque types to propagate by inference into other global decls'
+// types
+struct Test {
+  let inferredOpaque = bar() // expected-error{{inferred type}}
+  let inferredOpaqueStructural = Optional(bar()) // expected-error{{inferred type}}
+  let inferredOpaqueStructural2 = (bar(), bas()) // expected-error{{inferred type}}
+}
+
 //let zingle = {() -> __opaque P in 1 } // FIXME ex/pected-error{{'opaque' types are only implemented}}
 
 // Invalid positions