Don't add capture arguments to default argument generators.

If we ever add the ability for default arguments to refer to
enclosing variables, we should promote DefaultArgumentInitializer
to be an always-present entity, give it an independent capture
list, and make SILDeclRef refer to it directly.  In fact, we may
want to do that anyway.  Until then, it's a weird special case
in terms of not really being a reference to the function returned
by getAnyFunctionRef().

rdar://24242783

Author: rjmccall

lib/SIL/SILFunctionType.cpp

@@ -572,7 +572,10 @@ static CanSILFunctionType getSILFunctionType(SILModule &M,
   }
 
   // Lower the capture context parameters, if any.
-  if (constant)
+  // But note that default arg generators can't capture anything right now,
+  // and if we ever add that ability, it will be a different capture list
+  // from the function to which the argument is attached.
+  if (constant && !constant->isDefaultArgGenerator())
   if (auto function = constant->getAnyFunctionRef()) {
     auto &Types = M.Types;
     auto loweredCaptures = Types.getLoweredLocalCaptures(*function);

test/SILGen/default_arguments.swift

@@ -253,3 +253,14 @@ func test_r18400194() {
   (r18400194)(1)
 }
 
+// rdar://24242783
+//   Don't add capture arguments to local default argument generators.
+func localFunctionWithDefaultArg() {
+  var z = 5
+  func bar(x: Int? = nil) {
+    z += 1
+  }
+  bar()
+}
+// CHECK-LABEL: sil shared @_TIFF17default_arguments27localFunctionWithDefaultArgFT_T_L_3barFTGSqSi__T_A_
+// CHECK-SAME: $@convention(thin) () -> Optional<Int>