[AddressLowering] ParamDecl's context falls back.

When address lowering rewrites a return to use an indirect convention,
it creates a ParamDecl, using the function's decl context.  Some
functions, such as default argument getters, don't have such contexts,
though.  In such cases, fall back to module as decl context.

Author: nate-chandler

lib/SILOptimizer/Mandatory/AddressLowering.cpp

@@ -553,6 +553,12 @@ static unsigned insertIndirectReturnArgs(AddressLoweringState &pass) {
   auto &astCtx = pass.getModule()->getASTContext();
   auto typeCtx = pass.function->getTypeExpansionContext();
   auto *declCtx = pass.function->getDeclContext();
+  if (!declCtx) {
+    // Fall back to using the module as the decl context if the function
+    // doesn't have one.  The can happen with default argument getters, for
+    // example.
+    declCtx = pass.function->getModule().getSwiftModule();
+  }
 
   unsigned argIdx = 0;
   for (auto resultTy : pass.loweredFnConv.getIndirectSILResultTypes(typeCtx)) {

test/SILOptimizer/opaque_values_O_resilient.swift

@@ -0,0 +1,19 @@
+// RUN: %target-swift-frontend -enable-sil-opaque-values -parse-as-library -emit-sil -O -enable-library-evolution %s | %FileCheck %s
+
+public struct Gadget {
+// Verify the default accessor's arguments.  When AccessPathVerification runs,
+// it will be checked that the ParamDecl that AddressLowering synthesizes has a
+// non-null decl context.
+// CHECK-LABEL: sil @$s25opaque_values_O_resilient6GadgetV5whichA2C5WhichO_tcfC : {{.*}} {
+// CHECK:       bb0(%0 : $*Gadget, %1 : $*Gadget.Which, %2 : $@thin Gadget.Type):
+// CHECK-LABEL: } // end sil function '$s25opaque_values_O_resilient6GadgetV5whichA2C5WhichO_tcfC'
+  public init(which: Which = .that) {
+    fatalError()
+  }
+
+  public enum Which {
+    case that
+    case this(() -> Gadget)
+    case theOther
+  }
+}