[SILGen] Fix no-parameter subscript accessor emission more thoroughly.

Author: DougGregor

lib/SILGen/SILGenApply.cpp

@@ -5098,11 +5098,11 @@ void SILGenFunction::emitSetAccessor(SILLocation loc, SILDeclRef set,
       eltSources.emplace_back(loc, std::move(subscripts));
     }
 
-    if (inputTupleType) {
-      setValue = ArgumentSource(loc, inputTupleType, eltSources);
-    } else {
-      assert(eltSources.size() == 1);
+    if (eltSources.size() == 1) {
       setValue = std::move(eltSources.front());
+    } else {
+      assert(inputTupleType);
+      setValue = ArgumentSource(loc, inputTupleType, eltSources);
     }
 
   } else {

test/SILGen/subscript_accessor.swift

@@ -1,4 +1,4 @@
-// RUN: %target-swift-frontend -enable-sil-ownership -emit-silgen -primary-file %s | %FileCheck %s
+// RUN: %target-swift-frontend -enable-sil-ownership -O -emit-silgen -primary-file %s | %FileCheck %s
 
 // CHECK-LABEL: sil private [transparent] @$S18subscript_accessor1XVxSgycimytfU_
 // CHECK: [[SETTER:%.*]] = function_ref @$S18subscript_accessor1XVxSgycis
@@ -11,3 +11,9 @@ struct X<T> {
     set { }
   }
 }
+
+  // CHECK: sil{{.*}}S18subscript_accessor9testXRead1xxAA1XVyxG_tlF
+@_specialize(where T == (Int, Int))
+func testXRead<T>(x: X<T>) -> T {
+  return x[]!
+}