[5.0] IRGen: Empty fields do have an entry in the field offset vector

This is an error introduced as the result of a refactoring a while ago
and means that we will store dependently typed stored properties at the
wrong offset in a generic struct if it has stored properties of empty
types before said property.

rdar://36384871

Author: aschwaighofer

lib/IRGen/StructLayout.cpp

@@ -215,6 +215,7 @@ bool StructLayoutBuilder::addField(ElementLayout &elt,
   if (eltTI.isKnownEmpty(ResilienceExpansion::Maximal)) {
     addEmptyElement(elt);
     // If the element type is empty, it adds nothing.
+    NextNonFixedOffsetIndex++;
     return false;
   }
   // TODO: consider using different layout rules.

test/IRGen/generic_structs.swift

@@ -1,4 +1,4 @@
-// RUN: %target-swift-frontend -assume-parsing-unqualified-ownership-sil -primary-file %s -emit-ir
+// RUN: %target-swift-frontend -assume-parsing-unqualified-ownership-sil -primary-file %s -emit-ir | %FileCheck %s --check-prefix=CHECK-%target-ptrsize
 
 struct A<T1, T2>
 {
@@ -25,3 +25,35 @@ struct Foo<A1, A2>
 
 struct Bar<A1, A2> {
 }
+
+public protocol Proto { }
+
+public struct EmptyStruct {}
+
+public struct GenericStruct<T : Proto> {
+  var empty: EmptyStruct = EmptyStruct()
+  var dummy: Int = 0
+  var opt: Optional<T> = nil
+
+  public init() {}
+}
+
+// CHECK-32-LABEL: define{{.*}} swiftcc void @"$S15generic_structs13GenericStructVACyxGycfC"
+// CHECK-32:  [[TYPE:%.*]] =  call %swift.type* @"$S15generic_structs13GenericStructVMa"(%swift.type* %T, i8** %T.Proto)
+// CHECK-32:  [[PTR:%.*]] = bitcast %swift.type* %8 to i32*
+// CHECK-32:  [[FIELDOFFSETS:%.*]] = getelementptr inbounds i32, i32* [[PTR]], i32 2
+// CHECK-32:  [[FIELDOFFSET:%.*]] = getelementptr inbounds i32, i32* [[FIELDOFFSETS]], i32 2
+// CHECK-32:  [[OFFSET:%.*]] = load i32, i32* [[FIELDOFFSET]]
+// CHECK-32:  [[ADDROFOPT:%.*]] = getelementptr inbounds i8, i8* {{.*}}, i32 [[OFFSET]]
+// CHECK-32:  [[OPTPTR:%.*]] = bitcast i8* [[ADDROFOPT]] to %TSq*
+// CHECK-32:  call %TSq* @"$S15generic_structsytWb3_"(%TSq* {{.*}}, %TSq* [[OPTPTR]]
+
+// CHECK-64-LABEL: define{{.*}} swiftcc void @"$S15generic_structs13GenericStructVACyxGycfC"
+// CHECK-64:  [[TYPE:%.*]] =  call %swift.type* @"$S15generic_structs13GenericStructVMa"(%swift.type* %T, i8** %T.Proto)
+// CHECK-64:  [[PTR:%.*]] = bitcast %swift.type* %8 to i64*
+// CHECK-64:  [[FIELDOFFSETS:%.*]] = getelementptr inbounds i64, i64* [[PTR]], i64 2
+// CHECK-64:  [[FIELDOFFSET:%.*]] = getelementptr inbounds i64, i64* [[FIELDOFFSETS]], i32 2
+// CHECK-64:  [[OFFSET:%.*]] = load i64, i64* [[FIELDOFFSET]]
+// CHECK-64:  [[ADDROFOPT:%.*]] = getelementptr inbounds i8, i8* {{.*}}, i64 [[OFFSET]]
+// CHECK-64:  [[OPTPTR:%.*]] = bitcast i8* [[ADDROFOPT]] to %TSq*
+// CHECK-64:  call %TSq* @"$S15generic_structsytWb3_"(%TSq* {{.*}}, %TSq* [[OPTPTR]]

test/Interpreter/field_offset_generic.swift

@@ -0,0 +1,27 @@
+// RUN: %target-run-simple-swift | %FileCheck %s
+// REQUIRES: executable_test
+
+public protocol Proto { }
+
+public struct MyImpl: Proto { }
+
+public struct EmptyStruct {}
+
+private struct GenericStruct<T : Proto> {
+    var empty: EmptyStruct = EmptyStruct()
+    var dummy: Int = 0
+    var opt: Optional<T> = nil
+
+    init() {
+    }
+}
+
+public func test() {
+  let s = GenericStruct<MyImpl>()
+  assert(s.dummy == 0, "Expecting dummy == 0")
+  assert(s.opt == nil, "Expecting opt == nil")
+  // CHECK: dummy: 0
+  print("dummy: \(s.dummy)")
+}
+
+test()