Fixes Heap Allocations for Large alignMask

lib/IRGen/GenType.cpp

@@ -2032,6 +2032,10 @@ SILType irgen::getSingletonAggregateFieldType(IRGenModule &IGM,
     // If there's only one stored property, we have the layout of its field.
     auto allFields = structDecl->getStoredProperties();
     auto field = allFields.begin();
+    // If there are no other fields exit early to prevent calling std::next on
+    // an invalid iterator.
+    if (field == allFields.end())
+      return SILType();
     if (std::next(field) == allFields.end())
       return t.getFieldType(*field, *IGM.SILMod);
 

stdlib/public/runtime/Heap.cpp

@@ -23,9 +23,9 @@
 using namespace swift;
 
 void *swift::swift_slowAlloc(size_t size, size_t alignMask) {
+  void *p;
   // FIXME: use posix_memalign if alignMask is larger than the system guarantee.
   void *p = malloc(size);
-  if (!p) swift::crash("Could not allocate memory.");
   return p;
 }
 

test/IRGen/generic_structs.swift

@@ -0,0 +1,26 @@
+// RUN: %target-swift-frontend -primary-file %s -emit-ir | FileCheck %s
+
+struct A<T1, T2>
+{
+  var b: T1
+  var c: T2
+  var d: B<T1, T2>
+}
+struct B<T1, T2>
+{
+  var c: T1
+  var d: T2
+}
+
+struct C<T1>
+{}
+struct D<T2>
+{}
+
+struct Foo<A1, A2>
+{
+  var b: Bar<A1, A2>
+}
+
+struct Bar {
+}