[llvm] r336170 - [ADT] Try to work around a crash in MSVC.

[Chandler Carruth via llvm-commits]

Author: chandlerc
Date: Mon Jul  2 22:46:20 2018
New Revision: 336170

URL: http://llvm.org/viewvc/llvm-project?rev=336170&view=rev
Log:
[ADT] Try to work around a crash in MSVC.

Putting `sizeof(T) <= 16` into the parameter of a `std::conditional`
causes every version of MSVC I've tried to crash:

  https://godbolt.org/g/eqVULL

Really frustrating, but an extra layer of indirection through an
instantiated type gives a working way to access this computed constant.

Modified:
    llvm/trunk/include/llvm/ADT/FunctionExtras.h

Modified: llvm/trunk/include/llvm/ADT/FunctionExtras.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/ADT/FunctionExtras.h?rev=336170&r1=336169&r2=336170&view=diff
==============================================================================
--- llvm/trunk/include/llvm/ADT/FunctionExtras.h (original)
+++ llvm/trunk/include/llvm/ADT/FunctionExtras.h Mon Jul  2 22:46:20 2018
@@ -46,6 +46,13 @@ template <typename ReturnT, typename...
 class unique_function<ReturnT(ParamTs...)> {
   static constexpr int InlineStorageSize = sizeof(void *) * 3;
 
+  // MSVC has a bug and ICEs if we give it a particular dependent value
+  // expression as part of the `std::conditional` below. To work around this,
+  // we build that into a template struct's constexpr bool.
+  template <typename T> struct IsSizeLessThanThresholdT {
+    static constexpr bool value = sizeof(T) <= (2 * sizeof(void *));
+  };
+
   // Provide a type function to map parameters that won't observe extra copies
   // or moves and which are small enough to likely pass in register to values
   // and all other types to l-value reference types. We use this to compute the
@@ -60,7 +67,7 @@ class unique_function<ReturnT(ParamTs...
       !std::is_reference<T>::value &&
           llvm::is_trivially_copy_constructible<T>::value &&
           llvm::is_trivially_move_constructible<T>::value &&
-          sizeof(T) <= (2 * sizeof(void *)),
+          IsSizeLessThanThresholdT<T>::value,
       T, T &>::type;
 
   // The type of the erased function pointer we use as a callback to dispatch to