[llvm] [ADT] Simplify enable_if_struct_deref_supported (NFC) (PR #159939)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-adt

Author: Kazu Hirata (kazutakahirata)

<details>
<summary>Changes</summary>

This patch simplifies enable_if_struct_deref_supported by using a
default template parameter as a temporary type alias.  This way, we do
not have to repeat decltype(...).


---
Full diff: https://github.com/llvm/llvm-project/pull/159939.diff


1 Files Affected:

- (modified) llvm/include/llvm/ADT/fallible_iterator.h (+3-4) 


``````````diff
diff --git a/llvm/include/llvm/ADT/fallible_iterator.h b/llvm/include/llvm/ADT/fallible_iterator.h
index b42f62b67fd66..76fa054f2eb94 100644
--- a/llvm/include/llvm/ADT/fallible_iterator.h
+++ b/llvm/include/llvm/ADT/fallible_iterator.h
@@ -67,10 +67,9 @@ namespace llvm {
 /// without requiring redundant error checks.
 template <typename Underlying> class fallible_iterator {
 private:
-  template <typename T>
-  using enable_if_struct_deref_supported = std::enable_if_t<
-      !std::is_void<decltype(std::declval<T>().operator->())>::value,
-      decltype(std::declval<T>().operator->())>;
+  template <typename T, typename U = decltype(std::declval<T>().operator->())>
+  using enable_if_struct_deref_supported =
+      std::enable_if_t<!std::is_void_v<U>, U>;
 
 public:
   /// Construct a fallible iterator that *cannot* be used as an end-of-range

``````````

</details>


https://github.com/llvm/llvm-project/pull/159939