[llvm] [DenseMap] Fix MSVC buildbot failure in lookup_or (PR #142268)

[Jakub Kuderski via llvm-commits]

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@@ -220,7 +220,8 @@ class DenseMapBase : public DebugEpochBase {
   // Return the entry with the specified key, or \p Default. This variant is
   // useful, because `lookup` cannot be used with non-default-constructible
   // values.
-  ValueT lookup_or(const_arg_type_t<KeyT> Val, ValueT &&Default) const {
+  template <typename U = std::remove_cv_t<ValueT>>
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kuhar wrote:

Can we have a unit test that checks for the expected return type with `std::is_same_v`?

https://github.com/llvm/llvm-project/pull/142268