Add a SameAsOneOf helper (#5490)

Trying to make repeated `std::same_as` easier to write. Calling it
"concepts.h" because I figure we'll maybe have a couple more things like
this.

Was looking at this because I may add a couple more similar constructs.

common/BUILD

@@ -111,6 +111,11 @@ cc_test(
     ],
 )
 
+cc_library(
+    name = "concepts",
+    hdrs = ["concepts.h"],
+)
+
 cc_library(
     name = "enum_base",
     hdrs = ["enum_base.h"],
@@ -307,6 +312,7 @@ cc_library(
     hdrs = ["map.h"],
     deps = [
         ":check",
+        ":concepts",
         ":hashtable_key_context",
         ":raw_hashtable",
         "@llvm-project//llvm:Support",
@@ -374,6 +380,7 @@ cc_library(
     hdrs = ["raw_hashtable.h"],
     deps = [
         ":check",
+        ":concepts",
         ":hashing",
         ":hashtable_key_context",
         ":raw_hashtable_metadata_group",

common/concepts.h

@@ -0,0 +1,18 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef CARBON_COMMON_CONCEPTS_H_
+#define CARBON_COMMON_CONCEPTS_H_
+
+#include <concepts>
+
+namespace Carbon {
+
+// True if `T` is the same as one of `OtherT`.
+template <typename T, typename... OtherT>
+concept SameAsOneOf = (std::same_as<T, OtherT> || ...);
+
+}  // namespace Carbon
+
+#endif  // CARBON_COMMON_CONCEPTS_H_

common/map.h

@@ -10,6 +10,7 @@
 #include <utility>
 
 #include "common/check.h"
+#include "common/concepts.h"
 #include "common/hashtable_key_context.h"
 #include "common/raw_hashtable.h"
 #include "llvm/Support/Compiler.h"
@@ -90,10 +91,8 @@ class MapView
   template <typename OtherKeyT, typename OtherValueT>
   // NOLINTNEXTLINE(google-explicit-constructor)
   MapView(MapView<OtherKeyT, OtherValueT, KeyContextT> other_view)
-    requires(std::same_as<KeyT, OtherKeyT> ||
-             std::same_as<KeyT, const OtherKeyT>) &&
-            (std::same_as<ValueT, OtherValueT> ||
-             std::same_as<ValueT, const OtherValueT>)
+    requires(SameAsOneOf<KeyT, OtherKeyT, const OtherKeyT> &&
+             SameAsOneOf<ValueT, OtherValueT, const OtherValueT>)
       : ImplT(other_view) {}
 
   // Tests whether a key is present in the map.
@@ -193,10 +192,8 @@ class MapBase : protected RawHashtable::BaseImpl<InputKeyT, InputValueT,
   template <typename OtherKeyT, typename OtherValueT>
   // NOLINTNEXTLINE(google-explicit-constructor)
   operator MapView<OtherKeyT, OtherValueT, KeyContextT>() const
-    requires(std::same_as<KeyT, OtherKeyT> ||
-             std::same_as<const KeyT, OtherKeyT>) &&
-            (std::same_as<ValueT, OtherValueT> ||
-             std::same_as<const ValueT, OtherValueT>)
+    requires(SameAsOneOf<OtherKeyT, KeyT, const KeyT> &&
+             SameAsOneOf<OtherValueT, ValueT, const ValueT>)
   {
     return ViewT(*this);
   }

common/raw_hashtable.h

@@ -15,6 +15,7 @@
 #include <utility>
 
 #include "common/check.h"
+#include "common/concepts.h"
 #include "common/hashing.h"
 #include "common/raw_hashtable_metadata_group.h"
 #include "llvm/Support/Compiler.h"
@@ -374,10 +375,8 @@ class ViewImpl {
   template <typename OtherKeyT, typename OtherValueT>
   // NOLINTNEXTLINE(google-explicit-constructor)
   ViewImpl(ViewImpl<OtherKeyT, OtherValueT, KeyContextT> other_view)
-    requires(std::same_as<KeyT, OtherKeyT> ||
-             std::same_as<KeyT, const OtherKeyT>) &&
-                (std::same_as<ValueT, OtherValueT> ||
-                 std::same_as<ValueT, const OtherValueT>)
+    requires(SameAsOneOf<KeyT, OtherKeyT, const OtherKeyT> &&
+             SameAsOneOf<ValueT, OtherValueT, const OtherValueT>)
       : alloc_size_(other_view.alloc_size_), storage_(other_view.storage_) {}
 
   // Looks up an entry in the hashtable and returns its address or null if not