[llvm] [llvm][ADT] Add `getSingleElement` helper (PR #131508)

[Matthias Springer via llvm-commits]

https://github.com/matthias-springer updated https://github.com/llvm/llvm-project/pull/131508

>From 65159c4d7bf39c01c81b6c5e0ab946ce9c942116 Mon Sep 17 00:00:00 2001
From: Matthias Springer <mspringer at nvidia.com>
Date: Sun, 16 Mar 2025 11:21:49 +0100
Subject: [PATCH] [llvm][ADT] Add `getSingleElement` helper

---
 llvm/include/llvm/ADT/STLExtras.h    |  8 ++++++++
 llvm/unittests/ADT/STLExtrasTest.cpp | 29 ++++++++++++++++++++++++++++
 2 files changed, 37 insertions(+)

diff --git a/llvm/include/llvm/ADT/STLExtras.h b/llvm/include/llvm/ADT/STLExtras.h
index 78b7e94c2b3a1..dc0443c9244be 100644
--- a/llvm/include/llvm/ADT/STLExtras.h
+++ b/llvm/include/llvm/ADT/STLExtras.h
@@ -325,6 +325,14 @@ template <typename ContainerTy> bool hasSingleElement(ContainerTy &&C) {
   return B != E && std::next(B) == E;
 }
 
+/// Asserts that the given container has a single element and returns that
+/// element.
+template <typename ContainerTy>
+decltype(auto) getSingleElement(ContainerTy &&C) {
+  assert(hasSingleElement(C) && "expected container with single element");
+  return *adl_begin(C);
+}
+
 /// Return a range covering \p RangeOrContainer with the first N elements
 /// excluded.
 template <typename T> auto drop_begin(T &&RangeOrContainer, size_t N = 1) {
diff --git a/llvm/unittests/ADT/STLExtrasTest.cpp b/llvm/unittests/ADT/STLExtrasTest.cpp
index dbb094b0a3088..f69e892e94ea0 100644
--- a/llvm/unittests/ADT/STLExtrasTest.cpp
+++ b/llvm/unittests/ADT/STLExtrasTest.cpp
@@ -1016,6 +1016,35 @@ TEST(STLExtrasTest, hasSingleElement) {
   EXPECT_FALSE(hasSingleElement(S));
 }
 
+TEST(STLExtrasTest, getSingleElement) {
+  // Note: Asserting behavior of getSingleElement cannot be tested because the
+  // program would crash.
+
+  // Test const and non-const containers.
+  const std::vector<int> V1 = {7};
+  EXPECT_EQ(getSingleElement(V1), 7);
+  std::vector<int> V2 = {8};
+  EXPECT_EQ(getSingleElement(V2), 8);
+
+  // Test LLVM container.
+  SmallVector<int> V3 {9};
+  EXPECT_EQ(getSingleElement(V3), 9);
+
+  // Test that the returned element is a reference.
+  getSingleElement(V3) = 11;
+  EXPECT_EQ(V3[0], 11);
+
+  // Test non-random access container.
+  std::list<int> L1 = {10};
+  EXPECT_EQ(getSingleElement(L1), 10);
+
+  // Make sure that we use the `begin`/`end` functions from `some_namespace`,
+  // using ADL.
+  some_namespace::some_struct S;
+  S.data = V2;
+  EXPECT_EQ(getSingleElement(S), 8);
+}
+
 TEST(STLExtrasTest, hasNItems) {
   const std::list<int> V0 = {}, V1 = {1}, V2 = {1, 2};
   const std::list<int> V3 = {1, 3, 5};