[clang] [Clang] Implement C++26 P2748R5 "Disallow Binding a Returned Glvalue to a Temporary" (PR #89942)

[via cfe-commits]

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@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 -std=c++26 -fsyntax-only -verify %s
+
+auto&& f1() {
+  return 42; // expected-error{{returning reference to local temporary object}}
+}
+const double& f2() {
+  static int x = 42;
+  return x; // expected-error{{returning reference to local temporary object}}
+}
+auto&& id(auto&& r) {
+  return static_cast<decltype(r)&&>(r);
+}
+auto&& f3() {
+  return id(42);        // OK, but probably a bug
+}
+
+static_assert(__is_convertible(int, const int &));
+static_assert(__is_nothrow_convertible(int, const int &));
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yronglin wrote:

Agree to keep these 2 lines and I'll add more test in type-traits.cpp

https://github.com/llvm/llvm-project/pull/89942