[clang] [Clang] Implement __reference_converts_from_temporary (PR #91199)

[Erich Keane via cfe-commits]

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@@ -5627,6 +5627,77 @@ static bool EvaluateUnaryTypeTrait(Sema &Self, TypeTrait UTT,
 static bool EvaluateBinaryTypeTrait(Sema &Self, TypeTrait BTT, const TypeSourceInfo *Lhs,
                                     const TypeSourceInfo *Rhs, SourceLocation KeyLoc);
 
+static ExprResult CheckConvertibilityForTypeTraits(Sema &Self,
+                                                   const TypeSourceInfo *Lhs,
+                                                   const TypeSourceInfo *Rhs,
+                                                   SourceLocation KeyLoc) {
+
+  QualType LhsT = Lhs->getType();
+  QualType RhsT = Rhs->getType();
+
+  // C++0x [meta.rel]p4:
+  //   Given the following function prototype:
+  //
+  //     template <class T>
+  //       typename add_rvalue_reference<T>::type create();
+  //
+  //   the predicate condition for a template specialization
+  //   is_convertible<From, To> shall be satisfied if and only if
+  //   the return expression in the following code would be
+  //   well-formed, including any implicit conversions to the return
+  //   type of the function:
+  //
+  //     To test() {
+  //       return create<From>();
+  //     }
+  //
+  //   Access checking is performed as if in a context unrelated to To and
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erichkeane wrote:

Do you have a test for this access checking?  I would expect something conversion ops/ctors that are protected, but they are friends of eachother.

https://github.com/llvm/llvm-project/pull/91199