Adapt o3tl::temporary to C++23 P2266R1

With the recent implemenation of
<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2266r1.html> "P2266R1:
Simpler implicit move" in Clang 13 trunk as
<https://github.com/llvm/llvm-project/commit/bf20631782183cd19e0bb7219e908c2bbb01a75f>
"[clang] Implement P2266 Simpler implicit move", a --with-latest-c++ build
started to fail with

> In file included from sal/rtl/random.cxx:25:
> include/o3tl/temporary.hxx:21:62: error: non-const lvalue reference to type 'double' cannot bind to a temporary of type 'double'
> template <typename T> constexpr T& temporary(T&& x) { return x; }
>                                                              ^
> sal/rtl/random.cxx:97:37: note: in instantiation of function template specialization 'o3tl::temporary<double>' requested here
>     return std::modf(random, &o3tl::temporary(double()));
>                                     ^

etc.  (And fixing that by adding the recommended static_cast then triggered a
false loplugin:redundantcast warning.)

Change-Id: I222429e9872afdedf77a07014c0a2e9e06c60b50
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/117335
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

2 files changed, 29 insertions, 0 deletions

diff --git a/compilerplugins/clang/redundantcast.cxx b/compilerplugins/clang/redundantcast.cxx
index 088349fa1d9c..27566809494c 100644
--- a/compilerplugins/clang/redundantcast.cxx
+++ b/compilerplugins/clang/redundantcast.cxx
@@ -109,6 +109,14 @@ public:
                 expr->isSemanticForm() ? expr : expr->getSemanticForm(), queue);
     }
 
+    bool TraverseReturnStmt(ReturnStmt * stmt) {
+        auto const saved = returnExpr_;
+        returnExpr_ = stmt->getRetValue();
+        auto const ret = FilteringRewritePlugin::TraverseReturnStmt(stmt);
+        returnExpr_ = saved;
+        return ret;
+    }
+
     bool VisitImplicitCastExpr(ImplicitCastExpr const * expr);
 
     bool VisitCXXStaticCastExpr(CXXStaticCastExpr const * expr);
@@ -154,6 +162,8 @@ private:
         return compiler.getSourceManager().isMacroBodyExpansion(loc)
             && ignoreLocation(compiler.getSourceManager().getSpellingLoc(loc));
     }
+
+    Expr const * returnExpr_ = nullptr;
 };
 
 bool RedundantCast::VisitImplicitCastExpr(const ImplicitCastExpr * expr) {
@@ -502,6 +512,23 @@ bool RedundantCast::VisitCXXStaticCastExpr(CXXStaticCastExpr const * expr) {
     {
         return true;
     }
+    // For <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2266r1.html> "P2266R1: Simpler
+    // implicit move" (as implemented with <https://github.com/llvm/llvm-project/commit/
+    // bf20631782183cd19e0bb7219e908c2bbb01a75f> "[clang] Implement P2266 Simpler implicit move"
+    // towards Clang 13), don't warn about a static_cast in a return statement like
+    //
+    //   return static_cast<int &>(x);
+    //
+    // that needs an lvalue but where in a return statement like
+    //
+    //   return x;
+    //
+    // the expression would now be an xvalue:
+    if (k3 == VK_LValue && k1 == VK_LValue && returnExpr_ != nullptr
+        && expr == returnExpr_->IgnoreParens())
+    {
+        return true;
+    }
     // Don't warn if a static_cast on a pointer to function or member function is used to
     // disambiguate an overloaded function:
     if (fnptr) {
diff --git a/compilerplugins/clang/test/redundantcast.cxx b/compilerplugins/clang/test/redundantcast.cxx
index 97f4e6f73777..2a3721bb0c2e 100644
--- a/compilerplugins/clang/test/redundantcast.cxx
+++ b/compilerplugins/clang/test/redundantcast.cxx
@@ -286,6 +286,8 @@ void testStaticCast() {
     (void) static_cast<S const &&>(csr());
 }
 
+int & testReturnStaticCast(int && x) { return static_cast<int &>(x); }
+
 void testFunctionalCast() {
     (void) int(nir()); // expected-error {{redundant functional cast from 'int' to 'int' [loplugin:redundantcast]}}
     (void) S(nsr());