[libcxx-commits] [libcxx] e43c93d - Don't treat 'T &forward(T&&)' as builtin.

[Richard Smith via libcxx-commits]

Author: Richard Smith
Date: 2022-04-18T11:11:21-07:00
New Revision: e43c93dd63cca295ef26ab69cd305816a71d45fd

URL: https://github.com/llvm/llvm-project/commit/e43c93dd63cca295ef26ab69cd305816a71d45fd
DIFF: https://github.com/llvm/llvm-project/commit/e43c93dd63cca295ef26ab69cd305816a71d45fd.diff

LOG: Don't treat 'T &forward(T&&)' as builtin.

This allows the standard library to diagnose it properly. Suppress
warning in libc++ testsuite for unused result of call to std::forward.

Added: 
    

Modified: 
    clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
    clang/test/SemaCXX/builtin-std-move.cpp
    libcxx/test/std/utilities/utility/forward/forward.fail.cpp

Removed: 
    


################################################################################
diff  --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 8b0147fc66c36..f11a1748e5c68 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -621,6 +621,20 @@ static bool isRelevantAttr(Sema &S, const Decl *D, const Attr *A) {
     return true;
   }
 
+  if (const auto *BA = dyn_cast<BuiltinAttr>(A)) {
+    // Do not treat 'std::forward' as a builtin if it takes an rvalue reference
+    // type and returns an lvalue reference type. The library implementation
+    // will produce an error in this case; don't get in its way.
+    if (BA->getID() == Builtin::BIforward) {
+      const FunctionDecl *FD = dyn_cast<FunctionDecl>(D);
+      if (FD && FD->getNumParams() >= 1 &&
+          FD->getParamDecl(0)->getType()->isRValueReferenceType() &&
+          FD->getReturnType()->isLValueReferenceType()) {
+        return false;
+      }
+    }
+  }
+
   return true;
 }
 

diff  --git a/clang/test/SemaCXX/builtin-std-move.cpp b/clang/test/SemaCXX/builtin-std-move.cpp
index a53e57c86c14c..7f1b01e43d788 100644
--- a/clang/test/SemaCXX/builtin-std-move.cpp
+++ b/clang/test/SemaCXX/builtin-std-move.cpp
@@ -30,11 +30,18 @@ namespace std {
   template<typename T> struct remove_reference<T&> { using type = T; };
   template<typename T> struct remove_reference<T&&> { using type = T; };
 
+  template<typename T> struct is_lvalue_reference { static constexpr bool value = false; };
+  template<typename T> struct is_lvalue_reference<T&> { static constexpr bool value = true; };
+
   template<typename T> CONSTEXPR T &&forward(typename remove_reference<T>::type &x) {
     static_assert(T::moveable, "instantiated forward"); // expected-error {{no member named 'moveable' in 'B'}}
                                                         // expected-error at -1 {{no member named 'moveable' in 'C'}}
     return static_cast<T&&>(x);
   }
+  template<typename T> CONSTEXPR T &&forward(typename remove_reference<T>::type &&x) {
+    static_assert(!is_lvalue_reference<T>::value, "should not forward rval as lval"); // expected-error {{static_assert failed}}
+    return static_cast<T&&>(x);
+  }
 
   template<typename T> CONSTEXPR const T &as_const(T &x) {
     static_assert(T::moveable, "instantiated as_const"); // expected-error {{no member named 'moveable' in 'B'}}
@@ -76,6 +83,11 @@ static_assert(f({}), "should be constexpr");
 // expected-note@#call {{}}
 #endif
 
+A &forward_rval_as_lval() {
+  std::forward<A&&>(A()); // expected-warning {{const attribute}}
+  return std::forward<A&>(A()); // expected-note {{instantiation of}}
+}
+
 struct B {};
 B &&(*pMove)(B&) = std::move; // #1 expected-note {{instantiation of}}
 B &&(*pMoveIfNoexcept)(B&) = &std::move_if_noexcept; // #2 expected-note {{instantiation of}}

diff  --git a/libcxx/test/std/utilities/utility/forward/forward.fail.cpp b/libcxx/test/std/utilities/utility/forward/forward.fail.cpp
index 4d7790a751481..f913b835c2248 100644
--- a/libcxx/test/std/utilities/utility/forward/forward.fail.cpp
+++ b/libcxx/test/std/utilities/utility/forward/forward.fail.cpp
@@ -22,7 +22,7 @@ const A csource() {return A();}
 int main(int, char**)
 {
     {
-        std::forward<A&>(source());  // expected-note {{requested here}}
+        (void)std::forward<A&>(source());  // expected-note {{requested here}}
         // expected-error-re@*:* 1 {{static_assert failed{{.*}} "cannot forward an rvalue as an lvalue"}}
     }
     {