C++ PATCH for c++/88692 - -Wredundant-move false positive with *this

[Marek Polacek]

This patch fixes bogus -Wredundant-move warnings reported in 88692 and 87882.

To quickly recap, this warning is supposed to warn for cases like

struct T { };

T fn(T t)
{
  return std::move (t);
}

where NRVO isn't applicable for T because it's a parameter, but it's
a local variable and we're returning, so C++11 says activate move
semantics, so the std::move is redundant.  But, as these testcases show,
we're failing to realize that that is not the case when returning *this,
which is disguised as an ordinary PARM_DECL, and treat_lvalue_as_rvalue_p
was fooled by that.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2019-01-07  Marek Polacek  <pola...@redhat.com>

        PR c++/88692, c++/87882 - -Wredundant-move false positive with *this.
        * typeck.c (treat_lvalue_as_rvalue_p): Return false for 'this'.

        * g++.dg/cpp0x/Wredundant-move5.C: New test.
        * g++.dg/cpp0x/Wredundant-move6.C: New test.

diff --git gcc/cp/typeck.c gcc/cp/typeck.c
index e399cd3fe45..c6908d23a11 100644
--- gcc/cp/typeck.c
+++ gcc/cp/typeck.c
@@ -9371,6 +9371,9 @@ bool
 treat_lvalue_as_rvalue_p (tree retval, bool parm_ok)
 {
   STRIP_ANY_LOCATION_WRAPPER (retval);
+  /* *this remains an lvalue expression.  */
+  if (is_this_parameter (retval))
+    return false;
   return ((cxx_dialect != cxx98)
          && ((VAR_P (retval) && !DECL_HAS_VALUE_EXPR_P (retval))
              || (parm_ok && TREE_CODE (retval) == PARM_DECL))
diff --git gcc/testsuite/g++.dg/cpp0x/Wredundant-move5.C 
gcc/testsuite/g++.dg/cpp0x/Wredundant-move5.C
new file mode 100644
index 00000000000..b6a3b2296a8
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp0x/Wredundant-move5.C
@@ -0,0 +1,37 @@
+// PR c++/88692
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wredundant-move" }
+
+// Define std::move.
+namespace std {
+  template<typename _Tp>
+    struct remove_reference
+    { typedef _Tp   type; };
+
+  template<typename _Tp>
+    struct remove_reference<_Tp&>
+    { typedef _Tp   type; };
+
+  template<typename _Tp>
+    struct remove_reference<_Tp&&>
+    { typedef _Tp   type; };
+
+  template<typename _Tp>
+    constexpr typename std::remove_reference<_Tp>::type&&
+    move(_Tp&& __t) noexcept
+    { return static_cast<typename std::remove_reference<_Tp>::type&&>(__t); }
+}
+
+struct X {
+    X f() && {
+        return std::move(*this); // { dg-bogus "redundant move in return 
statement" }
+    }
+
+    X f2() & {
+        return std::move(*this); // { dg-bogus "redundant move in return 
statement" }
+    }
+
+    X f3() {
+        return std::move(*this); // { dg-bogus "redundant move in return 
statement" }
+    }
+};
diff --git gcc/testsuite/g++.dg/cpp0x/Wredundant-move6.C 
gcc/testsuite/g++.dg/cpp0x/Wredundant-move6.C
new file mode 100644
index 00000000000..5808a78638e
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp0x/Wredundant-move6.C
@@ -0,0 +1,43 @@
+// PR c++/87882
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wredundant-move" }
+
+// Define std::move.
+namespace std {
+  template<typename _Tp>
+    struct remove_reference
+    { typedef _Tp   type; };
+
+  template<typename _Tp>
+    struct remove_reference<_Tp&>
+    { typedef _Tp   type; };
+
+  template<typename _Tp>
+    struct remove_reference<_Tp&&>
+    { typedef _Tp   type; };
+
+  template<typename _Tp>
+    constexpr typename std::remove_reference<_Tp>::type&&
+    move(_Tp&& __t) noexcept
+    { return static_cast<typename std::remove_reference<_Tp>::type&&>(__t); }
+}
+
+struct Foo {
+   Foo Bar() {
+     return std::move(*this); // { dg-bogus "redundant move in return 
statement" }
+   }
+   Foo Baz() {
+     return *this;
+   }
+  int i;
+};
+
+void Move(Foo & f)
+{
+  f = Foo{}.Bar();
+}
+
+void NoMove(Foo & f)
+{
+  f = Foo{}.Baz();
+}