c++: tweak for -Wrange-loop-construct [PR116731]

This PR reports that the warning would be better off using a check
for trivially constructible rather than trivially copyable.

LLVM accepted a similar fix:
https://github.com/llvm/llvm-project/issues/47355

	PR c++/116731

gcc/cp/ChangeLog:

	* parser.cc (warn_for_range_copy): Check if TYPE is trivially
	constructible, not copyable.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wrange-loop-construct3.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

gcc/cp/parser.cc

@@ -14394,11 +14394,13 @@ warn_for_range_copy (tree decl, tree expr)
   else if (!CP_TYPE_CONST_P (type))
     return;
 
-  /* Since small trivially copyable types are cheap to copy, we suppress the
-     warning for them.  64B is a common size of a cache line.  */
+  /* Since small trivially constructible types are cheap to construct, we
+     suppress the warning for them.  64B is a common size of a cache line.  */
+  tree vec = make_tree_vec (1);
+  TREE_VEC_ELT (vec, 0) = TREE_TYPE (expr);
   if (TREE_CODE (TYPE_SIZE_UNIT (type)) != INTEGER_CST
       || (tree_to_uhwi (TYPE_SIZE_UNIT (type)) <= 64
-	  && trivially_copyable_p (type)))
+	  && is_trivially_xible (INIT_EXPR, type, vec)))
     return;
 
   /* If we can initialize a reference directly, suggest that to avoid the

gcc/testsuite/g++.dg/warn/Wrange-loop-construct3.C

@@ -0,0 +1,57 @@
+// PR c++/116731
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wrange-loop-construct" }
+
+void
+f0 ()
+{
+  struct S {
+    char a[64];
+    S& operator=(const S&) { return *this; };
+  };
+
+  S arr[8];
+  for (const auto r : arr)
+    (void) r;
+}
+
+void
+f1 ()
+{
+  struct S {
+    char a[65];
+    S& operator=(const S&) { return *this; };
+  };
+
+  S arr[8];
+  for (const auto r : arr) // { dg-warning "creates a copy" }
+    (void) r;
+}
+
+void
+f2 ()
+{
+  struct S {
+    char a[64];
+    S& operator=(const S&) { return *this; };
+    ~S() { }
+  };
+
+  S arr[8];
+  for (const auto r : arr) // { dg-warning "creates a copy" }
+    (void) r;
+}
+
+void
+f3 ()
+{
+  struct S {
+    char a[65];
+    S& operator=(const S&) { return *this; };
+    ~S() { }
+  };
+
+  S arr[8];
+  for (const auto r : arr) // { dg-warning "creates a copy" }
+    (void) r;
+}