Merge pull request #5800 from Darredevil/issue-6895-covariance

Fix issue 6895 std.traits.isCovariantWith doesn't work for function, …
merged-on-behalf-of: Andrei Alexandrescu <andralex@users.noreply.github.com>

std/traits.d

@@ -4885,7 +4885,9 @@ Determines whether the function type $(D F) is covariant with $(D G), i.e.,
 functions of the type $(D F) can override ones of the type $(D G).
  */
 template isCovariantWith(F, G)
-    if (is(F == function) && is(G == function))
+    if (is(F == function) && is(G == function) ||
+        is(F == delegate) && is(G == delegate) ||
+        isFunctionPointer!F && isFunctionPointer!G)
 {
     static if (is(F : G))
         enum isCovariantWith = true;
@@ -5031,6 +5033,15 @@ template isCovariantWith(F, G)
     static assert( isCovariantWith!(DerivA_1.test, DerivA_1.test));
     static assert( isCovariantWith!(DerivA_2.test, DerivA_2.test));
 
+     // function, function pointer and delegate
+     J function() derived_function;
+     I function() base_function;
+     J delegate() derived_delegate;
+     I delegate() base_delegate;
+     static assert(.isCovariantWith!(typeof(derived_function), typeof(base_function)));
+     static assert(.isCovariantWith!(typeof(*derived_function), typeof(*base_function)));
+     static assert(.isCovariantWith!(typeof(derived_delegate), typeof(base_delegate)));
+
     // scope parameter
     interface BaseB            {          void test(      int*,       int*); }
     interface DerivB_1 : BaseB { override void test(scope int*,       int*); }