Fix Issue 19226 - std.typecons.Nullable(T, T nullValue) doesn't fully…

… handle non-self-equal nullValue

std/typecons.d

@@ -3445,7 +3445,7 @@ Returns:
         }
         //Need to use 'is' if T is a float type
         //because NaN != NaN
-        else static if (isFloatingPoint!T)
+        else static if (__traits(compiles, { static assert(!(nullValue == nullValue)); }))
         {
             return _value is nullValue;
         }
@@ -3466,6 +3466,14 @@ Returns:
     assert(!ni.isNull);
 }
 
+@system unittest
+{
+    assert(typeof(this).init.isNull, typeof(this).stringof ~ 
+        ".isNull does not work correctly because " ~ typeof(T).stringof ~
+        " has an == operator that is non-reflexive and could not be " ~
+        " determined before runtime to be non-reflexive!");
+}
+
 // https://issues.dlang.org/show_bug.cgi?id=11135
 // disable test until https://issues.dlang.org/show_bug.cgi?id=15316 gets fixed
 version (none) @system unittest
@@ -3640,6 +3648,17 @@ if (is (typeof(nullValue) == T))
     assert(a.isNull);
 }
 
+@safe unittest
+{
+    // issue 19226 - fully handle non-self-equal nullValue
+    static struct S
+    {
+        float f;
+    }
+    alias N = Nullable!(S, S.init);
+    assert(N(S.init).isNull);
+}
+
 @safe unittest
 {
     static int f(scope const Nullable!(int, int.min) x) {