diff --git a/compiler/src/dmd/expressionsem.d b/compiler/src/dmd/expressionsem.d
index d97cee7769a3..bacb1729c449 100644
--- a/compiler/src/dmd/expressionsem.d
+++ b/compiler/src/dmd/expressionsem.d
@@ -14931,6 +14931,19 @@ private extern (C++) final class ExpressionSemanticVisitor : Visitor
     {
         //printf("EqualExp::semantic('%s')\n", exp.toErrMsg());
 
+        static Expression floatingNaNConst(Expression e)
+        {
+            auto optimized = e.optimize(WANTvalue);
+            if (!optimized || optimized.isConst() != 1)
+                return null;
+
+            if (optimized.type.isReal())
+                return CTFloat.isNaN(optimized.toReal()) ? optimized : null;
+            if (optimized.type.isImaginary())
+                return CTFloat.isNaN(optimized.toImaginary()) ? optimized : null;
+            return null;
+        }
+
         exp.setNoderefOperands();
 
         if (auto e = binSemanticProp(exp, sc))
@@ -15007,6 +15020,22 @@ private extern (C++) final class ExpressionSemanticVisitor : Visitor
         Type t1 = exp.e1.type.toBasetype();
         Type t2 = exp.e2.type.toBasetype();
 
+        if (t1.isFloating() && t2.isFloating())
+        {
+            auto nan1 = floatingNaNConst(exp.e1);
+            auto nan2 = floatingNaNConst(exp.e2);
+            auto nan = nan1 ? nan1 : nan2;
+            if (nan)
+            {
+                const(char)* cmpResult = exp.op == EXP.equal ? "false" : "true";
+                error(exp.loc, "comparison with `%s` is always %s; use `%s` instead",
+                    nan.toErrMsg(),
+                    cmpResult,
+                    "!is".ptr + (exp.op == EXP.equal));
+                return setError();
+            }
+        }
+
         static bool unifyArrayTypes(Type t1, Type t2, Scope* sc)
         {
             Type t1n = t1.nextOf().toBasetype();
diff --git a/compiler/test/compilable/test19227.d b/compiler/test/compilable/test19227.d
index 2a1d325d7580..0b282ac87595 100644
--- a/compiler/test/compilable/test19227.d
+++ b/compiler/test/compilable/test19227.d
@@ -20,8 +20,5 @@ struct T
 void main()
 {
     static assert(S.init is S.init);
-    static assert(S.init != S.init);
-
     static assert(T.init is T.init);
-    static assert(T.init != T.init);
 }
diff --git a/compiler/test/fail_compilation/fail18531.d b/compiler/test/fail_compilation/fail18531.d
new file mode 100644
index 000000000000..3b486653ceeb
--- /dev/null
+++ b/compiler/test/fail_compilation/fail18531.d
@@ -0,0 +1,23 @@
+/*
+TEST_OUTPUT:
+---
+fail_compilation/fail18531.d(16): Error: comparison with `float.nan` is always false; use `is` instead
+fail_compilation/fail18531.d(18): Error: comparison with `float.nan` is always true; use `!is` instead
+fail_compilation/fail18531.d(20): Error: comparison with `float.nan` is always false; use `is` instead
+fail_compilation/fail18531.d(22): Error: comparison with `double.nan` is always false; use `is` instead
+---
+*/
+
+enum myNaN = double.nan;
+
+void main()
+{
+    float a = float.nan;
+    assert(a == float.nan);
+    float b = 0.0;
+    assert(b != float.nan);
+    float c = float.init;
+    assert(c == float.init);
+    assert(a == c);
+    assert(myNaN == double.nan);
+}
diff --git a/compiler/test/runnable/constfold.d b/compiler/test/runnable/constfold.d
index 4aa7a968ae0a..38255e90a73b 100644
--- a/compiler/test/runnable/constfold.d
+++ b/compiler/test/runnable/constfold.d
@@ -148,13 +148,6 @@ static assert((7.2 != 6.2) == 1);
 static assert((7.2 == 7.2) == 1);
 static assert((7.2 != 7.2) == 0);
 
-static assert((7.2 == double.nan) == 0);
-static assert((7.2 != double.nan) == 1);
-static assert((double.nan == double.nan) == 0);
-static assert((double.nan != double.nan) == 1);
-static assert((double.nan == 7.2) == 0);
-static assert((double.nan != 7.2) == 1);
-
 static assert((5 is 5) == 1);
 static assert((5 is 4) == 0);
 static assert((5 !is 5) == 0);
@@ -228,6 +221,17 @@ void test3()
      assert(signbit(m) != 0);
 }
 
+void testNaN()
+{
+    double nan = double.nan;
+    assert((7.2 == nan) == 0);
+    assert((7.2 != nan) == 1);
+    assert((nan == nan) == 0);
+    assert((nan != nan) == 1);
+    assert((nan == 7.2) == 0);
+    assert((nan != 7.2) == 1);
+}
+
 /************************************/
 
 struct A4 { char [] a; }
@@ -587,6 +591,7 @@ int main()
     test1();
     test2();
     test3();
+    testNaN();
     test3697and();
     test3697or();
     test6077();
diff --git a/compiler/test/runnable/nan.d b/compiler/test/runnable/nan.d
index 9b97699b8bae..d878aecf8b71 100644
--- a/compiler/test/runnable/nan.d
+++ b/compiler/test/runnable/nan.d
@@ -2,8 +2,6 @@ import core.stdc.stdio;
 
 enum real er1 = real.nan;
 enum real er2 = 1;
-static assert(er1 != er2);
-static assert(!(er1 == er2));
 static assert(!(er1 < er2));
 static assert(!(er1 > er2));
 static assert(!(er1 >= er2));
@@ -11,8 +9,6 @@ static assert(!(er1 <= er2));
 
 enum double ed1 = real.nan;
 enum double ed2 = 1;
-static assert(ed1 != ed2);
-static assert(!(ed1 == ed2));
 static assert(!(ed1 < ed2));
 static assert(!(ed1 > ed2));
 static assert(!(ed1 >= ed2));
@@ -70,33 +66,33 @@ void test2(T)()
 
 bool test()
 {
-        real r1 = real.nan;
-        real r2 = 1;
-        b = (r1 != r2); assert(b);
-        b = (r1 == r2); assert(!b);
-        b = (r1 <  r2); assert(!b);
-        b = (r1 >  r2); assert(!b);
-        b = (r1 <= r2); assert(!b);
-        b = (r1 >= r2); assert(!b);
+    real r1 = real.nan;
+    real r2 = 1;
+    b = (r1 != r2); assert(b);
+    b = (r1 == r2); assert(!b);
+    b = (r1 <  r2); assert(!b);
+    b = (r1 >  r2); assert(!b);
+    b = (r1 <= r2); assert(!b);
+    b = (r1 >= r2); assert(!b);
 
-        double d1 = double.nan;
-        double d2 = 1;
-        b = (d1 != d2); assert(b);
-        b = (d1 == d2); assert(!b);
-        b = (d1 <  d2); assert(!b);
-        b = (d1 >  d2); assert(!b);
-        b = (d1 <= d2); assert(!b);
-        b = (d1 >= d2); assert(!b);
+    double d1 = double.nan;
+    double d2 = 1;
+    b = (d1 != d2); assert(b);
+    b = (d1 == d2); assert(!b);
+    b = (d1 <  d2); assert(!b);
+    b = (d1 >  d2); assert(!b);
+    b = (d1 <= d2); assert(!b);
+    b = (d1 >= d2); assert(!b);
 
-        float f1 = float.nan;
-        float f2 = 1;
-        b = (f1 != f2); assert(b);
-        b = (f1 == f2); assert(!b);
-        b = (f1 <  f2); assert(!b);
-        b = (f1 >  f2); assert(!b);
-        b = (f1 <= f2); assert(!b);
-        b = (f1 >= f2); assert(!b);
-        return true;
+    float f1 = float.nan;
+    float f2 = 1;
+    b = (f1 != f2); assert(b);
+    b = (f1 == f2); assert(!b);
+    b = (f1 <  f2); assert(!b);
+    b = (f1 >  f2); assert(!b);
+    b = (f1 <= f2); assert(!b);
+    b = (f1 >= f2); assert(!b);
+    return true;
 }
 
 void main()