Merge pull request #2764 from RazvanN7/Issue_3831

Fix Issue 3831 - writeln of a delegate typeid
merged-on-behalf-of: Nicholas Wilson <thewilsonator@users.noreply.github.com>

Author: dlang-bot

src/object.d

@@ -1534,7 +1534,20 @@ class TypeInfo_Delegate : TypeInfo
 {
     override string toString() const
     {
-        return cast(string)(next.toString() ~ " delegate()");
+        import core.demangle : demangleType;
+
+        alias SafeDemangleFunctionType = char[] function (const(char)[] buf, char[] dst = null) @safe nothrow pure;
+        SafeDemangleFunctionType demangle = ( () @trusted => cast(SafeDemangleFunctionType)(&demangleType) ) ();
+
+        return (() @trusted => cast(string)(demangle(deco))) ();
+    }
+
+    unittest
+    {
+        double sqr(double x) { return x * x; }
+        assert(typeid(typeof(&sqr)).toString() == "double delegate(double) pure nothrow @nogc @safe");
+        int g;
+        assert(typeid(typeof((int a, int b) => a + b + g)).toString() == "int delegate(int, int) pure nothrow @nogc @safe");
     }
 
     override bool opEquals(Object o)