Make toDelegate safe for function pointers

Co-authored-by: Elias Batek <0xEAB@users.noreply.github.com>

Author: Bolpat

std/functional.d

@@ -1814,13 +1814,42 @@ if (isCallable!(F))
     {
         return fp;
     }
+    else static if (is(F Func == Func*) && is(Func == function) && is(Func Params == __parameters))
+    {
+        return () @trusted
+        {
+            return buildDelegate(fp);
+        }();
+    }
     else static if (is(typeof(&F.opCall) == delegate)
                 || (is(typeof(&F.opCall) V : V*) && is(V == function)))
     {
         return toDelegate(&fp.opCall);
     }
     else
     {
+        return buildDelegate(fp);
+    }
+}
+
+///
+@safe unittest
+{
+    static int inc(ref uint num) {
+        num++;
+        return 8675309;
+    }
+
+    uint myNum = 0;
+    auto incMyNumDel = toDelegate(&inc);
+    auto returnVal = incMyNumDel(myNum);
+    assert(myNum == 1);
+}
+
+private template buildDelegate(F)
+if (isCallable!(F))
+{
+    auto buildDelegate(auto ref F fp) {
         alias DelType = typeof(&(new DelegateFaker!(F)).doIt);
 
         static struct DelegateFields {
@@ -1850,21 +1879,22 @@ if (isCallable!(F))
     }
 }
 
-///
-@system unittest
+@safe unittest
 {
     static int inc(ref uint num) {
         num++;
         return 8675309;
     }
 
-    uint myNum = 0;
-    auto incMyNumDel = toDelegate(&inc);
-    auto returnVal = incMyNumDel(myNum);
-    assert(myNum == 1);
+    uint myNum = 0x1337;
+    struct S1 { int opCall() { inc(myNum); return myNum; } }
+    static assert(!is(typeof(&s1.opCall) == delegate));
+    S1 s1;
+    auto getvals1 = toDelegate(s1);
+    assert(getvals1() == 0x1338);
 }
 
-@system unittest // not @safe due to toDelegate
+@system unittest
 {
     static int inc(ref uint num) {
         num++;