Add suggested fix to error message when casting struct to bool

Author: dkorpel

compiler/src/dmd/expressionsem.d

@@ -16843,8 +16843,12 @@ Expression toBoolean(Expression exp, Scope* sc)
             if (!t.isBoolean())
             {
                 if (tb != Type.terror)
+                {
                     error(exp.loc, "expression `%s` of type `%s` does not have a boolean value",
                               exp.toChars(), t.toChars());
+                    if (auto ts = tb.isTypeStruct())
+                        errorSupplemental(ts.sym.loc, "perhaps add Cast Operator Overloading with `bool opCast(T : bool)() => ...`");
+                }
                 return ErrorExp.get();
             }
             return e;

compiler/test/fail_compilation/diag13320.d

@@ -1,15 +1,23 @@
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/diag13320.d(14): Error: operator `++` not supported for `f` of type `Foo`
-fail_compilation/diag13320.d(9):        perhaps implement `auto opUnary(string op : "++")() {}` or `auto opOpAssign(string op : "+")(int) {}`
+fail_compilation/diag13320.d(20): Error: operator `++` not supported for `f` of type `Foo`
+fail_compilation/diag13320.d(14):        perhaps implement `auto opUnary(string op : "++")() {}` or `auto opOpAssign(string op : "+")(int) {}`
+fail_compilation/diag13320.d(21): Error: expression `f` of type `Foo` does not have a boolean value
+fail_compilation/diag13320.d(14):        perhaps add Cast Operator Overloading with `bool opCast(T : bool)() => ...`
+fail_compilation/diag13320.d(22): Error: expression `Foo()` of type `E` does not have a boolean value
+fail_compilation/diag13320.d(14):        perhaps add Cast Operator Overloading with `bool opCast(T : bool)() => ...`
 ---
 */
 
+
 struct Foo {}
+enum E : Foo { a = Foo.init }
 
 void main()
 {
     Foo f;
     ++f;
+    if (f) {}
+    assert(E.a);
 }