Run toString(null) for explicit nullable abstract

If the abstract is declared over Null<T>, then null is part of the type
so the user should be able to handle the null case within the toString
method. It is also useful to be able to have custom empty values for
an abstract that wraps a nullable type.

This is also in-line what null safety has to say, as it forces the user
to add a null check to toString when the concrete type is Null<T>.

src/typing/calls.ml

@@ -322,7 +322,7 @@ let rec needs_temp_var e =
 	| TField (e, _) | TParenthesis e -> needs_temp_var e
 	| _ -> true
 
-let call_to_string ctx ?(resume=false) e =
+let call_to_string ctx ?(resume=false) ?(no_null_check=false) e =
 	if not ctx.allow_transform then
 		{ e with etype = ctx.t.tstring }
 	else
@@ -333,7 +333,7 @@ let call_to_string ctx ?(resume=false) e =
 		ctx.f.meta <- List.tl ctx.f.meta;
 		build_call ctx acc [] (WithType.with_type ctx.t.tstring) e.epos
 	in
-	if ctx.com.config.pf_static && not (is_nullable e.etype) then
+	if no_null_check || (ctx.com.config.pf_static && not (is_nullable e.etype)) then
 		gen_to_string e
 	else begin (* generate `if(e == null) 'null' else e.toString()` *)
 		let string_null = mk (TConst (TString "null")) ctx.t.tstring e.epos in

src/typing/operators.ml

@@ -197,8 +197,8 @@ let make_binop ctx op e1 e2 is_assign_op p =
 	let tstring = ctx.t.tstring in
 	let to_string e =
 		let rec loop t = match classify t with
-			| KAbstract ({a_impl = Some c},_) when PMap.mem "toString" c.cl_statics ->
-				call_to_string ctx e
+			| KAbstract ({a_impl = Some c; a_this},_) when PMap.mem "toString" c.cl_statics ->
+				call_to_string ~no_null_check:(is_explicit_null a_this) ctx e
 			| KInt | KFloat | KString -> e
 			| KUnk | KDyn | KNumParam _ | KStrParam _ | KOther ->
 				Texpr.Builder.resolve_and_make_static_call ctx.com.std "string" [e] e.epos

src/typing/typer.ml

@@ -1603,7 +1603,8 @@ and type_meta ?(mode=MGet) ctx m e1 with_type p =
 		| (Meta.ToString,_,_) ->
 			let e = e() in
 			(match follow e.etype with
-				| TAbstract({a_impl = Some c},_) when PMap.mem "toString" c.cl_statics -> call_to_string ctx e
+				| TAbstract({a_impl = Some c; a_this},_) when PMap.mem "toString" c.cl_statics ->
+					call_to_string ~no_null_check:(is_explicit_null a_this) ctx e
 				| _ -> e)
 		| (Meta.Markup,_,_) ->
 			raise_typing_error "Markup literals must be processed by a macro" p
@@ -1725,8 +1726,8 @@ and type_call_builtin ctx e el mode with_type p =
 			let e = type_expr ctx e WithType.value in
 			let infos = type_expr ctx infos WithType.value in
 			let e = match follow e.etype with
-				| TAbstract({a_impl = Some c},_) when PMap.mem "toString" c.cl_statics ->
-					call_to_string ctx e
+				| TAbstract({a_impl = Some c; a_this},_) when PMap.mem "toString" c.cl_statics ->
+					call_to_string ~no_null_check:(is_explicit_null a_this) ctx e
 				| _ ->
 					e
 			in