Asserting `Duration` values between a range in `const` constructors gives an error only when calling

[FMorschel]

Write this in dartpad.dev:

class A{
  const A(this.d) : assert(d >= Duration.zero);
  
  final Duration d;
}

void main () {
  const a = A(Duration(days: 1));
}

You'll get this error only on line 8:

In constant expressions, operands of this operator must be of type 'num'.

I tested it on my local development machine and still got the same error.

My question is, why can't this error be shown in line 2? I do believe this should appear there since packages that do not have tests would never see this and since this example only requires a single Duration as a parameter (I have a package that could have a future case like this), this class should not be able to be constant.

Second question, why can't this be asserted in constant expressions even if Durations are constant classes? Can't this be converted to Duration.inMicroseconds to assert internally?

[mraleph]

This is working as intended though not very user friendly. For comparison CFE reports this much more clearly (though still leaks a bit of internal details in the formatting of the error message cc @johnniwinther )

lib/main.dart:8:13:
Error: Constant evaluation error:
  const a = A(Duration(days: 1));
            ^
lib/main.dart:2:30:
Info: The method '>=' can't be invoked on 'Duration {dart.core::_duration: 86400000000.0}' in a constant expression.
 - 'Duration' is from 'dart:core'.
  const A(this.d) : assert(d >= Duration.zero);
                             ^
lib/main.dart:8:9:
Info: While analyzing:
  const a = A(Duration(days: 1));

This is working as specified because it is not an error to declare such constructor it is error to use it in const context. You can only invoke it with new. I am not entirely sure why it is specified this way cc @eernstg @lrhn.

cc @bwilkerson @scheglov - I think this a commonly asked question. Do you know if there an umbrella issue for reporting a better error in the analyzer and/or reporting some sort of hint/warning that this const constructor is not valid.

[FMorschel]

This was somewhat strange to me because, in the following example, I get:

class A {
  const A(this.s);
  
  const A.asserted(this.s) : assert(s.isEmpty); //Invalid constant value. (about `s.isEmpty`)
  
  final List s;
}

So to me, there seems to be a missing case not accounted for that error or something else going on there.

[lrhn]

It's specified that way because the parser author at the time wanted to be able to recognize potentially constant expressions, without having types available, so potentially constant expressions are defined entirely by syntax.
Then it's a compile time error to evaluate them as constant if the values do not have the right type.

That was a bad idea.
It means you can write a potentially constant expression in a constant constructor which can never be invoked as const.
We should fix it, but it's never been a high priority.