Constructor parameters: `(a: Int = 0)(b: Int = 0)` is not equivalent to `()()`

[Sporarum]

Compiler version

Scala 3.3.0

Minimized code & Output

trait Obj1(a: Int = 0)(b: Int = 0)

trait Obj2()()

// Expected:
new Obj1()(){}
new Obj2()(){}

// Unexpected:
new Obj1{}      // missing arguments for constructor Obj1 in trait Obj1
new Obj2{}

https://scastie.scala-lang.org/SRf7kAOKQWuUAm8yegbFtg

Expectation

No error, or a similar error for Obj2{} (or better)

[Sporarum]

Has impact on trait parameters: #17988

[odersky]

It's because of this test in Typer.ensureConstrCall:

psym.primaryConstructor.info.takesParams

The test does not take default parameters into account (it cannot, for reasons given below). For Obj1 it returns false, for Obj2, true. That is the fork in the treatment. If the primaryConstructor takes parameters, we need to consstruct a constructor application to pass default arguments. But we can insert at most one (), which can then expand to default parameters, hence the error.

I don't think it's worth trying to fix this.