Unexpected ambiguous argument in inline given instance

[Iltotore]

Compiler version

3.1.0

Minimized code

class Test[A, B]

inline given [A, B](using A): Test[A, Test[A, B]] = new Test

given Int = 0
given String = ""

summon[Test[Int, Test[Int, String]]] //Working

Output

The above example works but if I try to summon[Test[Int, Test[String, Int]]], this produces an ambiguous implicit argument at using A.

I did some tests and noticed these behaviours:

• Works when removing the inline modifiers (skipped because Test[Int, Test[String, Int]] doesn't match Test[A, Test[A, B]])
• Works if there's no using A (skipped because Test[Int, Test[String, Int]] doesn't match Test[A, Test[A, B]])
• Works if A and B are covariant in Test
• Doesn't work if A and B are invariant or contravariant

From what I tested, the compiler seems to try to get the using A parameter where A = ? before checking the returned type.

Expectation

I would expect the compiler to just ignore inline given [A, B](using A): Test[A, Test[A, B]] = new Test when summoning Test[Int, Test[String, Int]] and search for another given instance (or raise a "no implicit found" error if there's no matching instance).

I actually see 2 potential solutions:

• Make the compiler verify the returned type when he tries to infer the generic types before looking for the using parameter(s)
• If a given instance produces an ambiguity error, check for another matching given instance before raising it.

Note: this is possibly related to #7586 and #14169

[Iltotore]

This issue seems to be fixed in 3.1.2