Can't properly disambiguate overloading when taking a lambda parameter

[anatoliykmetyuk]

Compiler version

3.1.2

Minimized code

trait Buffer
trait Arr

def dummy(pMessenger: Buffer): Int = ???
def dummy(pMessenger: Arr): Int = ???
def Test = query(dummy)

def query[Ptr <: Buffer](function: Ptr => Int) = ???

Output

[error] -- [E051] Reference Error: /Users/kmetiuk/Projects/GameDev/vulkan-tutorial/src/main/scala/LongBufferPointerBufferBug.scala:6:17
[error] 6 |def Test = query(dummy)
[error]   |                 ^^^^^
[error]   |Ambiguous overload. The overloaded alternatives of method dummy with types
[error]   | (pMessenger: Arr): Int
[error]   | (pMessenger: Buffer): Int
[error]   |both match expected type Ptr => Int
[error] one error found

Expectation

Should compile and resolve the overload to dummy taking Buffer. The following works:

trait Buffer
trait Arr

def dummy(pMessenger: Buffer): Int = ???
def dummy(pMessenger: Arr): Int = ???
def Test = query(dummy)

def query(function: Buffer => Int) = ???

[odersky]

Arr => Int matches Ptr => Int under the instantiation Ptr := Buffer & Arr. So the diagnosis is correct.

There could be a case made that Buffer & Arr is equivalent to Nothing, and we should try to avoid instantiatingPtr with such types, so in that sense the Buffer alternative is "better". But there is currently
no reasoning in spec or compiler along these lines, and it does not look trivial to add it. So I am re-classifying his as a feature request.