Probably regression in v3.7.x: false positive warning with `-Wunused:explicits`

[satorg]

Compiler version

The issue is reproducible starting with Scala v3.7.x: 3.7.0, 3.7.1 as well as 3.7.2-RC1-bin-20250609-41cf6eb-NIGHTLY.
It does not appear in Scala v3.3.x and v3.6.x.

Minimized code

//> using scala 3.7.nightly
//> using options -Wunused:explicits

// An external class that doesn't get its own `copy` method.
class Foo(val a: String, val b: Int)

//
// Example 1: add `copy` method via an extension method.
//
extension (self: Foo)
  def copy(a: String = self.a, b: Int = self.b): Foo = Foo(a, b)

//
// Example 2: implement `copyFoo` with parameter groups.
//
def copyFoo(foo: Foo)(a: String = foo.a, b: Int = foo.b): Foo = Foo(a, b)

Output

Starting with Scala v3.7.x only:

-- [E198] Unused Symbol Warning: .../unused-explicit-param.scala:10:11 
10 |extension (self: Foo)
   |           ^^^^
   |           unused explicit parameter in extension method copy
-- [E198] Unused Symbol Warning: .../unused-explicit-param.scala:16:12 
16 |def copyFoo(foo: Foo)(a: String = foo.a, b: Int = foo.b): Foo = Foo(a, b)
   |            ^^^
   |            unused explicit parameter
2 warnings found

No warnings in Scala v3.6.4 and v3.3.6.

Expectation

No warning whatsoever.

[SethTisue]

@som-snytt a bit of collateral damage from #20894 perhaps?

[som-snytt]

It's not designed to detect usages only in default args, but I also thought it doesn't warn for receivers of extensions.

Previous versions were false negatives anyway.

Edit: forgot to say, thanks for the report!

Also, thanks for the using directives, and for spelling "its" correctly. It's the little things!

Also thanks @SethTisue you are the whitelighter.

[satorg]

Btw, just realized that it's also the case with implict parameters too:

//> using scala 3.7.nightly
//> using options -Wunused:implicits

class Foo(val a: String)

def copyFoo(using foo: Foo)(a: String = foo.a): Foo = Foo(a)

which triggers

-- [E198] Unused Symbol Warning: .../unused-implicit-params.scala:7:18
7 |def copyFoo(using foo: Foo)(a: String = foo.a): Foo = Foo(a)
  |                  ^^^
  |                  unused implicit parameter

in Scala 3.7 only.