Inconsistent type-checking for implicits with inferred return type

[scabug]

An implicit with an inferred return type is (sometimes) visible before it's point of definition, despite the comments on #801.

Example in the wild is in specs2

• a clean compile succeeds
• an incremental compile that touches only that file fails

[error] specs2/matcher/src/main/scala/org/specs2/matcher/MatchResultMessages.scala:13: could not find implicit value for parameter mm: scalaz.Monoid[Product with Serializable with MatchResultMessages.this.MatchResultMessage]
[error]     Reducer.unitReducer { r: MatchResult[T] => r match {
[error]                         ^
[error] one error found
[error] (matcher/compile:compileIncremental) Compilation failed

See discussion on Scala contributors

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-10229?orig=1
Reporter: @dragos
Affected Versions: 2.12.1
See #801

[SethTisue]

Note that Scala 2.13 now warns by default when an implicit doesn't have an explicitly declared type.

Scala 3 errors.