Bogus Compile error on implicit ambiguity

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Code located here: https://github.com/axos88/scala-bug-implicit-ambiguity
Unfortunately I did not find an easy way to reduce it further, or remove the dependency on spray-json.

Compiler currently errs with:

$ sbt ';clean;compile'
[info] Set current project to implicit-bug (in build file:///)
[success] Total time: 0 s, completed Jul 14, 2016 4:42:01 PM
[info] Resolving jline#jline;2.12.1 ...
[info] Done updating.
[info] Compiling 2 Scala sources to <pwd>/target/scala-2.11/classes...
[error] <pwd>/src/main/scala/example/json/JsonProtocol.scala:30: could not find implicit value for evidence parameter of type spray.json.DefaultJsonProtocol.JF[example.Foo]
[error]     implicit val bazFormat = jsonFormat2(Baz)
[error]                                         ^
[error] one error found
[error] (compile:compileIncremental) Compilation failed
[error] Total time: 3 s, completed Jul 14, 2016 4:42:04 PM

Take a look at the file: https://github.com/axos88/scala-bug-implicit-ambiguity/blob/master/src/main/scala/example/json/JsonProtocol.scala

The following things fix the error:

1. CHANGING THE ORDER of line 30 and 31 (! wtf?)
2. Removing the T: TypeTag restriction from line 11 (resulting in def stringFormat[T])
3. Removing line 22, or the implicit from Foo's companion class (makes sense).

The fact that #1 fixes it suggests it is a compiler bug, or an undetected undefined behaviour.

Expected Behaviour:

Compilation should succeed, and fooFormatHighPrio should take precedence over fooFormatLowPrio and FooFormatLowestPrio when one has JsonProtocol._ in scope.

OR

Error with ambigous implicits for JsonFormat[Foo]

Current Behaviour:

Misterious error message about NOT finding an implicit, whereas there are multiple candidates with different priorities.

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Imported From: https://issues.scala-lang.org/browse/SI-9861?orig=1
Reporter: Akos Vandra (axos88)
Affected Versions: 2.11.8
See #8697

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@som-snytt said:
I haven't tried, but I see your implicits don't have explicit types. Relying on inferred types creates textual ordering dependencies, so it's not a good idea and eventually may be disallowed.

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Akos Vandra (axos88) said:
Hello.

Explicitly stating the type of the quxFormat fixes the compilation error. Now this is AGAIN something that seems unrelated to the line for which the implicitnot found error is thrown.

I hope you agree that this is not expected behaviour, and that the raise error does not help at all identifying the issue.
Can you please explain what is happening here?

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Akos Vandra (axos88) said:
The compiler should be abel to imply the type of the implicits easily.

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Akos Vandra (axos88) said:
This is not the duplicate of #8697.

The implicits whose order is changed in the file are NOT related, and have nothing in common with the implicit that is NOT being found.
Basically adding a new line with an implicit value AFTER the questioned file creates an error in the line before.

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@som-snytt said (edited on Jul 16, 2016 7:48:21 PM UTC):
Moving stringFormat to object package lets it compile without other changes. In fact, importing only JsonProtocol.stringFormat suffices.

Sorry for the edit noise. Just following typer-debug output.

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@SethTisue said:
I'm afraid it's unlikely this will get further attention unless you can isolate and minimize it further. I'm provisionally closing the ticket; we can reopen if you can successfully isolate it.

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@som-snytt said:
This is the one where the call to stringFormat in object Foo induces implicit search (augmentString and a TypeTag), which causes the imported symbols to be checked. Conversely to my previous comment,

import JsonProtocol.{ stringFormat, barFormat, bazFormat } 

suffices to break it. barFormat uses stringFormat, baz uses formatter for Foo, Bar.