Nowarn for first typecheck of macro invocation

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It would be useful if macro invocations could get a free pass on warnings before expansion.

That way, a build can run with warnings turned high and fatal, even if a macro does something like turn "a pure expression [that] does nothing in statement position" into a productive member of society, for example, by wrapping a boolean in an assert.

Perhaps enable it with -Ymacro-no-warn, and perhaps try to buffer the warnings and emit them if there are warnings after expansion. (The old warnings may or may not be meaningful for a given macro.)

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Imported From: https://issues.scala-lang.org/browse/SI-8077?orig=1
Reporter: @som-snytt
Affected Versions: 2.11.0-M7

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@xeno-by said:
Could you provide a use case?

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@som-snytt said:
Sorry, I think my test was bad that day. In fact, I don't see the warnings.

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Teo Klestrup Röijezon (teozkr) said:
Running into this in a project of mine. It runs a block of statements, and replaces it with the REPL output. Using this triggers warnings for pure statements (like literals) in positions other than the last statement.

Declaration
Usage

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@xeno-by said:
Even something as simple as this triggers the decribed issue:

import scala.reflect.macros.whitebox._
import scala.language.experimental.macros

object Macros {
  def impl(c: Context)(x: c.Tree) = {
    import c.universe._
    q"()"
  }

  def foo(x: Any): Any = macro impl
}

object Test extends App {
  Macros.foo {
    1
    2
  }
}

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@som-snytt said:
Thanks for reopening. I was faked out that day because the warning is emitted on { 2 == 1 } but not on

{ val x = 1 ; 2 == x }

[SethTisue]

scala/scala#8995 improves the situation w/r/t to "a pure expression does nothing in statement position" warnings in particular