Better error messages for common Function/Tuple mistakes

Firstly, for `((a, b) => c): (Tuple2[A, B] => C)`, we currently
just offer "missing parameter type." Is something of a rite of
passage to know that you need `{ case (...)}`

This commit stops short DWIM, but does offer a diagnostic to guide
the user towards the supported way of destructuring a `Tuple` in
the sole argument of a `Function1`.

Secondly, another (less common?) way one might try to write a function
to destructure a single tuple argument is:

    (((a, b)) => c)

The parser now matches offers a specific error message for this, and
points out the alternatives.

In both cases, we avoid offering syntactically invalid alternatives,
by detecting names that aren't valid as variable-patterns, and
falling back to generic "paramN" in the error message.

A handly utility function to sequence a list of options is liberated
from the pattern matcher for broader use.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 6d799b0098..4973346eff 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -2906,6 +2906,7 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
       else if (argpts.lengthCompare(numVparams) != 0)
         WrongNumberOfParametersError(fun, argpts)
       else {
+        var issuedMissingParameterTypeError = false
         foreach2(fun.vparams, argpts) { (vparam, argpt) =>
           if (vparam.tpt.isEmpty) {
             vparam.tpt.tpe =
@@ -2923,7 +2924,8 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
                     }
                   case _ =>
                 }
-                MissingParameterTypeError(fun, vparam, pt)
+                MissingParameterTypeError(fun, vparam, pt, withTupleAddendum = !issuedMissingParameterTypeError)
+                issuedMissingParameterTypeError = true
                 ErrorType
               }
             if (!vparam.tpt.pos.isDefined) vparam.tpt setPos vparam.pos.focus