Simplify (ambiguous) error issuing.

The two functional differences are:
  - always add the diagnostics string
  - check erroneousness in `issueAmbiguousTypeErrorUnlessErroneous`,
    before even constructing the error message.

Consider this nugget:

```
-    def issueAmbiguousError(pre: Type, sym1: Symbol, sym2: Symbol, err: AbsTypeError) {
-      issueCommon(err) { case _ if ambiguousErrors =>
-        if (!pre.isErroneous && !sym1.isErroneous && !sym2.isErroneous)
```

I'd like to state for the record that the if-erroneous in the
case of the partial function looked super-dodgy: it meant that,
when `ambiguousErrors`, `issueCommon` would not get to the `else` branches
that buffer or throw, and if the erroneous condition was met,
nothing would be issued/buffered/thrown.

This refactoring checks this condition up front.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
index 73c3e6f016..6b5a70bffb 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
@@ -1381,6 +1381,7 @@ trait Implicits {
         def maybeInvalidConversionError(msg: String) {
           // We have to check context.ambiguousErrors even though we are calling "issueAmbiguousError"
           // which ostensibly does exactly that before issuing the error. Why? I have no idea. Test is pos/t7690.
+          // AM: I would guess it's because ambiguous errors will be buffered in silent mode if they are not reported
           if (context.ambiguousErrors)
             context.issueAmbiguousError(AmbiguousImplicitTypeError(tree, msg))
         }