SI-7752 Don't disambiguate type parameters of overloaded alts

These are passed through from `InferencerContextErrors#applyErrorMsg`
to `withDisambiguation` as the `locals` parameter, which is promptly
ignored. This looks to be an unintended change in 39938bcc299.

Without this patch, the enclosed test case enters a pathalogical
disambiguation session, that not only flirts with unpleasant big-O
complexities, but also flails about appending "(in method foo)"
only to find that *all* occurences of the same-named type parameter
come from some method named "foo".

    [snipping error message 40 seconds in the making]
    method foo), O(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo), P(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo), Q(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo), R(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo), S(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo), T, U(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo)(in method foo), V)
     cannot be applied to (Int)
      foo((1))

1 files changed, 7 insertions, 4 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala b/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala
index 4950a7efef..2270e812eb 100644
--- a/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala
@@ -366,11 +366,14 @@ trait TypeDiagnostics {
     val strings = mutable.Map[String, Set[TypeDiag]]() withDefaultValue Set()
     val names   = mutable.Map[Name, Set[TypeDiag]]() withDefaultValue Set()
 
-    def record(t: Type, sym: Symbol) = {
-      val diag = TypeDiag(t, sym)
+    val localsSet = locals.toSet
 
-      strings("" + t) += diag
-      names(sym.name) += diag
+    def record(t: Type, sym: Symbol) = {
+      if (!localsSet(sym)) {
+        val diag = TypeDiag(t, sym)
+        strings("" + t) += diag
+        names(sym.name) += diag
+      }
     }
     for (tpe <- types ; t <- tpe) {
       t match {