Refine pretypeArgs

It worked more or less by accident before. Now it's more complicated,
but we also have tests.

1 files changed, 25 insertions, 4 deletions

diff --git a/src/dotty/tools/dotc/typer/Applications.scala b/src/dotty/tools/dotc/typer/Applications.scala
index b3a71408b..d655b25f6 100644
--- a/src/dotty/tools/dotc/typer/Applications.scala
+++ b/src/dotty/tools/dotc/typer/Applications.scala
@@ -1151,9 +1151,30 @@ trait Applications extends Compatibility { self: Typer =>
         }
         arg match {
           case arg: untpd.Function if arg.args.exists(isUnknownParamType) =>
-            val commonFormal = altFormals.map(_.head).reduceLeft(_ | _)
-            overload.println(i"pretype arg $arg with expected type $commonFormal")
-            pt.typedArg(arg, commonFormal)
+            def isUniform[T](xs: List[T])(p: (T, T) => Boolean) = xs.forall(p(_, xs.head))
+            val formalsForArg: List[Type] = altFormals.map(_.head)
+            // For alternatives alt_1, ..., alt_n, test whether formal types for current argument are of the form
+            //   (p_1_1, ..., p_m_1) => r_1
+            //   ...
+            //   (p_1_n, ..., p_m_n) => r_n
+            val decomposedFormalsForArg: List[Option[(List[Type], Type)]] =
+              formalsForArg.map(defn.FunctionOf.unapply)
+            if (decomposedFormalsForArg.forall(_.isDefined)) {
+              val formalParamTypessForArg: List[List[Type]] =
+                decomposedFormalsForArg.map(_.get._1)
+              if (isUniform(formalParamTypessForArg)((x, y) => x.length == y.length)) {
+                val commonParamTypes = formalParamTypessForArg.transpose.map(ps =>
+                  // Given definitions above, for i = 1,...,m,
+                  //   ps(i) = List(p_i_1, ..., p_i_n)  -- i.e. a column
+                  // If all p_i_k's are the same, assume the type as formal parameter
+                  // type of the i'th parameter of the closure.
+                  if (isUniform(ps)(ctx.typeComparer.isSameTypeWhenFrozen(_, _))) ps.head
+                  else WildcardType)
+                val commonFormal = defn.FunctionOf(commonParamTypes, WildcardType)
+                overload.println(i"pretype arg $arg with expected type $commonFormal")
+                pt.typedArg(arg, commonFormal)
+              }
+            }
           case _ =>
         }
         recur(altFormals.map(_.tail), args1)
@@ -1161,7 +1182,7 @@ trait Applications extends Compatibility { self: Typer =>
     }
     def paramTypes(alt: Type): List[Type] = alt match {
       case mt: MethodType => mt.paramTypes
-      case mt: PolyType => paramTypes(mt.resultType).map(wildApprox(_))
+      case mt: PolyType => paramTypes(mt.resultType)
       case _ => Nil
     }
     recur(alts.map(alt => paramTypes(alt.widen)), pt.args)