Proparage function result constrains when inferring parameter types

If an application has functions with implicit parameter types we need to be
more aggressive about propagating knowledge of the expected result type into
the constraint.

Fixes #1378.

1 files changed, 10 insertions, 0 deletions

diff --git a/src/dotty/tools/dotc/typer/Applications.scala b/src/dotty/tools/dotc/typer/Applications.scala
index 6e78a570d..7a742112b 100644
--- a/src/dotty/tools/dotc/typer/Applications.scala
+++ b/src/dotty/tools/dotc/typer/Applications.scala
@@ -553,6 +553,16 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
       // a modified tree but this would be more convoluted and less efficient.
       if (proto.isTupled) proto = proto.tupled
 
+      // If some of the application's arguments are function literals without explicitly declared
+      // parameter types, and the expected type is a value type, relate the
+      // normalized result type of the application with the expected type through `<:<`.
+      // This can add more constraints which help sharpen the inferred parameter
+      // types for the argument function literal(s).
+      // This tweak is needed to make i1348 compile.
+      if (tree.args.exists(untpd.isFunctionWithImplicitParamType(_)))
+        if (!constrainResult(fun1.tpe.widen, proto.derivedFunProto(resultType = pt)))
+          typr.println(i"result failure for $tree with type ${fun1.tpe.widen}, expected = $pt")
+
       fun1.tpe match {
         case ErrorType => tree.withType(ErrorType)
         case TryDynamicCallType =>