Less eager tvar interpolation: wait until method calls are fully applied

Fix #738

Author: smarter

src/dotty/tools/dotc/typer/Typer.scala

@@ -563,7 +563,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
             val pos = params indexWhere (_.name == param.name)
             if (pos < mtpe.paramTypes.length) {
               val ptype = mtpe.paramTypes(pos)
-              if (isFullyDefined(ptype, ForceDegree.none)) return ptype
+              if (isFullyDefined(ptype, ForceDegree.noBottom)) return ptype
             }
           case _ =>
         }
@@ -1265,7 +1265,16 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
 
   def adapt(tree: Tree, pt: Type, original: untpd.Tree = untpd.EmptyTree)(implicit ctx: Context) = /*>|>*/ track("adapt") /*<|<*/ {
     /*>|>*/ ctx.traceIndented(i"adapting $tree of type ${tree.tpe} to $pt", typr, show = true) /*<|<*/ {
-      interpolateUndetVars(tree, if (tree.isDef) tree.symbol else NoSymbol)
+      val isMethodCall =
+        tree.tpe.widen match {
+          case (_: MethodType) | (_: PolyType) if !tree.isDef =>
+            true
+          case _ =>
+            false
+        }
+      if (!isMethodCall) // Delay tvar interpolation in method calls until they're fully applied
+        interpolateUndetVars(tree, if (tree.isDef) tree.symbol else NoSymbol)
+
       tree.overwriteType(tree.tpe.simplified)
       adaptInterpolated(tree, pt, original)
     }

tests/pos/tparam_inf.scala

@@ -0,0 +1,12 @@
+object Test {
+  def foo1[T](x: T)(implicit ev: T): Nothing = ???
+  def foo2[T](x: T)(implicit ev: T): T = ???
+
+  def test: Unit = {
+    implicit val ii: Int = 42
+
+    foo1(10)
+    foo2(10)
+  }
+}
+