Fix #6734: Suppress interpolattion for extension methods

Suppress interpolation in the middle of an ExtMethodApply. The reason is that
the ExtMethodApply has a WildcardType, which hides information about bound type
variables in the result type.

Author: odersky

compiler/src/dotty/tools/dotc/typer/Inferencing.scala

@@ -384,7 +384,9 @@ trait Inferencing { this: Typer =>
     // `qualifying`.
 
     val ownedVars = state.ownedVars
-    if ((ownedVars ne locked) && !ownedVars.isEmpty) {
+    if ((ownedVars ne locked) && !ownedVars.isEmpty &&
+        !tree.isInstanceOf[Applications.IntegratedTypeArgs] // suppress interpolation in the middle of an extension method application
+        ) {
       val qualifying = ownedVars -- locked
       if (!qualifying.isEmpty) {
         typr.println(i"interpolate $tree: ${tree.tpe.widen} in $state, owned vars = ${state.ownedVars.toList}%, %, previous = ${locked.toList}%, % / ${state.constraint}")

tests/pos/i6734.scala

@@ -0,0 +1,11 @@
+object Bug {
+
+  def (ab: (A, B)) pipe2[A, B, Z](f: (A, B) => Z): Z = f(ab._1, ab._2)
+
+  def (a: A) leftErr[A, B](b: B): A = (a, b).pipe2((a, b) => a) //Did not compile before.
+  def (a: A) leftOk1[A, B](b: B): A = Tuple2(a, b).pipe2((a, b) => a) //Compiles
+  def (a: A) leftOk2[A, B](b: B): A = {
+    val t = (a, b)
+    t.pipe2((a, b) => a) //Compiles
+  }
+}