Add explanation

Author: odersky

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

@@ -258,6 +258,28 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
         // method could be constructed as the definition site of the type variable for
         // that default constructor. This would interpolate type variables too early,
         // causing lots of tests (among them tasty_unpickleScala2) to fail.
+        //
+        // The test case is in i1757.scala. Here we have a variable `s` and a method `cpy`
+        // defined like this:
+        //
+        //      var s
+        //      def cpy[X](b: List[Int] = b): B[X] = new B[X](b)
+        //
+        // The call `s.cpy()` then gets expanded to
+        //
+        //      { val $1$: B[Int] = this.s
+        //        $1$.cpy[X']($1$.cpy$default$1[X']
+        //      }
+        //
+        // A type variable gets interpolated if it does not appear in the type
+        // of the current tree and the current tree contains the variable's "definition".
+        // Previously, the polymorphic function tree to which the variable was first added
+        // was taken as the variable's definition. But that fails here because that
+        // tree was `s.cpy` but got transformed into `$1$.cpy`. We now take the type argument
+        // [X'] of the variable as its definition tree, which is more robust. But then
+        // it's crucial that the type tree is not copied directly as argument to
+        // `cpy$default$1`. If it was, the variable `X'` would already be interpolated
+        // when typing the default argument, which is too early.
         spliceMeth(meth, fn).appliedToTypes(targs.tpes)
       case _ => meth
     }