add a few comments

Author: SethTisue

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -740,7 +740,8 @@ class Definitions {
   @tu lazy val MethodHandlesLookupClass: ClassSymbol = requiredClass("java.lang.invoke.MethodHandles.Lookup")
   @tu lazy val VarHandleClass: ClassSymbol           = requiredClass("java.lang.invoke.VarHandle")
 
-  // from the Java language spec: https://docs.oracle.com/javase/specs/jls/se11/html/jls-15.html#jls-15.12.3
+  // Java language spec: https://docs.oracle.com/javase/specs/jls/se11/html/jls-15.html#jls-15.12.3
+  // Scala 2 spec: https://scala-lang.org/files/archive/spec/2.13/06-expressions.html#signature-polymorphic-methods
   def isPolymorphicSignature(sym: Symbol) = sym.is(JavaDefined) && {
     val owner = sym.maybeOwner
     (owner == MethodHandleClass || owner == VarHandleClass)

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

@@ -947,6 +947,10 @@ trait Applications extends Compatibility {
                 else expectedResultType match
                   case SelectionProto(nme.asInstanceOf_, PolyProto(_, resTp), _, _) => resTp
                   case _ => defn.ObjectType
+              // synthesize a method type based on the types at the call site.
+              // one can imagine the original signature-polymorphic method as
+              // being infinitely overloaded, with each individual overload only
+              // being brought into existence as needed
               val info = MethodType(proto.typedArgs().map(_.tpe.widen), resultType)
               val fun2 = fun1.withType(funRef.symbol.copy(info = info).termRef)
               simpleApply(fun2, proto)

tasty/src/dotty/tools/tasty/TastyFormat.scala

@@ -86,7 +86,7 @@ Standard-Section: "ASTs" TopLevelStat*
                   IDENT                 NameRef Type                               -- Used when term ident’s type is not a TermRef
                   SELECT                possiblySigned_NameRef qual_Term           -- qual.name
                   SELECTin       Length possiblySigned_NameRef qual_Term owner_Type -- qual.name, referring to a symbol declared in owner that has the given signature (see note below)
-                  SELECTinPoly   Length possiblySigned_NameRef qual_Term owner_Type method_Type -- like SELECTin, but with the method_Type too
+                  SELECTinPoly   Length possiblySigned_NameRef qual_Term owner_Type method_Type -- like SELECTin, but with the method_Type too (because signature polymorphic)
                   QUALTHIS              typeIdent_Tree                             -- id.this, different from THIS in that it contains a qualifier ident with position.
                   NEW                   clsType_Term                               -- new cls
                   THROW                 throwableExpr_Term                         -- throw throwableExpr