Only rename method type params shadowing enclosing class type params

compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala

@@ -49,24 +49,21 @@ object GenericSignatures {
     val builder = new StringBuilder(64)
     val isTraitSignature = sym0.enclosingClass.is(Trait)
 
-    // Collect class-level type parameter names to avoid conflicts with method-level type parameters
-    val usedNames = collection.mutable.Set.empty[String]
-    if(sym0.is(Method)) {
-      sym0.enclosingClass.typeParams.foreach { tp =>
-        usedNames += sanitizeName(tp.name)
-      }
-    }
+    // Track class type parameter names that are shadowed by method type parameters
+    // Used to trigger renaming of method type parameters to avoid conflicts
+    val shadowedClassTypeParamNames = collection.mutable.Set.empty[String]
     val methodTypeParamRenaming = collection.mutable.Map.empty[String, String]
+
     def freshTypeParamName(sanitizedName: String): String = {
-      if !usedNames.contains(sanitizedName) then sanitizedName
+      if !shadowedClassTypeParamNames.contains(sanitizedName) then sanitizedName
       else {
         var i = 1
         var newName = sanitizedName + i
-        while usedNames.contains(newName) do
+        while shadowedClassTypeParamNames.contains(newName) do
           i += 1
           newName = sanitizedName + i
         methodTypeParamRenaming(sanitizedName) = newName
-        usedNames += newName
+        shadowedClassTypeParamNames += newName
         newName
       }
     }
@@ -347,7 +344,22 @@ object GenericSignatures {
 
         case mtd: MethodOrPoly =>
           val (tparams, vparams, rte) = collectMethodParams(mtd)
-          if (toplevel && !sym0.isConstructor) polyParamSig(tparams)
+          if (toplevel && !sym0.isConstructor) {
+            if (sym0.is(Method)) {
+              val (usedMethodTypeParamNames, usedClassTypeParams) = collectUsedTypeParams(vparams :+ rte, sym0)
+              val methodTypeParamNames = tparams.map(tp => sanitizeName(tp.paramName.lastPart)).toSet
+              // Only add class type parameters to shadowedClassTypeParamNames if they are:
+              // 1. Referenced in the method signature, AND
+              // 2. Shadowed by a method type parameter with the same name
+              // This will trigger renaming of the method type parameter
+              usedClassTypeParams.foreach { classTypeParam =>
+                val classTypeParamName = sanitizeName(classTypeParam.name)
+                if methodTypeParamNames.contains(classTypeParamName) then
+                  shadowedClassTypeParamNames += classTypeParamName
+              }
+            }
+            polyParamSig(tparams)
+          }
           builder.append('(')
           for vparam <- vparams do jsig1(vparam)
           builder.append(')')
@@ -448,6 +460,12 @@ object GenericSignatures {
         (initialSymbol.is(Method) && initialSymbol.typeParams.contains(sym))
       )
 
+  private def isTypeParameterInMethSig(sym: Symbol, initialSymbol: Symbol)(using Context) =
+    !sym.maybeOwner.isTypeParam &&
+      sym.isTypeParam && (
+        (initialSymbol.is(Method) && initialSymbol.typeParams.contains(sym))
+      )
+
   // @M #2585 when generating a java generic signature that includes
   // a selection of an inner class p.I, (p = `pre`, I = `cls`) must
   // rewrite to p'.I, where p' refers to the class that directly defines
@@ -528,4 +546,27 @@ object GenericSignatures {
     val rte = recur(mtd)
     (tparams.toList, vparams.toList, rte)
   end collectMethodParams
+
+  /** Collect type parameters that are actually used in the given types. */
+  private def collectUsedTypeParams(types: List[Type], initialSymbol: Symbol)(using Context): (Set[Name], Set[Symbol]) =
+    val usedMethodTypeParamNames = collection.mutable.Set.empty[Name]
+    val usedClassTypeParams = collection.mutable.Set.empty[Symbol]
+    val collector = new TypeTraverser:
+      def traverse(tp: Type) = tp.dealias match
+        case ref @ TypeParamRef(_: PolyType, _) =>
+          usedMethodTypeParamNames += ref.paramName
+        case TypeRef(pre, _) =>
+          val sym = tp.typeSymbol
+          if isTypeParameterInMethSig(sym, initialSymbol) then
+            usedMethodTypeParamNames += sym.name
+          else if sym.isTypeParam && sym.isContainedIn(initialSymbol.topLevelClass) then
+            usedClassTypeParams += sym
+          else
+            traverse(pre)
+        case _ =>
+          traverseChildren(tp)
+
+    types.foreach(collector.traverse)
+    (usedMethodTypeParamNames.toSet, usedClassTypeParams.toSet)
+  end collectUsedTypeParams
 }

tests/generic-java-signatures/shadowedTypeParam.check

@@ -0,0 +1,9 @@
+bar: public <T> T Foo.bar(T)
+baz: public <T> T Foo.baz(T,Foo<T>)
+qux: public <U> U Foo.qux(U,Foo<T>)
+quux: public <T,U> scala.Tuple2<T, U> Foo.quux(T,U,Foo<T>)
+copy: public <T> Bar<T> Bar.copy(T)
+copy$default$1: public <T1> T Bar.copy$default$1()
+m: public <T1> T C.m()
+compose: public <A1> scala.Function1<A1, B> JavaPartialFunction.compose(scala.Function1<A1, A>)
+compose: public <R> scala.PartialFunction<R, B> JavaPartialFunction.compose(scala.PartialFunction<R, A>)

tests/generic-java-signatures/shadowedTypeParam.scala

@@ -0,0 +1,53 @@
+// All of type params shouldn't rename
+class Foo[T]:
+  def bar[T](x: T): T = x
+  def baz[T](x: T, y: Foo[T]): T = x
+  def qux[U](x: U, y: Foo[T]): U = x
+  def quux[T, U](x: T, y: U, z: Foo[T]): (T, U) = (x, y)
+
+// https://github.com/scala/scala3/issues/24671
+final case class Bar[+T](t: T)
+
+// `m: public <T1> T C.m()` rather than `m: public <T> T C.m()`
+// that was wrong and could be crashed with
+// `val c = C[String]; String x = c.<Object>m();`.
+abstract class C[T]:
+  def x: T
+  def m[T] = x
+
+// https://github.com/scala/scala3/issues/24134
+// The mixin forwarders for compose in Function1 method  has signature
+// `def compose[A](g: A => T1): A => R`
+// Where the JavaPartialFunction[A, B] has type parameter A (name clash),
+// The type parameter A in method should be renamed to avoid name duplication.
+abstract class JavaPartialFunction[A, B] extends PartialFunction[A, B]
+
+@main def Test(): Unit =
+  val fooMethods = classOf[Foo[_]].getDeclaredMethods()
+  printMethodSig(fooMethods, "bar")
+  printMethodSig(fooMethods, "baz")
+  printMethodSig(fooMethods, "qux")
+  printMethodSig(fooMethods, "quux")
+
+  val barMethods = classOf[Bar[_]].getDeclaredMethods()
+  printMethodSig(barMethods, "copy")
+  // copy$default$1 have `<T1> T Bar.copy$default$1` rather than `<T> T Bar.copy$default$1`
+  // as reported in https://github.com/scala/scala3/issues/24671
+  // The type parameter rename occurs because the return type T refers the enclosing class's type param T.
+  printMethodSig(barMethods, "copy$default$1")
+
+  val cMethods = classOf[C[_]].getDeclaredMethods()
+  printMethodSig(cMethods, "m")
+
+  val jpfMethods = classOf[JavaPartialFunction[_, _]].getDeclaredMethods()
+  printMethodSigs(jpfMethods, "compose")
+
+def printMethodSig(methods: Array[java.lang.reflect.Method], name: String): Unit =
+  methods.find(_.getName.endsWith(name)).foreach { m =>
+    println(s"$name: ${m.toGenericString}")
+  }
+
+def printMethodSigs(methods: Array[java.lang.reflect.Method], name: String): Unit =
+  methods.filter(_.getName == name).sortBy(_.toGenericString).foreach { m =>
+    println(s"$name: ${m.toGenericString}")
+  }