Only rename method type params shadowing enclosing class type params

Previously, method type parameters were always renamed if they had the same name as any class type parameter, regardless of whether the class type parameter was actually used in the method signature.

As a result, we rename methods's type params for unnecessary names like: #24671
(Note that renaming actually doesn't cause binary incompatibility, and this change is just for make generic signature a bit clear and silence false-positive MIMA reporting).

For example, in an example below, the Scala compiler rename the generic signature of `bar` to something like `bar[T1](x: T1): T1` because `T` is used by `Foo[T]`. However, this is unnessary rename because none of T in method signature refer the `T` of `Foo[T]`.

```scala
class Foo[T]:
  def bar[T](x: T): T = ???
```

This commit makes the renaming conditional:
Method type parameters are only renamed when
- (1) A class type parameter is referenced in the method signature, and
- (2) That class type parameter is shadowed by a method type parameter

Author: tanishiking

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 +546,48 @@ 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]
+
+    def collect(tp: Type): Unit = 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
+          collect(pre)
+      case AppliedType(tycon, args) =>
+        collect(tycon)
+        args.foreach(collect)
+      case AndType(tp1, tp2) =>
+        collect(tp1)
+        collect(tp2)
+      case OrType(tp1, tp2) =>
+        collect(tp1)
+        collect(tp2)
+      case RefinedType(parent, _, refinedInfo) =>
+        collect(parent)
+        collect(refinedInfo)
+      case TypeBounds(lo, hi) =>
+        collect(lo)
+        collect(hi)
+      case ExprType(res) =>
+        collect(res)
+      case AnnotatedType(tpe, _) =>
+        collect(tpe)
+      case defn.ArrayOf(elemtp) =>
+        collect(elemtp)
+      case _ =>
+        ()
+
+    types.foreach(collect)
+    (usedMethodTypeParamNames.toSet, usedClassTypeParams.toSet)
+  end collectUsedTypeParams
 }

tests/generic-java-signatures/shadowedTypeParam.check

@@ -0,0 +1,7 @@
+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)
+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 +1,42 @@
+// 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)
+
+// 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$1 still have `<T1> T Bar.copy$default$1`
+  // but it should be `Bar<T> Bar.copy$default$1`
+  // (It shouldn't have unused type param, and return type should be Bar<T>)
+
+  val jpfMethods = classOf[JavaPartialFunction[_, _]].getDeclaredMethods()
+  printMethodSigs(jpfMethods, "compose")
+
+def printMethodSig(methods: Array[java.lang.reflect.Method], name: String): Unit =
+  methods.find(_.getName.endsWith(name)).foreach { m =>
+    println(s"$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}")
+  }