Make SigPoly symbols resistant to withPrefix changes

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -1238,8 +1238,7 @@ class TreeUnpickler(reader: TastyReader,
               val fn = readTerm()
               val methType = readType()
               val args = until(end)(readTerm())
-              val sym2 = fn.symbol.copy(info = methType) // symbol not entered (same as in simpleApply)
-              val fun2 = fn.withType(sym2.termRef)
+              val fun2 = typer.Applications.retypeSignaturePolymorphicFn(fn, methType)
               tpd.Apply(fun2, args)
             case TYPED =>
               val expr = readTerm()

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

@@ -341,6 +341,12 @@ object Applications {
     val getter = findDefaultGetter(fn, n, testOnly)
     if getter.isEmpty then getter
     else spliceMeth(getter.withSpan(fn.span), fn)
+
+  def retypeSignaturePolymorphicFn(fun: Tree, methType: Type)(using Context): Tree =
+    val sym1 = fun.symbol
+    val flags2 = sym1.flags | NonMember // ensures Select typing doesn't let TermRef#withPrefix revert the type
+    val sym2 = sym1.copy(info = methType, flags = flags2) // symbol not entered, to avoid overload resolution problems
+    fun.withType(sym2.termRef)
 }
 
 trait Applications extends Compatibility {
@@ -950,8 +956,7 @@ trait Applications extends Compatibility {
                 case resTp if isFullyDefined(resTp, ForceDegree.all) => resTp
                 case _ => defn.ObjectType
             val methType = MethodType(proto.typedArgs().map(_.tpe.widen), resultType)
-            val sym2 = funRef.symbol.copy(info = methType) // symbol not entered, to avoid overload resolution problems
-            val fun2 = fun1.withType(sym2.termRef)
+            val fun2 = Applications.retypeSignaturePolymorphicFn(fun1, methType)
             simpleApply(fun2, proto)
           case funRef: TermRef =>
             val app = ApplyTo(tree, fun1, funRef, proto, pt)

tests/explicit-nulls/pos/i11332.scala

@@ -0,0 +1,18 @@
+// scalajs: --skip
+import scala.language.unsafeNulls
+
+import java.lang.invoke._, MethodType.methodType
+
+// A copy of tests/run/i11332.scala
+// to test the bootstrap minimisation which failed
+// (because bootstrap runs under explicit nulls)
+class Foo:
+  def neg(x: Int): Int = -x
+
+  val l    = MethodHandles.lookup()
+  val self = new Foo()
+
+  val test = // testing as a expression tree - previously derivedSelect broke the type
+    l
+      .findVirtual(classOf[Foo], "neg", methodType(classOf[Int], classOf[Int]))
+      .invokeExact(self, 4): Int

tests/run/i11332.scala

@@ -16,6 +16,7 @@ class Foo {
   def id[T](x: T): T = x
 
   val l = MethodHandles.lookup()
+  val self    = new Foo()
   val mhNeg   = l.findVirtual(classOf[Foo], "neg", methodType(classOf[Int], classOf[Int]))
   val mhRev   = l.findVirtual(classOf[Foo], "rev", methodType(classOf[String], classOf[String]))
   val mhOverL = l.findVirtual(classOf[Foo], "over", methodType(classOf[String], classOf[Long]))
@@ -24,25 +25,25 @@ class Foo {
   val mhObj   = l.findVirtual(classOf[Foo], "obj", methodType(classOf[Any], classOf[String]))
   val mhCL    = l.findStatic(classOf[ClassLoader], "getPlatformClassLoader", methodType(classOf[ClassLoader]))
 
-  val testNeg1 = assert(-42 == (mhNeg.invokeExact(this, 42): Int))
-  val testNeg2 = assert(-33 == (mhNeg.invokeExact(this, 33): Int))
+  val testNeg1 = assert(-42 == (mhNeg.invokeExact(self, 42): Int))
+  val testNeg2 = assert(-33 == (mhNeg.invokeExact(self, 33): Int))
 
-  val testRev1  = assert("oof" == (mhRev.invokeExact(this, "foo"): String))
-  val testRev2  = assert("rab" == (mhRev.invokeExact(this, "bar"): String))
+  val testRev1  = assert("oof" == (mhRev.invokeExact(self, "foo"): String))
+  val testRev2  = assert("rab" == (mhRev.invokeExact(self, "bar"): String))
 
-  val testOverL = assert("long" == (mhOverL.invokeExact(this, 1L): String))
-  val testOVerI = assert("int" == (mhOverI.invokeExact(this, 1): String))
+  val testOverL = assert("long" == (mhOverL.invokeExact(self, 1L): String))
+  val testOVerI = assert("int" == (mhOverI.invokeExact(self, 1): String))
 
-  val testNeg_tvar = assert(-3 == (id(mhNeg.invokeExact(this, 3)): Int))
-  val testNeg_obj  = expectWrongMethod(mhNeg.invokeExact(this, 4))
+  val testNeg_tvar = assert(-3 == (id(mhNeg.invokeExact(self, 3)): Int))
+  val testNeg_obj  = expectWrongMethod(mhNeg.invokeExact(self, 4))
 
-  val testUnit_exp = { mhUnit.invokeExact(this, "hi"): Unit; () }
-  val testUnit_val = { val hi2: Unit = mhUnit.invokeExact(this, "hi2"); assert((()) == hi2) }
-  val testUnit_def = { def hi3: Unit = mhUnit.invokeExact(this, "hi3"); assert((()) == hi3) }
+  val testUnit_exp = { mhUnit.invokeExact(self, "hi"): Unit; () }
+  val testUnit_val = { val hi2: Unit = mhUnit.invokeExact(self, "hi2"); assert((()) == hi2) }
+  val testUnit_def = { def hi3: Unit = mhUnit.invokeExact(self, "hi3"); assert((()) == hi3) }
 
-  val testObj_exp = { mhObj.invokeExact(this, "any"); () }
-  val testObj_val = { val any2 = mhObj.invokeExact(this, "any2"); assert("any2" == any2) }
-  val testObj_def = { def any3 = mhObj.invokeExact(this, "any3"); assert("any3" == any3) }
+  val testObj_exp = { mhObj.invokeExact(self, "any"); () }
+  val testObj_val = { val any2 = mhObj.invokeExact(self, "any2"); assert("any2" == any2) }
+  val testObj_def = { def any3 = mhObj.invokeExact(self, "any3"); assert("any3" == any3) }
 
   val testCl_pass  = assert(null != (mhCL.invoke(): ClassLoader))
   val testCl_cast  = assert(null != (mhCL.invoke().asInstanceOf[ClassLoader]: ClassLoader))
@@ -51,10 +52,10 @@ class Foo {
 
   val testNeg_inline_obj = expectWrongMethod(l
     .findVirtual(classOf[Foo], "neg", methodType(classOf[Int], classOf[Int]))
-    .invokeExact(this, 3))
+    .invokeExact(self, 3))
   val testNeg_inline_pass = assert(-4 == (l
     .findVirtual(classOf[Foo], "neg", methodType(classOf[Int], classOf[Int]))
-    .invokeExact(this, 4): Int))
+    .invokeExact(self, 4): Int))
 
   def expectWrongMethod(op: => Any) = try {
     op