Lift all non trivial prefixes for default parameters

Checking if the prefix is pure is not enough to know if we need to list
the prefix. In the case of default parameters, the prefix tree might be
used several times to compute the default values. This expression should
only be computed once and therefore it should be lifted if there is some
computation/allocation involved. Furthermore, if the prefix contains a
local definition, it must be lifted to avoid duplicating the definition.

A similar situation could happen with dependent default parameters. This
currently works as expected.

Fixes #15315

Author: nicolasstucki

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

@@ -122,7 +122,10 @@ abstract class Lifter {
     case TypeApply(fn, targs) =>
       cpy.TypeApply(tree)(liftApp(defs, fn), targs)
     case Select(pre, name) if isPureRef(tree) =>
-      cpy.Select(tree)(liftPrefix(defs, pre), name)
+      val liftedPrefix =
+        if tree.symbol.is(HasDefaultParams) then liftPrefix(defs, pre)
+        else liftNonIdempotentPrefix(defs, pre)
+      cpy.Select(tree)(liftedPrefix, name)
     case Block(stats, expr) =>
       liftApp(defs ++= stats, expr)
     case New(tpt) =>
@@ -138,8 +141,26 @@ abstract class Lifter {
    *
    *  unless `pre` is idempotent.
    */
-  def liftPrefix(defs: mutable.ListBuffer[Tree], tree: Tree)(using Context): Tree =
+  def liftNonIdempotentPrefix(defs: mutable.ListBuffer[Tree], tree: Tree)(using Context): Tree =
     if (isIdempotentExpr(tree)) tree else lift(defs, tree)
+
+  /** Lift prefix `pre` of an application `pre.f(...)` to
+   *
+   *     val x0 = pre
+   *     x0.f(...)
+   *
+   *  unless `pre` is idempotent reference, a `this` reference, a literal value, or a or the prefix of an `init` (`New` tree).
+   *
+   *  Note that default arguments will refer to the prefix, we do not want
+   *  to re-evaluate a complex expression each time we access a getter.
+   */
+  def liftPrefix(defs: mutable.ListBuffer[Tree], tree: Tree)(using Context): Tree =
+    tree match
+      case tree: Literal => tree
+      case tree: This => tree
+      case tree: New => tree // prefix of <init> call
+      case tree: RefTree if isIdempotentExpr(tree) => tree
+      case _ => lift(defs, tree)
 }
 
 /** No lifting at all */

compiler/test/dotty/tools/backend/jvm/DottyBytecodeTests.scala

@@ -1733,6 +1733,58 @@ class DottyBytecodeTests extends DottyBytecodeTest {
       assertSameCode(instructions, expected)
     }
   }
+
+  @Test def newInPrefixesOfDefaultParam = {
+    val source =
+      s"""class A:
+         |  def f(x: Int = 1): Int = x
+         |
+         |class Test:
+         | def meth1() = (new A).f()
+         | def meth2() = { val a = new A; a.f() }
+         """.stripMargin
+
+    checkBCode(source) { dir =>
+      val clsIn      = dir.lookupName("Test.class", directory = false).input
+      val clsNode    = loadClassNode(clsIn)
+      val meth1      = getMethod(clsNode, "meth1")
+      val meth2      = getMethod(clsNode, "meth2")
+
+      val instructions1 = instructionsFromMethod(meth1)
+      val instructions2 = instructionsFromMethod(meth2)
+
+      assert(instructions1 == instructions2,
+        "`assert` was not properly inlined in `meth1`\n" +
+        diffInstructions(instructions1, instructions2))
+    }
+  }
+
+  @Test def newInDependentOfDefaultParam = {
+    val source =
+      s"""class A:
+         |  def i: Int = 1
+         |
+         |class Test:
+         |  def f(a: A)(x: Int = a.i): Int = x
+         |  def meth1() = f(new A)()
+         |  def meth2() = { val a = new A; f(a)() }
+         """.stripMargin
+
+    checkBCode(source) { dir =>
+      val clsIn      = dir.lookupName("Test.class", directory = false).input
+      val clsNode    = loadClassNode(clsIn)
+      val meth1      = getMethod(clsNode, "meth1")
+      val meth2      = getMethod(clsNode, "meth2")
+
+      val instructions1 = instructionsFromMethod(meth1)
+      val instructions2 = instructionsFromMethod(meth2)
+
+      assert(instructions1 == instructions2,
+        "`assert` was not properly inlined in `meth1`\n" +
+        diffInstructions(instructions1, instructions2))
+    }
+  }
+
 }
 
 object invocationReceiversTestCode {

tests/run/i15315.scala

@@ -0,0 +1,5 @@
+class A:
+  def f(x: Int = 1): Int = x
+
+@main def Test() =
+  (new A{}).f()