Fix #8881: Lift lets over applications

This is needed to correctly deal with bunched arguments arisign from IFTs.
It's necessary in particular for let-bindings introduced by the compiler,
e.g. when passing default araguments. User-defined blocks are not the
problem since they get already eta expanded in typer.

Author: odersky

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -93,6 +93,7 @@ class Compiler {
          new FunctionXXLForwarders,  // Add forwarders for FunctionXXL apply method
          new ParamForwarding,        // Add forwarders for aliases of superclass parameters
          new TupleOptimizations,     // Optimize generic operations on tuples
+         new LetOverApply,            // Lift blocks from receivers of applications
          new ArrayConstructors) ::   // Intercept creation of (non-generic) arrays and intrinsify.
     List(new Erasure) ::             // Rewrite types to JVM model, erasing all type parameters, abstract types and refinements.
     List(new ElimErasedValueType,    // Expand erased value types to their underlying implmementation types

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

@@ -0,0 +1,37 @@
+package dotty.tools
+package dotc
+package transform
+
+import core._
+import MegaPhase._
+import Contexts.Context
+import Symbols._, Decorators._, Types._
+import ast.Trees._
+import dotty.tools.dotc.ast.tpd
+
+
+import scala.collection.immutable.::
+
+
+/** Rewrite `{ stats; expr}.f(args) }` to `{ stats; expr.f(args) }` before
+ *  proceeding, but leave closures alone. This is necessary to be able to
+ *  collapse applies of IFTs.
+ */
+class LetOverApply extends MiniPhase:
+  import ast.tpd._
+
+  override def phaseName: String = "letOverApply"
+
+  override def transformApply(tree: tpd.Apply)(using Context): tpd.Tree =
+    tree.fun match
+      case Select(blk @ Block(stats, expr), name) if !expr.isInstanceOf[Closure] =>
+        cpy.Block(blk)(stats,
+          cpy.Apply(tree)(
+            cpy.Select(tree.fun)(expr, name), tree.args))
+      case Block(stats, expr) =>
+        cpy.Block(tree.fun)(stats,
+          cpy.Apply(tree)(expr, tree.args))
+      case _ =>
+        tree
+
+end LetOverApply

tests/pos/i8881.scala

@@ -0,0 +1,5 @@
+object Crash {
+  def f(a: String, b: String, c: Int = 0): Int ?=> String = ""
+  given Int = ???
+  f(b = "b", a = "a")
+}