Fix rewrite logic for old <function> _ syntax (#21715)

A rewrite would previously produce uncompilable
code if the access path to the eta-expanded
function goes through at least one `def`.

Fixes #21394

Author: bracevac

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

@@ -71,10 +71,13 @@ trait Migrations:
     }
     nestedCtx.typerState.commit()
 
+    def functionPrefixSuffix(arity: Int) = if (arity > 0) ("", "") else ("(() => ", "())")
+
     lazy val (prefix, suffix) = res match {
-      case Block(mdef @ DefDef(_, vparams :: Nil, _, _) :: Nil, _: Closure) =>
-        val arity = vparams.length
-        if (arity > 0) ("", "") else ("(() => ", "())")
+      case Block(DefDef(_, vparams :: Nil, _, _) :: Nil, _: Closure) =>
+        functionPrefixSuffix(vparams.length)
+      case Block(ValDef(_, _, _) :: Nil, Block(DefDef(_, vparams :: Nil, _, _) :: Nil, _: Closure)) =>
+        functionPrefixSuffix(vparams.length)
       case _ =>
         ("(() => ", ")")
     }

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -63,6 +63,7 @@ class CompilationTests {
       compileFile("tests/rewrites/rewrites.scala", defaultOptions.and("-source", "3.0-migration").and("-rewrite", "-indent")),
       compileFile("tests/rewrites/rewrites3x.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
       compileFile("tests/rewrites/rewrites3x-fatal-warnings.scala", defaultOptions.and("-rewrite", "-source", "future-migration", "-Xfatal-warnings")),
+      compileFile("tests/rewrites/i21394.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
       compileFile("tests/rewrites/uninitialized-var.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
       compileFile("tests/rewrites/with-type-operator.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
       compileFile("tests/rewrites/private-this.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),

tests/rewrites/i21394.check

@@ -0,0 +1,17 @@
+trait Container:
+    def loopDef: Container
+    val loopVal: Container
+    def fun(x: Int)(y: Int): Int
+
+def test(c: Container): Int =
+    use(c.fun)
+    + use(c.loopDef.fun)
+    + use(c.loopVal.fun)
+    + use(c.loopDef.loopDef.fun)
+    + use(c.loopVal.loopVal.fun)
+    + use(c.loopVal.loopDef.fun)
+    + use(c.loopDef.loopVal.fun)
+    + use(c.loopVal.loopDef.loopVal.fun)
+    + use(c.loopVal.loopDef.loopVal.loopDef.fun)
+
+def use(f: Int => Int => Int): Int = ???

tests/rewrites/i21394.scala

@@ -0,0 +1,17 @@
+trait Container:
+    def loopDef: Container
+    val loopVal: Container
+    def fun(x: Int)(y: Int): Int
+
+def test(c: Container): Int =
+    use(c.fun _)
+    + use(c.loopDef.fun _)
+    + use(c.loopVal.fun _)
+    + use(c.loopDef.loopDef.fun _)
+    + use(c.loopVal.loopVal.fun _)
+    + use(c.loopVal.loopDef.fun _)
+    + use(c.loopDef.loopVal.fun _)
+    + use(c.loopVal.loopDef.loopVal.fun _)
+    + use(c.loopVal.loopDef.loopVal.loopDef.fun _)
+
+def use(f: Int => Int => Int): Int = ???