From e42d883c99c8f6747d78650e3683a228e9962342 Mon Sep 17 00:00:00 2001
From: Oliver Bracevac <bracevac.git@gmail.com>
Date: Sun, 6 Oct 2024 23:53:06 +0200
Subject: [PATCH] Fix rewrite logic for old `<function> _` syntax

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

Fixes #21394
---
 .../src/dotty/tools/dotc/typer/Migrations.scala |  9 ++++++---
 .../dotty/tools/dotc/CompilationTests.scala     |  1 +
 tests/rewrites/i21394.check                     | 17 +++++++++++++++++
 tests/rewrites/i21394.scala                     | 17 +++++++++++++++++
 4 files changed, 41 insertions(+), 3 deletions(-)
 create mode 100644 tests/rewrites/i21394.check
 create mode 100644 tests/rewrites/i21394.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Migrations.scala b/compiler/src/dotty/tools/dotc/typer/Migrations.scala
index 7f27f27112a0..f0d1d235a19c 100644
--- a/compiler/src/dotty/tools/dotc/typer/Migrations.scala
+++ b/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 _ =>
         ("(() => ", ")")
     }
diff --git a/compiler/test/dotty/tools/dotc/CompilationTests.scala b/compiler/test/dotty/tools/dotc/CompilationTests.scala
index 0c5d5764949a..d7ef7f6f6085 100644
--- a/compiler/test/dotty/tools/dotc/CompilationTests.scala
+++ b/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")),
diff --git a/tests/rewrites/i21394.check b/tests/rewrites/i21394.check
new file mode 100644
index 000000000000..50e6ab0901b6
--- /dev/null
+++ b/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 = ???
\ No newline at end of file
diff --git a/tests/rewrites/i21394.scala b/tests/rewrites/i21394.scala
new file mode 100644
index 000000000000..33f183fbed48
--- /dev/null
+++ b/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 = ???
\ No newline at end of file