Fix undercompilation when case class field type changes (issue #26231)

In Scala 3, the compiler generates `def unapply(x: C): C = x` for case
classes.  Its signature is always `(C): C` regardless of field types, so
its API hash never changes when a field is renamed or retyped.

The PatternMatcher phase (which runs after ExtractDependencies) lowers
`case C(x)` to a direct call to the product selector `_1()`, `_2()`, etc.
Because ExtractDependencies ran before PatternMatcher, those selector
calls were never in the tree, so `_1` was never recorded as a used name
in the dependent file.  Zinc therefore saw no reason to recompile the
file when the field type changed, producing a NoSuchMethodError at
runtime.

Fix: in `AbstractExtractDependenciesCollector.recordTree`, add a case for
`UnApply` that records each product selector (`_1`, `_2`, …) found on
the unapply's result type as a member-reference used name.  When the
selector's return type changes (because the field type changed) its name
hash changes and Zinc correctly invalidates the dependent file.

The key detail is that `fun.tpe` in an `UnApply` node is a `TermRef`;
we must call `.widen.finalResultType` to reach the underlying case-class
type before asking `Applications.productSelectors` for the `_N` members.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: retronym

compiler/src/dotty/tools/dotc/sbt/ExtractDependencies.scala

@@ -4,7 +4,6 @@ package sbt
 import java.io.File
 import java.nio.file.Path
 import java.util.EnumSet
-
 import dotty.tools.dotc.ast.tpd
 import dotty.tools.dotc.classpath.FileUtils.{hasClassExtension, hasTastyExtension}
 import dotty.tools.dotc.core.Contexts.*
@@ -16,16 +15,15 @@ import dotty.tools.dotc.core.Phases.*
 import dotty.tools.dotc.core.Symbols.*
 import dotty.tools.dotc.core.Denotations.StaleSymbol
 import dotty.tools.dotc.core.Types.*
-
-import dotty.tools.dotc.util.{SrcPos, NoSourcePosition}
+import dotty.tools.dotc.typer.Applications.*
+import dotty.tools.dotc.util.{NoSourcePosition, SrcPos}
 import dotty.tools.io
-import dotty.tools.io.{AbstractFile, PlainFile, ZipArchive, NoAbstractFile, FileExtension}
+import dotty.tools.io.{AbstractFile, FileExtension, NoAbstractFile, PlainFile, ZipArchive}
 import xsbti.UseScope
 import xsbti.api.DependencyContext
 import xsbti.api.DependencyContext.*
 
 import scala.jdk.CollectionConverters.*
-
 import scala.collection.{Set, mutable}
 import scala.compiletime.uninitialized
 
@@ -265,6 +263,18 @@ trait AbstractExtractDependenciesCollector(rec: DependencyRecorder) extends tpd.
         addInheritanceDependencies(t)
       case t: Template =>
         addInheritanceDependencies(t)
+      case UnApply(fun, implicits, patterns) =>
+        // For a case-class unapply of the form `def unapply(x: C): C = x`, the
+        // compiler emits calls to the product selector methods `_1`, `_2`, … in
+        // the generated bytecode (via PatternMatcher, which runs after this phase).
+        // Those selectors are never explicit in the typed tree, so they would not
+        // normally be recorded as used names.  We record them here so that Zinc
+        // knows to recompile this file if the return type of any selector changes.
+        //
+        // fun.tpe is a TermRef; widen first to reach the underlying MethodType,
+        // then take finalResultType to get the case class type (e.g. Customer2).
+        val selectors = productSelectors(fun.tpe.widen.finalResultType)
+        selectors.foreach(addMemberRefDependency)
       case _ => ()
 
   /**Reused EqHashSet, safe to use as each TypeDependencyTraverser is used atomically

sbt-bridge/test/xsbt/ExtractAPISpecification.scala

@@ -258,8 +258,7 @@ class ExtractAPISpecification {
       usedNames("Test").contains("unapply"))
 
     // `_1` must also be recorded because it is what the generated bytecode calls.
-    // TODO: this currently FAILS, documenting the undercompilation bug.
-    assertTrue("_1 must be a used name in Test (currently missing — undercompilation bug)",
+    assertTrue("_1 must be a used name in Test",
       usedNames("Test").contains("_1"))
   }