Do not create forwarders when an error has already occurred (#26220)

Fixes #26037

Author: SolalPirelli

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

@@ -1268,7 +1268,9 @@ class Namer { typer: Typer =>
                it suffices to check if symbol is the same class. */
             cls == id.symbol
           case _ => false
-        if !sym.isAccessibleFrom(pathType) then
+        if mbr.info.isInstanceOf[ErrorType] then
+          No("already has an error")
+        else if !sym.isAccessibleFrom(pathType) then
           No("is not accessible")
         else if sym.isConstructor || sym.is(ModuleClass) || sym.is(Bridge) || sym.is(PhantomSymbol) || sym.isAllOf(JavaModule) then
           Skip

compiler/test/dotc/neg-best-effort-pickling.excludelist

@@ -20,6 +20,7 @@ i11226.scala # missing type
 i974.scala # cyclic reference
 i13864.scala # missing symbol in pickling
 type-params.scala # recursion limit exceeded
+26037.scala # recursion limit exceeded
 
 unsafe-dollar-in-name.scala # missing owner in unpickler
 unsafe-dollar-in-name-2.scala # missing owner in unpickler

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

@@ -10,8 +10,8 @@ import org.junit.Ignore
   import CompilationTests.{*, given}
   import CompilationTest.aggregateTests
 
-  @Test def example: Unit =
-    implicit val testGroup: TestGroup = TestGroup("single-test")
+  @Test def example(): Unit =
+    given testGroup: TestGroup = TestGroup("single-test")
     // can add, e.g., .and("-some-option")
     val options = defaultOptions
     // can also use `compileDir` (single test as a dir), `compileFilesInDir` (all tests within a dir)
@@ -21,3 +21,14 @@ import org.junit.Ignore
     val compilationTest = withCoverage(aggregateTests(test))
     runWithCoverageOrFallback[TestKind](compilationTest, testGroup.name)
 
+  @Test def bestEffortTasty(): Unit =
+    given testGroup: TestGroup = TestGroup("single-test")
+    // can add, e.g., .and("-some-option")
+    val options = TestConfiguration.bestEffortBaselineOptions
+    // make sure we only test one thing at a time, despite the framework requiring an entire dir + filters
+    val testFile = "26037.scala"
+    dotc.comptest.compileBestEffortTastyInDir(
+      "tests/neg", options,
+      picklingFilter = FileFilter.include(List(testFile)),
+      unpicklingFilter = FileFilter.include(List(testFile))
+    ).checkNoCrash()

compiler/test/dotty/tools/vulpix/ParallelTesting.scala

@@ -1684,7 +1684,7 @@ trait ParallelTesting extends RunnerOrchestration with CoverageSupport:
     assert(!flags.options.contains(bestEffortFlag), "Best effort compilation flag should not be added manually")
 
     val outDir = new JFile(defaultOutputDir, testGroup.name)
-    val sourceDir = new JFile(f)
+    val sourceDir = TestSources.getPath(f).toFile
     checkRequirements(f, sourceDir, outDir)
 
     val (dirsStep1, filteredPicklingFiles) = compilationTargets(sourceDir, picklingFilter)

tests/neg/26037.scala

@@ -0,0 +1,8 @@
+object core:
+  opaque type T = String | List[T] // error: illegal cyclic type
+  object T:
+    def make(s: String): T = s
+
+object api:
+  export core.T
+  export core.T.*