fix(#17255): cannot find Scala companion module from Java (#19773)

To find Scala companion mudule from Java, we should strip module suffix
`$`.
This ~provides workaround~ fixes #17255 as well as forward-port
scala/scala#10644. ~, but it requires some
refinment to fix it because not-fully-qualified type like the following
example still fails to compile due to missing symbol.~

Related to scala/scala#10644

Author: bishabosha

compiler/src/dotty/tools/dotc/core/ContextOps.scala

@@ -41,7 +41,10 @@ object ContextOps:
             else pre.findMember(name, pre, required, excluded)
           }
           else // we are in the outermost context belonging to a class; self is invisible here. See inClassContext.
-            ctx.owner.findMember(name, ctx.owner.thisType, required, excluded)
+            if ctx.isJava then
+              javaFindMember(name, ctx.owner.thisType, lookInCompanion = true,required, excluded)
+            else
+              ctx.owner.findMember(name, ctx.owner.thisType, required, excluded)
         else
           ctx.scope.denotsNamed(name).filterWithFlags(required, excluded).toDenot(NoPrefix)
       }
@@ -55,11 +58,20 @@ object ContextOps:
     final def javaFindMember(name: Name, pre: Type, lookInCompanion: Boolean, required: FlagSet = EmptyFlags, excluded: FlagSet = EmptyFlags): Denotation =
       assert(ctx.isJava)
       inContext(ctx) {
-
+        import dotty.tools.dotc.core.NameOps.*
         val preSym = pre.typeSymbol
-
         // 1. Try to search in current type and parents.
-        val directSearch = pre.findMember(name, pre, required, excluded)
+        val directSearch =
+          def asModule =
+            if name.isTypeName && name.endsWith(StdNames.str.MODULE_SUFFIX) then
+              pre.findMember(name.stripModuleClassSuffix.moduleClassName, pre, required, excluded) match
+                case NoDenotation => NoDenotation
+                case symDenot: SymDenotation =>
+                  symDenot.companionModule.denot
+            else NoDenotation
+          pre.findMember(name, pre, required, excluded) match
+            case NoDenotation => asModule
+            case denot => denot
 
         // 2. Try to search in companion class if current is an object.
         def searchCompanionClass = if lookInCompanion && preSym.is(Flags.Module) then

compiler/test/dotc/pos-test-pickling.blacklist

@@ -29,6 +29,7 @@ i16649-irrefutable.scala
 strict-pattern-bindings-3.0-migration.scala
 i17186b.scala
 i11982a.scala
+i17255
 
 # Tree is huge and blows stack for printing Text
 i7034.scala

compiler/test/dotc/run-test-pickling.blacklist

@@ -44,3 +44,5 @@ t6138
 t6138-2
 i12656.scala
 trait-static-forwarder
+i17255
+

tests/pos/i17255/Bar.java

@@ -0,0 +1,6 @@
+package example;
+
+
+public class Bar {
+	private static final example.Foo$ MOD = example.Foo$.MODULE$;
+}

tests/pos/i17255/Baz.java

@@ -0,0 +1,7 @@
+package example;
+
+import example.Foo$;
+
+public class Baz {
+	private static final Foo$ MOD = Foo$.MODULE$;
+}

tests/pos/i17255/Foo.scala

@@ -0,0 +1,5 @@
+package example
+
+case class Foo(i: Int)
+
+object Foo

tests/run/i17255/J.java

@@ -0,0 +1,16 @@
+package p;
+
+public class J {
+	public static J j = new J();
+	public static p.J f() {
+		return p.J.j;
+	}
+	public static Module$ module2() {
+		return p.Module$.MODULE$;
+	}
+	public static p.Module$ module() {
+		return p.Module$.MODULE$;
+	}
+
+	public String toString() { return "J"; }
+}

tests/run/i17255/Module.scala

@@ -0,0 +1,13 @@
+// scalajs: --skip
+
+package p {
+  object Module {
+    override def toString = "Module"
+  }
+}
+
+object Test extends App {
+  assert(p.J.f().toString == "J")
+  assert(p.J.module().toString == "Module")
+  assert(p.J.module2().toString == "Module")
+}