Mitigate change in status of scala.caps (#22956)

`scala.caps` was an object until 3.6, it is a package from 3.7. Without
special handling this would cause a TypeError to be thrown if a build
has several versions of the Scala standard library on the classpath.
This was the case for 29 projects in the open CB. These projects should
be updated. But until that's the case we issue a warning instead of a
hard failure.

Author: WojciechMazur

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

@@ -78,7 +78,7 @@ object SymbolLoaders {
    *  and give them `completer` as type.
    */
   def enterPackage(owner: Symbol, pname: TermName, completer: (TermSymbol, ClassSymbol) => PackageLoader)(using Context): Symbol = {
-    val preExisting = owner.info.decls lookup pname
+    val preExisting = owner.info.decls.lookup(pname)
     if (preExisting != NoSymbol)
       // Some jars (often, obfuscated ones) include a package and
       // object with the same name. Rather than render them unusable,
@@ -95,6 +95,18 @@ object SymbolLoaders {
           s"Resolving package/object name conflict in favor of object ${preExisting.fullName}.  The package will be inaccessible.")
         return NoSymbol
       }
+      else if pname == nme.caps && owner == defn.ScalaPackageClass then
+        // `scala.caps`` was an object until 3.6, it is a package from 3.7. Without special handling
+        // this would cause a TypeError to be thrown below if a build has several versions of the
+        // Scala standard library on the classpath. This was the case for 29 projects in OpenCB.
+        // These projects should be updated. But until that's the case we issue a warning instead
+        // of a hard failure.
+        report.warning(
+          em"""$owner contains object and package with same name: $pname.
+             |This indicates that there are several versions of the Scala standard library on the classpath.
+             |The build should be reconfigured so that only one version of the standard library is on the classpath.""")
+        owner.info.decls.openForMutations.unlink(preExisting)
+        owner.info.decls.openForMutations.unlink(preExisting.moduleClass)
       else
         throw TypeError(
           em"""$owner contains object and package with same name: $pname

tests/neg/i22890.check

@@ -0,0 +1,7 @@
+-- [E161] Naming Error: tests/neg/i22890/caps_1.java:3:0 ---------------------------------------------------------------
+3 |class caps { // error: caps is already defined as package caps
+  |^
+  |caps is already defined as package scala.caps
+package scala contains object and package with same name: caps.
+This indicates that there are several versions of the Scala standard library on the classpath.
+The build should be reconfigured so that only one version of the standard library is on the classpath.

tests/neg/i22890/Test_2.scala

@@ -0,0 +1,3 @@
+@main def Test =
+  println("hello")
+// nopos-warn

tests/neg/i22890/caps_1.java

@@ -0,0 +1,5 @@
+package scala;
+
+class caps { // error: caps is already defined as package caps
+  static public void foo() {}
+}