Don't check import selectors for Java code

Moving import selector checking from PostTyper to Typer had the
side-effect that they were also checked in Java code. But that does
not work since the Java files are compiled before the Scala files
so the selected reference has not been seen yet.

Fixed by disabling import selector checks for Java files.

Fixes #15608

Author: odersky

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

@@ -1460,8 +1460,9 @@ trait Checking {
         report.error(ImportRenamedTwice(sel.imported), sel.imported.srcPos)
       seen += sel.name
 
-    for sel <- selectors do
-      if !sel.isWildcard then checkIdent(sel)
+    if !ctx.compilationUnit.isJava then
+      for sel <- selectors do
+        if !sel.isWildcard then checkIdent(sel)
   end checkImportSelectors
 }
 

tests/pos/i15608/JavaMDCBinder.java

@@ -0,0 +1,20 @@
+package foo.slf4j.impl;
+
+import bar.slf4j.MDCAdapter;
+import bar.slf4j.ScalaMDCAdapter;
+import bar.slf4j.ScalaMDCAdapter$;
+
+public class JavaMDCBinder {
+    public static final JavaMDCBinder SINGLETON = new JavaMDCBinder();
+
+    private JavaMDCBinder() {
+    }
+
+    public MDCAdapter getMDCA() {
+        return ScalaMDCAdapter$.MODULE$;
+    }
+
+    public String getMDCAdapterClassStr() {
+        return ScalaMDCAdapter.class.getName();
+    }
+}

tests/pos/i15608/Test.scala

@@ -0,0 +1,6 @@
+package bar.slf4j
+
+trait MDCAdapter
+
+object ScalaMDCAdapter extends MDCAdapter {
+}