Introduce Context#isJava

Safer to use than ctx.compilationUnit.isJava since compilationUnit is
currently nullable (and ends up being null at least in some of our tests).

Author: smarter

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1625,7 +1625,7 @@ object desugar {
         Apply(Select(Apply(scalaDot(nme.StringContext), strs), id).withSpan(tree.span), elems)
       case PostfixOp(t, op) =>
         if ((ctx.mode is Mode.Type) && !isBackquoted(op) && op.name == tpnme.raw.STAR) {
-          if ctx.compilationUnit.isJava then
+          if ctx.isJava then
             AppliedTypeTree(ref(defn.RepeatedParamType), t)
           else
             Annotated(

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

@@ -363,6 +363,13 @@ object Contexts {
     /** Does current phase use an erased types interpretation? */
     final def erasedTypes = phase.erasedTypes
 
+    /** Are we in a Java compilation unit? */
+    final def isJava: Boolean =
+      // FIXME: It would be much nicer if compilationUnit was non-nullable,
+      // perhaps we need to introduce a `NoCompilationUnit` compilation unit
+      // to be used as a default value.
+      compilationUnit != null && compilationUnit.isJava
+
     /** Is current phase after FrontEnd? */
     final def isAfterTyper = base.isAfterTyper(phase)
 

compiler/src/dotty/tools/dotc/transform/Erasure.scala

@@ -542,7 +542,7 @@ object Erasure {
     private def checkValue(tree: Tree)(using Context): Unit =
       val sym = tree.tpe.termSymbol
       if (sym is Flags.Package)
-         || (sym.isAllOf(Flags.JavaModule) && !ctx.compilationUnit.isJava)
+         || (sym.isAllOf(Flags.JavaModule) && !ctx.isJava)
       then
         report.error(JavaSymbolIsNotAValue(sym), tree.srcPos)
 

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

@@ -943,7 +943,7 @@ trait Checking {
 
   /** Check that `tpt` does not define a higher-kinded type */
   def checkSimpleKinded(tpt: Tree)(using Context): Tree =
-    if (!tpt.tpe.hasSimpleKind && !ctx.compilationUnit.isJava)
+    if (!tpt.tpe.hasSimpleKind && !ctx.isJava)
         // be more lenient with missing type params in Java,
         // needed to make pos/java-interop/t1196 work.
       errorTree(tpt, MissingTypeParameterFor(tpt.tpe))

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

@@ -110,7 +110,7 @@ trait TypeAssigner {
             if (tpe.isError) tpe
             else errorType(ex"$whatCanNot be accessed as a member of $pre$where.$whyNot", pos)
           }
-        else if ctx.compilationUnit != null && ctx.compilationUnit.isJava && tpe.isAnyRef then
+        else if ctx.isJava && tpe.isAnyRef then
           defn.FromJavaObjectType
         else
           TypeOps.makePackageObjPrefixExplicit(tpe withDenot d)

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

@@ -576,7 +576,7 @@ class Typer extends Namer
       val qual1 = typedType(tree.qualifier, selectionProto(tree.name, pt, this))
       assignType(cpy.Select(tree)(qual1, tree.name), qual1)
     }
-    else if (ctx.compilationUnit.isJava && tree.name.isTypeName)
+    else if (ctx.isJava && tree.name.isTypeName)
       // SI-3120 Java uses the same syntax, A.B, to express selection from the
       // value A and from the type A. We have to try both.
       selectWithFallback(tryJavaSelectOnType) // !!! possibly exponential bcs of qualifier retyping
@@ -1760,7 +1760,7 @@ class Typer extends Namer
       else if (tpt1.symbol == defn.orType)
         checkedArgs = checkedArgs.mapconserve(arg =>
           checkSimpleKinded(checkNoWildcard(arg)))
-      else if (ctx.compilationUnit.isJava)
+      else if (ctx.isJava)
         if (tpt1.symbol eq defn.ArrayClass) then
           checkedArgs match {
             case List(arg) =>
@@ -3524,7 +3524,7 @@ class Typer extends Namer
         if ((pt eq AnyTypeConstructorProto) || tp.typeParamSymbols.isEmpty) tree
         else {
           val tp1 =
-            if (ctx.compilationUnit.isJava)
+            if (ctx.isJava)
               // Cook raw type
               AppliedType(tree.tpe, tp.typeParams.map(Function.const(TypeBounds.empty)))
             else