Fix #7694: Set a compilation unit for syntax highlighting

Author: smarter

compiler/src/dotty/tools/dotc/printing/SyntaxHighlighting.scala

@@ -1,4 +1,5 @@
-package dotty.tools.dotc.printing
+package dotty.tools.dotc
+package printing
 
 import dotty.tools.dotc.ast.untpd
 import dotty.tools.dotc.core.Contexts.Context
@@ -32,8 +33,10 @@ object SyntaxHighlighting {
     def freshCtx = ctx.fresh.setReporter(Reporter.NoReporter)
     if (in.isEmpty || ctx.settings.color.value == "never") in
     else {
-      implicit val ctx = freshCtx
       val source = SourceFile.virtual("<highlighting>", in)
+
+      implicit val ctx = freshCtx.setCompilationUnit(CompilationUnit(source, mustExist = false)(freshCtx))
+
       val colorAt = Array.fill(in.length)(NoColor)
 
       def highlightRange(from: Int, to: Int, color: String) =

compiler/test/dotty/tools/dotc/printing/SyntaxHighlightingTests.scala

@@ -110,6 +110,8 @@ class SyntaxHighlightingTests extends DottyTest {
 
     test("def f1(x: Int) = 123", "<K|def> <V|f1>(<V|x>: <T|Int>) = <L|123>")
     test("def f2[T](x: T) = { 123 }", "<K|def> <V|f2>[<T|T>](<V|x>: <T|T>) = { <L|123> }")
+
+    test("def f3[T[_", "<K|def> <V|f3>[<T|T>[<T|_>")
   }
 
   @Test