Improve error message for inline val null and Unit

Fixes #12177

Author: nicolasstucki

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

@@ -42,6 +42,8 @@ class InlineVals extends MiniPhase:
           if tp.derivesFrom(defn.StringClass) || defn.ScalaValueClasses().exists(tp.derivesFrom(_)) then
             val pos = if tpt.span.isZeroExtent then rhs.srcPos else tpt.srcPos
             report.error(em"inline value must have a literal constant type", pos)
+          else if tp.derivesFrom(defn.NullClass) then
+            report.error(em"`inline val` with `null` is no supported.\n\nTo inline a `null` consider using `inline def`", rhs)
           else
             report.error(em"inline value must contain a literal constant value.\n\nTo inline more complex types consider using `inline def`", rhs)
   }

tests/neg/i12177.check

@@ -0,0 +1,6 @@
+-- Error: tests/neg/i12177.scala:2:17 ----------------------------------------------------------------------------------
+2 |  inline val v = null // error
+  |                 ^^^^
+  |                 `inline val` with `null` is no supported.
+  |
+  |                 To inline a `null` consider using `inline def`

tests/neg/i12177.scala

@@ -0,0 +1,4 @@
+object Test1 {
+  inline val v = null // error
+  inline def d = null
+}