Improve error message for inline val null and Unit

Fixes #12177

Author: nicolasstucki

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

@@ -958,7 +958,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
 
   /** Extractors for splices */
   object Spliced {
-    /** Extracts the content of a spliced expresion tree.
+    /** Extracts the content of a spliced expression tree.
      *  The result can be the contents of a term splice, which
      *  will return a term tree.
      */

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

@@ -39,9 +39,13 @@ class InlineVals extends MiniPhase:
             val details = if enclosingInlineds.isEmpty then "" else em"but was: $rhs"
             report.error(s"inline value must be pure$details", rhs.srcPos)
         case tp =>
-          if tp.derivesFrom(defn.StringClass) || defn.ScalaValueClasses().exists(tp.derivesFrom(_)) then
+          if tp.derivesFrom(defn.UnitClass) then
+            report.error(em"`inline val` of type `Unit` is not supported.\n\nTo inline a `Unit` consider using `inline def`", rhs)
+          else 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 not 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 +1,24 @@
+-- Error: tests/neg/i12177.scala:2:17 ----------------------------------------------------------------------------------
+2 |  inline val v = null // error
+  |                 ^^^^
+  |                 `inline val` with `null` is not supported.
+  |
+  |                 To inline a `null` consider using `inline def`
+-- Error: tests/neg/i12177.scala:4:17 ----------------------------------------------------------------------------------
+4 |  inline val u = () // error
+  |                 ^^
+  |                 `inline val` of type `Unit` is not supported.
+  |
+  |                 To inline a `Unit` consider using `inline def`
+-- Error: tests/neg/i12177.scala:6:18 ----------------------------------------------------------------------------------
+6 |  inline val u2 = { println(); () } // error
+  |                  ^^^^^^^^^^^^^^^^^
+  |                  `inline val` of type `Unit` is not supported.
+  |
+  |                  To inline a `Unit` consider using `inline def`
+-- Error: tests/neg/i12177.scala:7:18 ----------------------------------------------------------------------------------
+7 |  inline val u3 = { } // error
+  |                  ^^^
+  |                  `inline val` of type `Unit` is not supported.
+  |
+  |                  To inline a `Unit` consider using `inline def`

tests/neg/i12177.scala

@@ -0,0 +1,8 @@
+object Test1 {
+  inline val v = null // error
+  inline def d = null
+  inline val u = () // error
+  inline def e = ()
+  inline val u2 = { println(); () } // error
+  inline val u3 = { } // error
+}