Fix scala#8841: Improve inline val error message

Author: nicolasstucki

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

@@ -888,9 +888,12 @@ trait Checking {
       // final vals can be marked inline even if they're not pure, see Typer#patchFinalVals
       val purityLevel = if (sym.is(Final)) Idempotent else Pure
       tpt.tpe.widenTermRefExpr match
-        case tp: ConstantType if exprPurity(tree) >= purityLevel => // ok
+        case tp: ConstantType =>
+          if !(exprPurity(tree) >= purityLevel) then
+            ctx.error(em"inline value must be pure", tree.sourcePos)
         case _ =>
-          ctx.error(em"type of inline must be a known value", tree.sourcePos)
+          val pos = if tpt.span.isZeroExtent then tree.sourcePos else tpt.sourcePos
+          ctx.error(em"inline value must have a literal constant type", pos)
   }
 
   /** A hook to exclude selected symbols from double declaration check */

tests/neg/i8841.check

@@ -0,0 +1,12 @@
+-- Error: tests/neg/i8841.scala:2:20 -----------------------------------------------------------------------------------
+2 |  inline val log1 : Boolean = false // error
+  |                    ^^^^^^^
+  |                    inline value must have a literal constant type
+-- Error: tests/neg/i8841.scala:3:20 -----------------------------------------------------------------------------------
+3 |  inline val log2 = true: Boolean // error
+  |                    ^^^^^^^^^^^^^
+  |                    inline value must have a literal constant type
+-- Error: tests/neg/i8841.scala:4:28 -----------------------------------------------------------------------------------
+4 |  inline val log3: false  = { println(); false } // error
+  |                            ^^^^^^^^^^^^^^^^^^^^
+  |                            inline value must be pure

tests/neg/i8841.scala

@@ -0,0 +1,5 @@
+object Foo {
+  inline val log1 : Boolean = false // error
+  inline val log2 = true: Boolean // error
+  inline val log3: false  = { println(); false } // error
+}