Fix #8841: Improve inline val error message

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.dealias 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 */

docs/docs/reference/metaprogramming/inline.md

@@ -228,6 +228,25 @@ constant expressions in the sense defined by the [SLS §
 including _platform-specific_ extensions such as constant folding of pure
 numeric computations.
 
+An inline value must have a literal type such as `1` or `true`.
+```scala
+inline val four = 4
+// equivalent to
+inline val four: 4 = 4
+```
+
+It is also possible to have inline vals of types that do not have a syntax, such as `Short(4)`.
+
+```scala
+trait InlineConstants {
+  inline val myShort: Short
+}
+
+object Constants extends InlineConstants {
+  inline val myShort/*: Short(4)*/ = 4
+}
+```
+
 ## Transparent Inline Methods
 
 Inline methods can additionally be declared `transparent`.

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
+}

tests/pos/inline-vals.scala

@@ -0,0 +1,23 @@
+trait InlineConstants {
+  inline val myInlinedBoolean: Boolean
+  inline val myInlinedByte: Byte
+  inline val myInlinedShort: Short
+  inline val myInlinedInt: Int
+  inline val myInlinedLong: Long
+  inline val myInlinedFloat: Float
+  inline val myInlinedDouble: Double
+  inline val myInlinedChar: Char
+  inline val myInlinedString: String
+}
+
+object Constants extends InlineConstants {
+  inline val myInlinedBoolean = true
+  inline val myInlinedByte = 1
+  inline val myInlinedShort = 2
+  inline val myInlinedInt = 3
+  inline val myInlinedLong = 4
+  inline val myInlinedFloat = 5
+  inline val myInlinedDouble = 6
+  inline val myInlinedChar = 'a'
+  inline val myInlinedString = "abc"
+}