Refine the type of inline vals with literal rhs

Author: mbovel

compiler/src/dotty/tools/dotc/inlines/Inlines.scala

@@ -619,4 +619,32 @@ object Inlines:
           // the opaque type itself. An example is in pos/opaque-inline1.scala.
     end expand
   end InlineCall
+
+  /** An inline val is refinable if it has an explicit type that is not a
+   *  singleton type.
+   */
+  def mightRefineInlineVal(mdef: untpd.ValOrDefDef, sym: Symbol)(using Context): Boolean =
+    sym.isInlineVal && !sym.is(Final)
+    && !mdef.tpt.isEmpty && !mdef.tpt.isInstanceOf[untpd.SingletonTypeTree]
+
+  /** Refine the type of an inline val to a constant type if its right hand
+   *  side is a literal. See tests/pos/i24321.scala and tests/printing/i24321.scala.
+   */
+  def refineInlineVal(mdef: untpd.ValOrDefDef, sym: Symbol, tp: Type)(using Context): Type =
+    if !Inlines.mightRefineInlineVal(mdef, sym) then
+      return tp
+
+    // Only refine if the explicit type is a primitive value types and String.
+    // We don't include opaque types. See tests/neg/i13851b.scala.
+    val tpSym = tp.dealiasKeepOpaques.typeSymbol
+    if !tpSym.isPrimitiveValueClass && tpSym != defn.StringClass then
+      return tp
+
+    untpd.stripBlock(mdef.rhs) match
+      case rhs: (untpd.Literal | untpd.Number) =>
+        ctx.typer.typedAheadExpr(rhs, tp) match
+          case tpd.ConstantValue(c) => ConstantType(Constant(c))
+          case _ => tp
+      case _ => tp
+
 end Inlines

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

@@ -1927,7 +1927,10 @@ class Namer { typer: Typer =>
           sym.setFlag(Deferred | HasDefault)
         case _ =>
 
-    val mbrTpe = paramFn(checkSimpleKinded(typedAheadType(mdef.tpt, tptProto)).tpe)
+    val mbrTpe0 = checkSimpleKinded(typedAheadType(mdef.tpt, tptProto)).tpe
+    val mbrTpe1 = Inlines.refineInlineVal(mdef, sym, mbrTpe0)
+    val mbrTpe = paramFn(mbrTpe1)
+
     // Add an erased to the using clause generated from a `: Singleton` context bound
     mdef.tpt match
       case tpt: untpd.ContextBoundTypeTree if mbrTpe.typeSymbol == defn.SingletonClass =>

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

@@ -3038,7 +3038,14 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         sym.resetFlag(Lazy)
       checkErasedOK(sym)
       sym.setFlag(Erased)
-    val tpt1 = checkSimpleKinded(typedType(tpt))
+
+    val isRefinedInlineVal =
+      Inlines.mightRefineInlineVal(vdef, sym)
+      && sym.info.isInstanceOf[ConstantType]
+
+    val tpt0 = checkSimpleKinded(typedType(tpt))
+    val tpt1 = if isRefinedInlineVal then TypeTree(sym.info) else tpt0
+
     val rhs1 = vdef.rhs match
       case rhs @ Ident(nme.WILDCARD) =>
         rhs.withType(tpt1.tpe)

tests/neg/i8841.check

@@ -1,7 +1,3 @@
--- 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
   |                    ^^^^^^^^^^^^^

tests/neg/i8841.scala

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

tests/neg/inline-val.scala

@@ -1,4 +1,4 @@
 
 inline val a = 1 : Int // error
-inline val b: Int = 1 // error
-inline val c = b // error
+inline val b: Int = 1 // ok
+inline val c = b // ok

tests/pos/i24321.scala

@@ -0,0 +1,4 @@
+object Bar:
+  inline val MAX: Byte = 10
+
+val y = Bar.MAX to Bar.MAX

tests/printing/i24321.check

@@ -0,0 +1,15 @@
+[[syntax trees at end of                     typer]] // tests/printing/i24321.scala
+package <empty> {
+  final lazy module val i24321$package: i24321$package = new i24321$package()
+  final module class i24321$package() extends Object() {
+    this: i24321$package.type =>
+    inline val BOOL_CONST: (true : Boolean) = true
+    inline val CHAR_CONST: ('A' : Char) = 'A'
+    inline val INT_CONST: (100000 : Int) = 100000
+    inline val LONG_CONST: (100000L : Long) = 100000L
+    inline val FLOAT_CONST: (3.14f : Float) = 3.14f
+    inline val DOUBLE_CONST: (3.14159d : Double) = 3.14159d
+    inline val STRING_CONST: ("Hello, Scala 3!" : String) = "Hello, Scala 3!"
+  }
+}
+

tests/printing/i24321.scala

@@ -0,0 +1,7 @@
+inline val BOOL_CONST: Boolean = true
+inline val CHAR_CONST: Char = 'A'
+inline val INT_CONST: Int = 100000
+inline val LONG_CONST: Long = 100000L
+inline val FLOAT_CONST: Float = 3.14f
+inline val DOUBLE_CONST: Double = 3.14159
+inline val STRING_CONST: String = "Hello, Scala 3!"