Merge pull request scala#1599 from dotty-staging/fix-#1570

Fix scala#1570: Allow inline parameters as inline args

src/dotty/tools/dotc/core/Flags.scala

@@ -544,6 +544,9 @@ object Flags {
   /** An inline method */
   final val InlineMethod = allOf(Inline, Method)
 
+  /** An inline parameter */
+  final val InlineParam = allOf(Inline, Param)
+
   /** A parameter or parameter accessor */
   final val ParamOrAccessor = Param | ParamAccessor
 

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

@@ -482,10 +482,13 @@ trait Checking {
 
   /** Check that `tree` is a pure expression of constant type */
   def checkInlineConformant(tree: Tree, what: => String)(implicit ctx: Context): Unit =
-    tree.tpe.widenTermRefExpr match {
-      case tp: ConstantType if isPureExpr(tree) => // ok
-      case tp if defn.isFunctionType(tp) && isPureExpr(tree) => // ok
-      case _ => ctx.error(em"$what must be a constant expression or a function", tree.pos)
+    tree.tpe match {
+      case tp: TermRef if tp.symbol.is(InlineParam) => // ok
+      case tp => tp.widenTermRefExpr match {
+        case tp: ConstantType if isPureExpr(tree) => // ok
+        case tp if defn.isFunctionType(tp) && isPureExpr(tree) => // ok
+        case _ => ctx.error(em"$what must be a constant expression or a function", tree.pos)
+      }
     }
 
   /** Check that class does not define same symbol twice */

tests/pos/i1570.scala

@@ -0,0 +1,4 @@
+object Test {
+  inline def foo(inline n: Int) = bar(n)
+  inline def bar(inline n: Int) = n
+}