Fix #7972: Disallow signatures with _:* return types

Author: nicolasstucki

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

@@ -938,6 +938,11 @@ trait Checking {
       errorTree(tpt, MissingTypeParameterFor(tpt.tpe))
     else tpt
 
+  /** Check that the signature of the class mamber does not return a repeated parameter type */
+  def checkSignatureRepeatedParam(sym: Symbol)(implicit ctx: Context): Unit =
+    if (sym.maybeOwner.isClass && sym.info.finalResultType.isRepeatedParam)
+      ctx.error(em"Cannot return repeated parameter type ${sym.info.finalResultType}", sym.sourcePos)
+
   /** Verify classes extending AnyVal meet the requirements */
   def checkDerivedValueClass(clazz: Symbol, stats: List[Tree])(implicit ctx: Context): Unit =
     Checking.checkDerivedValueClass(clazz, stats)

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

@@ -1609,6 +1609,7 @@ class Typer extends Namer
       case rhs => typedExpr(rhs, tpt1.tpe.widenExpr)
     }
     val vdef1 = assignType(cpy.ValDef(vdef)(name, tpt1, rhs1), sym)
+    checkSignatureRepeatedParam(sym)
     if (sym.is(Inline, butNot = DeferredOrTermParamOrAccessor))
       checkInlineConformant(rhs1, isFinal = sym.is(Final), em"right-hand side of inline $sym")
     patchFinalVals(vdef1)
@@ -1694,7 +1695,9 @@ class Typer extends Namer
       checkThisConstrCall(rhs1)
     }
 
-    assignType(cpy.DefDef(ddef)(name, tparams1, vparamss1, tpt1, rhs1), sym).setDefTree
+    val ddef2 = assignType(cpy.DefDef(ddef)(name, tparams1, vparamss1, tpt1, rhs1), sym)
+    checkSignatureRepeatedParam(sym)
+    ddef2.setDefTree
       //todo: make sure dependent method types do not depend on implicits or by-name params
   }
 

tests/neg/i7972.scala

@@ -0,0 +1,18 @@
+object O {
+  def m1(a: Int*) = (a: _*) // error: Cannot return repeated parameter type Int*
+  def m2(a: Int*) = { // error: Cannot return repeated parameter type Int*
+    val b = (a: _*)
+    b
+  }
+  def m3(a: Int*): Any = {
+    val b = (a: _*) // Ok, does not affect the signature
+    b
+  }
+  def m4(a: 2*) = (a: _*) // error: Cannot return repeated parameter type Int*
+
+}
+
+class O(a: Int*) {
+  val m = (a: _*) // error: Cannot return repeated parameter type Int*
+}
+