Only allow erased parameters in erased definitions

So far, we do not have any use case for them. We could enable them in a
later version.

The current implementation does not handle correctly the non-erased
arguments to erased definitions. These should always be evaluated, but
in some cases we can dorp them by mistake.

compiler/src/dotty/tools/dotc/transform/PostTyper.scala

@@ -537,7 +537,14 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
         report.error("classes that extend MacroAnnotation must not be inner/local classes", sym.srcPos)
 
     private def checkErasedDef(tree: ValOrDefDef)(using Context): Unit =
+      def checkOnlyErasedParams(): Unit = tree match
+        case tree: DefDef =>
+          for params <- tree.paramss; param <- params if !param.symbol.isType && !param.symbol.is(Erased) do
+            report.error("erased definition can only have erased parameters", param.srcPos)
+        case _ =>
+
       if tree.symbol.is(Erased, butNot = Macro) then
+        checkOnlyErasedParams()
         val tpe = tree.rhs.tpe
         if tpe.derivesFrom(defn.NothingClass) then
           report.error("`erased` definition cannot be implemented with en expression of type Nothing", tree.srcPos)

tests/coverage/run/erased/test.scala

@@ -2,7 +2,7 @@ import scala.language.experimental.erasedDefinitions
 
 erased def parameterless: String = "y"
 
-erased def e(x: String): String = "x"
+erased def e(erased x: String): String = "x"
 def foo(erased a: String)(b: String): String =
   println(s"foo(a)($b)")
   b

tests/coverage/run/erased/test.scoverage.check

@@ -25,8 +25,8 @@ test$package
 Object
 <empty>.test$package
 foo
-181
-203
+188
+210
 7
 println
 Apply
@@ -42,8 +42,8 @@ test$package
 Object
 <empty>.test$package
 foo
-189
-202
+196
+209
 7
 s
 Apply
@@ -59,8 +59,8 @@ test$package
 Object
 <empty>.test$package
 foo
-132
 139
+146
 6
 foo
 DefDef
@@ -76,8 +76,8 @@ test$package
 Object
 <empty>.test$package
 identity
-245
-269
+252
+276
 11
 println
 Apply
@@ -93,8 +93,8 @@ test$package
 Object
 <empty>.test$package
 identity
-253
-268
+260
+275
 11
 s
 Apply
@@ -110,8 +110,8 @@ test$package
 Object
 <empty>.test$package
 identity
-209
-221
+216
+228
 10
 identity
 DefDef
@@ -127,8 +127,8 @@ test$package
 Object
 <empty>.test$package
 Test
-300
-323
+307
+330
 16
 foo
 Apply
@@ -144,8 +144,8 @@ test$package
 Object
 <empty>.test$package
 Test
-326
-342
+333
+349
 17
 foo
 Apply
@@ -161,8 +161,8 @@ test$package
 Object
 <empty>.test$package
 Test
-345
-374
+352
+381
 18
 foo
 Apply
@@ -178,8 +178,8 @@ test$package
 Object
 <empty>.test$package
 Test
-357
-373
+364
+380
 18
 identity
 Apply
@@ -195,8 +195,8 @@ test$package
 Object
 <empty>.test$package
 Test
-275
-289
+282
+296
 15
 Test
 DefDef

tests/neg/erased-1.scala

@@ -12,25 +12,14 @@ object Test {
     })
     foo1(a) // OK
     foo2( // error
-      a // error
-    )
-    foo3( // error
-      a
+      a // Ok
     )
     a // error
   }
-  erased def foo2(a: Int): Int = {
-    foo0(a) // OK
-    foo1(a) // OK
-    foo2(a) // OK
-    foo3(a) // OK
-    a // OK
-  }
-  erased def foo3(erased a: Int): Int = {
+  erased def foo2(erased a: Int): Int = {
     foo0(a) // OK
     foo1(a) // OK
     foo2(a) // OK
-    foo3(a) // OK
     a // OK
   }
-}
+}

tests/neg/erased-2.scala

@@ -8,39 +8,26 @@ object Test {
     )
     foo1(u) // OK
     foo2( // error
-      u // error
-    )
-    foo3( // error
-      u
+      u // Ok
     )
     u // error
     u // error
   }
-  erased def foo2(a: Int): Int = {
-    foo0(u) // OK
-    foo1(u) // OK
-    foo2(u) // OK
-    foo3(u) // OK
-    u // warn
-    u // OK
-  }
-  erased def foo3(erased a: Int): Int = {
+  erased def foo2(erased a: Int): Int = {
     foo0(u) // OK
     foo1(u) // OK
     foo2(u) // OK
-    foo3(u) // OK
     u // warn
     u // OK
   }
 
-  erased val foo4: Int = {
+  erased val foo3: Int = {
     foo0(u) // OK
     foo1(u) // OK
     foo2(u) // OK
-    foo3(u) // OK
     u // warn
     u // OK
   }
 
   erased def u: Int = 42
-}
+}

tests/neg/erased-3.scala

@@ -8,40 +8,27 @@ object Test {
     )
     foo1(u()) // OK
     foo2( // error
-      u() // error
-    )
-    foo3( // error
-      u()
+      u() // Ok
     )
     u() // error
     u() // error
   }
-  erased def foo2(a: Int): Int = {
-    foo0(u()) // OK
-    foo1(u()) // OK
-    foo2(u()) // OK
-    foo3(u()) // OK
-    u() // warn
-    u() // OK
-  }
-  erased def foo3(erased a: Int): Int = {
+  erased def foo2(erased a: Int): Int = {
     foo0(u()) // OK
     foo1(u()) // OK
     foo2(u()) // OK
-    foo3(u()) // OK
     u() // warn
     u() // OK
   }
 
-  erased val foo4: Int = {
+  erased val foo3: Int = {
     foo0(u()) // OK
     foo1(u()) // OK
     foo2(u()) // OK
-    foo3(u()) // OK
     println()
     u() // warn
     u() // OK
   }
 
   erased def u(): Int = 42
-}
+}

tests/neg/erased-args-lifted.scala

@@ -2,7 +2,7 @@
 
 object Test {
   def foo(a: Int)(b: Int, c: Int) = 42
-  erased def bar(i: Int): Int = {
+  erased def bar(erased i: Int): Int = {
     println(1)
     42
   }

tests/neg/erased-params.scala

@@ -0,0 +1,12 @@
+import scala.language.experimental.erasedDefinitions
+
+erased def test1(x: Int): Int = x // error
+erased def test2(erased x: Int): Int = x
+erased def test3(erased x: Int, erased y: Int): Int = x
+erased def test4(erased x: Int, y: Int): Int = x // error
+erased def test5(x: Int, erased y: Int): Int = y // error
+erased def test6(x: Int, y: Int): Int = y // error // error
+erased def test7(erased x: Int)(erased y: Int): Int = x
+erased def test8(erased x: Int)(y: Int): Int = x // error
+erased def test9(x: Int)(erased y: Int): Int = y // error
+erased def test10(x: Int)(y: Int): Int = y // error // error