Fix #6009: Emit error when erased is inside the parameter list

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -908,7 +908,9 @@ object Parsers {
         case FORSOME => syntaxError(ExistentialTypesNoLongerSupported()); t
         case _ =>
           if (imods.is(Implicit) && !t.isInstanceOf[FunctionWithMods])
-            syntaxError("Types with implicit keyword can only be function types", implicitKwPos(start))
+            syntaxError("Types with implicit keyword can only be function types `implicit (...) => ...`", implicitKwPos(start))
+          if (imods.is(Erased) && !t.isInstanceOf[FunctionWithMods])
+            syntaxError("Types with erased keyword can only be function types `erased (...) => ...`", implicitKwPos(start))
           t
       }
     }

docs/docs/reference/other-new-features/erased-terms.md

@@ -171,13 +171,13 @@ Rules
 
 
 3. Functions
-   * `(erased x1: T1, x2: T2, ..., xN: TN) => y : (erased T1, T2, ..., TN) => R`
-   * `(implicit erased x1: T1, x2: T2, ..., xN: TN) => y : (implicit erased T1, T2, ..., TN) => R`
-   * `implicit erased T1 => R  <:<  erased T1 => R`
-   * `(implicit erased T1, T2) => R  <:<  (erased T1, T2) => R`
+   * `erased (x1: T1, x2: T2, ..., xN: TN) => y : erased (T1, T2, ..., TN) => R`
+   * `given erased (x1: T1, x2: T2, ..., xN: TN) => y : given erased (T1, T2, ..., TN) => R`
+   * `given erased T1 => R  <:<  erased T1 => R`
+   * `given erased (T1, T2) => R  <:<  erased (T1, T2) => R`
    *  ...
 
-   Note that there is no subtype relation between `erased T => R` and `T => R` (or `implicit erased T => R` and `implicit T => R`)
+   Note that there is no subtype relation between `erased T => R` and `T => R` (or `given erased T => R` and `given T => R`)
 
 
 4. Eta expansion
@@ -189,7 +189,7 @@ Rules
    * All `erased` parameters are removed from the function
    * All argument to `erased` parameters are not passed to the function
    * All `erased` definitions are removed
-   * All `(erased T1, T2, ..., TN) => R` and `(implicit erased T1, T2, ..., TN) => R` become `() => R`
+   * All `(erased T1, T2, ..., TN) => R` and `(given erased T1, T2, ..., TN) => R` become `() => R`
 
 
 6. Overloading

tests/neg/i6009.scala

@@ -0,0 +1,3 @@
+class Foo {
+  def foo(f: (erased Int) => Int): Int = ??? // error: Types with erased keyword can only be function types `erased (...) => ...`
+}

tests/pos/i6009a.scala

@@ -0,0 +1,6 @@
+class Foo {
+  def foo(f: erased Int => Int): Int = {
+    erased val ctx = 1
+    f(ctx)
+  }
+}

tests/pos/i6009b.scala

@@ -0,0 +1,6 @@
+class Foo {
+  def foo(f: erased (Int) => Int): Int = {
+    erased val ctx = 1
+    f(ctx)
+  }
+}

tests/pos/i6009c.scala

@@ -0,0 +1,6 @@
+class Foo {
+  def foo(f: given erased Int => Int): Int = {
+    implicit erased val ctx = 1
+    f
+  }
+}