Fix #10546: Parse singleton applications only under a command line flag

This now needs -language:experimental.dependent

Fixes #10546.
Fixes #10536.

Author: odersky

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

@@ -27,7 +27,7 @@ import util.Chars
 import scala.annotation.{tailrec, switch}
 import rewrites.Rewrites.{patch, overlapsPatch}
 import reporting._
-import config.Feature.{sourceVersion, migrateTo3}
+import config.Feature.{sourceVersion, migrateTo3, dependentEnabled}
 import config.SourceVersion._
 import config.SourceVersion
 
@@ -1613,7 +1613,7 @@ object Parsers {
         typeIdent()
       else
         def singletonArgs(t: Tree): Tree =
-          if in.token == LPAREN
+          if in.token == LPAREN && dependentEnabled
           then singletonArgs(AppliedTypeTree(t, inParens(commaSeparated(singleton))))
           else t
         singletonArgs(simpleType1())

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -163,6 +163,7 @@ class CompilationTests {
       compileFile("tests/neg-custom-args/kind-projector.scala", defaultOptions.and("-Ykind-projector")),
       compileFile("tests/neg-custom-args/typeclass-derivation2.scala", defaultOptions.and("-Yerased-terms")),
       compileFile("tests/neg-custom-args/i5498-postfixOps.scala", defaultOptions withoutLanguageFeature "postfixOps"),
+      compileFile("tests/neg-custom-args/deptypes.scala", defaultOptions.and("-language:experimental.dependent")),
     ).checkExpectedErrors()
   }
 

library/src/scala/runtime/stdLibPatches/language.scala

@@ -18,8 +18,12 @@ object language:
    */
   object experimental:
 
-    /** Experimental support for richer dependent types */
-    object dependent
+    /* Experimental support for richer dependent types (disabled for now)
+     * One can still run the compiler with support for parsing singleton applications
+     * using command line option `-language:experimental.dependent`.
+     * But one cannot use a feature import for this as long as this entry is commented out.
+     */
+    //object dependent
 
     /** Experimental support for named type arguments */
     object namedTypeArguments

tests/neg-custom-args/deptypes.scala

@@ -0,0 +1,10 @@
+
+type Vec[T] = (n: Int) =>> Array[T] // error: not yet implemented
+
+type Matrix[T](m: Int, n: Int) = Vec[Vec[T](n)](m)  // error: not yet implemented
+
+type Tensor2[T](m: Int)(n: Int) = Matrix[T](m, n)   // error: not yet implemented
+
+val x: Vec[Int](10) = ???  // error: not yet implemented
+val n = 10
+type T = Vec[String](n)  // error: not yet implemented

tests/neg/deptypes.scala

@@ -0,0 +1,5 @@
+object test:
+  def times(num : Int)(block : => Unit) : Unit = ()
+    times(10): println("ah") // error: end of statement expected but '(' found // error
+
+  def foo: Set(Int) = Set(1) // error: end of statement expected but '(' found // error // error