Fix stability check for inline parameters

Inline parameters are not stable. Two references to the same parameter
may not be idempotent.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -739,7 +739,7 @@ object SymDenotations {
       * So the first call to a stable member might fail and/or produce side effects.
       */
     final def isStableMember(using Context): Boolean = {
-      def isUnstableValue = isOneOf(UnstableValueFlags) || info.isInstanceOf[ExprType]
+      def isUnstableValue = isOneOf(UnstableValueFlags) || info.isInstanceOf[ExprType] || isAllOf(InlineParam)
       isType || is(StableRealizable) || exists && !isUnstableValue
     }
 

library/src/scala/quoted/Quotes.scala

@@ -14,7 +14,7 @@ import scala.reflect.TypeTest
  *  }
  *  ```
  */
-transparent inline def quotes(using inline q: Quotes): q.type = q
+transparent inline def quotes(using inline q: Quotes): Quotes = q
 
 /** Quotation context provided by a macro expansion or in the scope of `scala.quoted.staging.run`.
  *  Used to perform all operations on quoted `Expr` or `Type`.

library/src/scala/runtime/stdLibPatches/Predef.scala

@@ -31,7 +31,7 @@ object Predef:
    *  @tparam T the type of the value to be summoned
    *  @return the given value typed: the provided type parameter
    */
-  transparent inline def summon[T](using inline x: T): x.type = x
+  transparent inline def summon[T](using inline x: T): T = x
 
   // Extension methods for working with explicit nulls
 

tests/pos/inline-param-unstable-path.scala

@@ -0,0 +1,6 @@
+inline val a = 3
+inline def f(inline x: Int, y: Int, z: => Int): Unit =
+  val x2: x.type = x // error: (x : Int) is not a valid singleton type, since it is not an immutable path
+  val y2: y.type = y
+  val z2: z.type = z // error: (z : Int) is not a valid singleton type, since it is not an immutable path
+  val a2: a.type = a