Re-use isConcrete checking in match types for NamedTyple.From

 - Move isConcrete to a new object `MatchTypes`. We should also move other MatchType-related
   stuff from Types and TypeComparer here. Type and TypeComparer are already unconfortably big,
   and MatchTypes are a coherent topic where everything should work together.
 - Streamline isConcrete a bit.
 - Re-use isConcrete for a similar test in CheckRealizable.
 - Re-use isConcrete for evaluating NamedTuple.From

Fixes #20517

Author: odersky

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

@@ -116,15 +116,7 @@ class CheckRealizable(using Context) {
     case _: SingletonType | NoPrefix =>
       Realizable
     case tp =>
-      def isConcrete(tp: Type): Boolean = tp.dealias match {
-        case tp: TypeRef => tp.symbol.isClass
-        case tp: TypeParamRef => false
-        case tp: TypeProxy => isConcrete(tp.underlying)
-        case tp: AndType => isConcrete(tp.tp1) && isConcrete(tp.tp2)
-        case tp: OrType  => isConcrete(tp.tp1) && isConcrete(tp.tp2)
-        case _ => false
-      }
-      if (!isConcrete(tp)) NotConcrete
+      if !MatchTypes.isConcrete(tp) then NotConcrete
       else boundsRealizability(tp).andAlso(memberRealizability(tp))
   }
 

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

@@ -0,0 +1,59 @@
+package dotty.tools
+package dotc
+package core
+
+import Types.*, Contexts.*, Symbols.*, Flags.*, Decorators.*
+
+object MatchTypes:
+
+  /* Concreteness checking
+   *
+   * When following a baseType and reaching a non-wildcard, in-variant-pos type capture,
+   * we have to make sure that the scrutinee is concrete enough to uniquely determine
+   * the values of the captures. This comes down to checking that we do not follow any
+   * upper bound of an abstract type.
+   *
+   * See notably neg/wildcard-match.scala for examples of this.
+   *
+   * See neg/i13780.scala, neg/i13780-1.scala and neg/i19746.scala for
+   * ClassCastException reproducers if we disable this check.
+   */
+  def isConcrete(tp: Type)(using Context): Boolean =
+    val tp1 = tp.normalized
+
+    tp1 match
+      case tp1: TypeRef =>
+        if tp1.symbol.isClass then true
+        else
+          tp1.info match
+            case info: AliasingBounds => isConcrete(info.alias)
+            case _                    => false
+      case tp1: AppliedType =>
+        isConcrete(tp1.tycon) && isConcrete(tp1.superType)
+      case tp1: HKTypeLambda =>
+        true
+      case tp1: TermRef =>
+        !tp1.symbol.is(Param) && isConcrete(tp1.underlying)
+      case tp1: TermParamRef =>
+        false
+      case tp1: SingletonType =>
+        isConcrete(tp1.underlying)
+      case tp1: ExprType =>
+        isConcrete(tp1.underlying)
+      case tp1: AnnotatedType =>
+        isConcrete(tp1.parent)
+      case tp1: RefinedOrRecType =>
+        isConcrete(tp1.underlying)
+      case tp1: AndOrType =>
+        isConcrete(tp1.tp1) && isConcrete(tp1.tp2)
+      case tp1: TypeVar =>
+        isConcrete(tp1.underlying)
+      case tp1: LazyRef =>
+        isConcrete(tp1.ref)
+      case tp1: FlexibleType =>
+        isConcrete(tp1.hi)
+      case _ =>
+        false
+  end isConcrete
+
+end MatchTypes

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

@@ -24,6 +24,7 @@ import reporting.trace
 import annotation.constructorOnly
 import cc.*
 import NameKinds.WildcardParamName
+import MatchTypes.isConcrete
 
 /** Provides methods to compare types.
  */
@@ -3409,58 +3410,6 @@ class MatchReducer(initctx: Context) extends TypeComparer(initctx) {
 
     // See https://docs.scala-lang.org/sips/match-types-spec.html#matching
     def matchSpeccedPatMat(spec: MatchTypeCaseSpec.SpeccedPatMat): MatchResult =
-      /* Concreteness checking
-       *
-       * When following a baseType and reaching a non-wildcard, in-variant-pos type capture,
-       * we have to make sure that the scrutinee is concrete enough to uniquely determine
-       * the values of the captures. This comes down to checking that we do not follow any
-       * upper bound of an abstract type.
-       *
-       * See notably neg/wildcard-match.scala for examples of this.
-       *
-       * See neg/i13780.scala, neg/i13780-1.scala and neg/i19746.scala for
-       * ClassCastException reproducers if we disable this check.
-       */
-
-      def isConcrete(tp: Type): Boolean =
-        val tp1 = tp.normalized
-
-        tp1 match
-          case tp1: TypeRef =>
-            if tp1.symbol.isClass then true
-            else
-              tp1.info match
-                case info: AliasingBounds => isConcrete(info.alias)
-                case _                    => false
-          case tp1: AppliedType =>
-            isConcrete(tp1.tycon) && isConcrete(tp1.superType)
-          case tp1: HKTypeLambda =>
-            true
-          case tp1: TermRef =>
-            !tp1.symbol.is(Param) && isConcrete(tp1.underlying)
-          case tp1: TermParamRef =>
-            false
-          case tp1: SingletonType =>
-            isConcrete(tp1.underlying)
-          case tp1: ExprType =>
-            isConcrete(tp1.underlying)
-          case tp1: AnnotatedType =>
-            isConcrete(tp1.parent)
-          case tp1: RefinedType =>
-            isConcrete(tp1.underlying)
-          case tp1: RecType =>
-            isConcrete(tp1.underlying)
-          case tp1: AndOrType =>
-            isConcrete(tp1.tp1) && isConcrete(tp1.tp2)
-          case tp1: FlexibleType =>
-            isConcrete(tp1.hi)
-          case _ =>
-            val tp2 = tp1.stripped.stripLazyRef
-            (tp2 ne tp) && isConcrete(tp2)
-      end isConcrete
-
-      // Actual matching logic
-
       val instances = Array.fill[Type](spec.captureCount)(NoType)
       val noInstances = mutable.ListBuffer.empty[(TypeName, TypeBounds)]
 

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

@@ -101,7 +101,7 @@ object TypeEval:
         expectArgsNum(1)
         val arg = tp.args.head
         val cls = arg.classSymbol
-        if cls.is(CaseClass) then
+        if MatchTypes.isConcrete(arg) && cls.is(CaseClass) then
           val fields = cls.caseAccessors
           val fieldLabels = fields.map: field =>
             ConstantType(Constant(field.name.toString))

tests/neg/i20517.check

@@ -0,0 +1,7 @@
+-- [E007] Type Mismatch Error: tests/neg/i20517.scala:10:43 ------------------------------------------------------------
+10 |  def dep(foo: Foo[Any]): From[foo.type] = (elem = "") // error
+   |                                           ^^^^^^^^^^^
+   |                                           Found:    (elem : String)
+   |                                           Required: NamedTuple.From[(foo : Foo[Any])]
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/i20517.scala

@@ -0,0 +1,17 @@
+import scala.language.experimental.namedTuples
+import NamedTuple.From
+
+case class Foo[+T](elem: T)
+
+trait Base[M[_]]:
+  def dep(foo: Foo[Any]): M[foo.type]
+
+class SubAny extends Base[From]:
+  def dep(foo: Foo[Any]): From[foo.type] = (elem = "") // error
+
+object Test:
+  @main def run =
+    val f: Foo[Int] = Foo(elem = 1)
+    val b: Base[From] = SubAny()
+    val nt: (elem: Int) = b.dep(f)
+    val x: Int = nt.elem // was ClassCastException