Constrain pattern types when typing a match type case

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1713,6 +1713,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       val pat1 = withMode(Mode.Pattern) {
         checkSimpleKinded(typedType(cdef.pat, mapPatternBounds = true))
       }
+      if !ctx.isAfterTyper && pt != defn.ImplicitScrutineeTypeRef then
+        withMode(Mode.GadtConstraintInference) {
+          TypeComparer.constrainPatternType(pat1.tpe, selType)
+        }
       val pat2 = indexPattern(cdef).transform(pat1)
       var body1 = typedType(cdef.body, pt)
       if !body1.isType then

tests/pos/i14151.scala

@@ -0,0 +1,2 @@
+class BoundedPair[A, B <: A]:
+  type Second[A, T <: BoundedPair[A, _ <: A]] <: A = T match { case BoundedPair[A, b] => b }

tests/pos/i14151b.scala

@@ -0,0 +1,2 @@
+class Single[B](val value: B):
+  type Retrieve[A, T <: Single[_ <: A]] <: A = T match { case Single[b] => b }

tests/pos/i14151c.scala

@@ -0,0 +1,2 @@
+class Single[B](val value: B):
+  type Retrieve[A] <: A = Single[A] match { case Single[b] => b }