Fixes #24914 Treat equivalent candidates as divergent in implicit search

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

@@ -1863,15 +1863,33 @@ trait Implicits:
       // is larger (see `typeSize`) and is constructed using the same set of types and type
       // constructors (see `coveringSet`).
       //
+      // We also treat two candidates as equivalent for this check when they are sibling
+      // givens declared in the same owner that share a declared signature. Such siblings
+      // (a common pattern when modeling type class hierarchies, see i24914) demand the
+      // same context parameters, so exploring one and then another under the same
+      // prototype shape makes no progress and would otherwise lead to a factorial
+      // blow-up of the search before the expression-count limit eventually kicks in.
+      // We use `frozen_=:=` so the check does not commit constraint updates.
+      //
       // We are able to tie a recursive knot if there is compatible term already under
       // construction which is separated from this context by at least one by name argument
       // as we ascend the chain of open implicits to the outermost search context.
 
+      def equivalentCandidates(c1: Candidate, c2: Candidate): Boolean =
+        if c1.ref eq c2.ref then true
+        else
+          val sym1 = c1.ref.symbol
+          val sym2 = c2.ref.symbol
+          sym1.exists
+            && sym2.exists
+            && sym1.maybeOwner == sym2.maybeOwner
+            && c1.ref.info.frozen_=:=(c2.ref.info)
+
       @tailrec
       def loop(history: SearchHistory, belowByname: Boolean): Boolean =
         history match
           case prev @ OpenSearch(cand1, tp, outer) =>
-            if cand1.ref eq cand.ref then
+            if equivalentCandidates(cand1, cand) then
               lazy val wildTp = wildApprox(tp.widenExpr)
               if belowByname && (wildTp <:< wildPt) then
                 fullyDefinedType(tp, "by-name implicit parameter", srcPos)

tests/neg/i24914.scala

@@ -0,0 +1,31 @@
+// Regression test for #24914: make sure that a forest of sibling givens
+// sharing the same signature cannot blow up the implicit search into
+// factorial territory. Before the divergence check considered candidates
+// with identical declared types equivalent, this compiled for tens of
+// seconds and eventually failed with a "search problem too large" warning.
+// The search should now be cut short by ordinary divergence detection.
+
+trait Functor[F[_]]
+trait Monad[F[_]] extends Functor[F]
+
+class T[F[_], A, X]
+
+object Functor:
+  given t1[F[_]: Functor, A]: Functor[[X] =>> T[F, A, X]] = ???
+  given t2[F[_]: Functor, A]: Functor[[X] =>> T[F, A, X]] = ???
+  given t3[F[_]: Functor, A]: Functor[[X] =>> T[F, A, X]] = ???
+  given t4[F[_]: Monad, A]: Functor[[X] =>> T[F, A, X]] = ???
+  given t5[F[_]: Functor, A]: Functor[[X] =>> T[F, A, X]] = ???
+  given t6[F[_]: Functor, A]: Functor[[X] =>> T[F, A, X]] = ???
+
+object Monad:
+  given t1[F[_]: Monad, A]: Monad[[X] =>> T[F, A, X]] = ???
+  given t2[F[_]: Monad, A]: Monad[[X] =>> T[F, A, X]] = ???
+  given t3[F[_]: Monad, A]: Monad[[X] =>> T[F, A, X]] = ???
+  given t4[F[_]: Monad, A]: Monad[[X] =>> T[F, A, X]] = ???
+  given t5[F[_]: Monad, A]: Monad[[X] =>> T[F, A, X]] = ???
+  given t6[F[_]: Monad, A]: Monad[[X] =>> T[F, A, X]] = ???
+
+def m[F[_]: Functor]: F[String] = ???
+
+val x = for _ <- m yield () // error