Add doc comment that explains what checkLooingImplicits does

Author: odersky

compiler/src/dotty/tools/dotc/transform/CheckLoopingImplicits.scala

@@ -12,7 +12,22 @@ object CheckLoopingImplicits:
   val name: String = "checkLoopingImplicits"
   val description: String = "check that implicit defs do not call themselves in an infinite loop"
 
-/** Checks that implicit defs do not call themselves in an infinite loop */
+/** Checks that some definitions do not call themselves in an infinite loop
+ *  This is an incomplete check, designed to catch some likely bugs instead
+ *  of being exhaustive. The situations where infinite loops are diagnosed are
+ *   1. A given method should not directly call itself
+ *   2. An apply method in a given object should not directly call itself
+ *   3. A lazy val should not directly force itself
+ *   4. An extension method should not directly call itself
+ *
+ *  In all these cases, there are some situations which would not lead to
+ *  an infinite loop at runtime. For instance, the call could go at runtime to an
+ *  overriding version of the method or val which breaks the loop. That's why
+ *  this phase only issues warnings, not errors, and also why we restrict
+ *  checks to the 4 cases above, where a recursion is somewhat hidden.
+ *  There are also other more complicated calling patterns that could also
+ *  be diagnosed as loops with more effort. This could be improved in the future.
+ */
 class CheckLoopingImplicits extends MiniPhase:
   thisPhase =>
   import tpd._

tests/neg-custom-args/fatal-warnings/i15474.scala

@@ -1,7 +1,11 @@
 import scala.language.implicitConversions
 
-given Conversion[ String, Int ] with
-  def apply(from: String): Int = from.toInt   // error
+object Test1:
+  given c: Conversion[ String, Int ] with
+    def apply(from: String): Int = from.toInt   // error
+
+object Test2:
+  given c: Conversion[ String, Int ] = _.toInt   // loop not detected, could be used as a fallback to avoid the warning.
 
 object Prices {
   opaque type Price = BigDecimal