add a useful helper for debugging typeRel problems

compiler/sigmatch.nim

@@ -302,9 +302,31 @@ proc describeArgs*(c: PContext, n: PNode, startIdx = 1;
     add(result, argTypeToString(arg, prefer))
     if i != sonsLen(n) - 1: add(result, ", ")
 
-proc typeRel*(c: var TCandidate, f, aOrig: PType,
+proc typeRelImpl*(c: var TCandidate, f, aOrig: PType,
                   flags: TTypeRelFlags = {}): TTypeRelation
 
+var nextTypeRel = 0
+
+template typeRel*(c: var TCandidate, f, aOrig: PType,
+                 flags: TTypeRelFlags = {}): TTypeRelation =
+  const traceTypeRel = false
+
+  when traceTypeRel:
+    var enteringAt = nextTypeRel
+    if mdbg:
+      inc nextTypeRel
+      echo "----- TYPE REL ", enteringAt
+      debug f
+      debug aOrig
+
+  let r = typeRelImpl(c, f, aOrig, flags)
+
+  when traceTypeRel:
+    if enteringAt != nextTypeRel:
+      echo "----- TYPE REL ", enteringAt, " RESULT: ", r
+
+  r
+
 proc concreteType(c: TCandidate, t: PType): PType =
   case t.kind
   of tyNil:
@@ -871,7 +893,7 @@ proc isCovariantPtr(c: var TCandidate, f, a: PType): bool =
   else:
     return false
 
-proc typeRel(c: var TCandidate, f, aOrig: PType,
+proc typeRelImpl(c: var TCandidate, f, aOrig: PType,
                  flags: TTypeRelFlags = {}): TTypeRelation =
   # typeRel can be used to establish various relationships between types:
   #