fixes #4537

compiler/sigmatch.nim

@@ -1031,6 +1031,7 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
         if x < isSubtype: return isNone
         # 'and' implies minimum matching result:
         if x < result: result = x
+      if result > isGeneric: result = isGeneric
       bindingRet result
 
   of tyOr:
@@ -1041,6 +1042,7 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
         # 'or' implies maximum matching result:
         if x > result: result = x
       if result >= isSubtype:
+        if result > isGeneric: result = isGeneric
         bindingRet result
       else:
         result = isNone

tests/converter/tor_in_converter.nim

@@ -0,0 +1,23 @@
+discard """
+  output: '''test
+test'''
+"""
+# bug #4537
+
+# nim js --d:nodejs
+
+type
+  Str = distinct string
+
+when true:
+  # crashes
+  converter convert(s: string | cstring): Str = Str($s)
+else:
+  # works!
+  converter convert(s: string): Str = Str($s)
+  converter convert(s: cstring): Str = Str($s)
+
+proc echoStr(s: Str) = echo s.string
+
+echoStr("test")
+echoStr("test".cstring)