adds procParamTypeBackendAliases to opt out c type aliases

compiler/options.nim

@@ -259,6 +259,9 @@ type
       ## Old transformation for closures in JS backend
     noPanicOnExcept
       ## don't panic on bare except
+    procParamTypeBackendAliases
+      ## Keep the old proc type compatibility rules that ignore backend
+      ## c type aliases.
 
   SymbolFilesOption* = enum
     disabledSf, writeOnlySf, readOnlySf, v2Sf, stressTest

compiler/sigmatch.nim

@@ -784,7 +784,8 @@ proc procParamTypeRel(c: var TCandidate; f, a: PType): TTypeRelation =
     # if f is metatype.
     result = typeRel(c, f, a)
 
-  if result == isEqual:
+  if result == isEqual and
+      procParamTypeBackendAliases notin c.c.config.legacyFeatures:
     # Ensure types that are semantically equal also match at the backend level.
     # E.g. reject assigning proc(csize_t) to proc(uint) since these map to
     # different C types (size_t vs unsigned long long).