fixes #19015 [backport:1.6] (#19204)

(cherry picked from commit d584dd5b99)

changelog.md

@@ -4,6 +4,10 @@
 ## Changes affecting backward compatibility
 
 
+- Optional parameters in combination with `: body` syntax (RFC #405) are now opt-in via
+  `experimental:flexibleOptionalParams`.
+
+## Standard library additions and changes
 
 ## Standard library additions and changes
 

compiler/options.nim

@@ -207,7 +207,8 @@ type
     strictNotNil,
     overloadableEnums,
     strictEffects,
-    unicodeOperators
+    unicodeOperators,
+    flexibleOptionalParams
 
   LegacyFeature* = enum
     allowSemcheckedAstModification,

compiler/sigmatch.nim

@@ -2461,7 +2461,8 @@ proc matchesAux(c: PContext, n, nOrig: PNode, m: var TCandidate, marker: var Int
         if m.callee.n[f].kind != nkSym:
           internalError(c.config, n[a].info, "matches")
           noMatch()
-        if a >= firstArgBlock: f = max(f, m.callee.n.len - (n.len - a))
+        if flexibleOptionalParams in c.features and a >= firstArgBlock:
+          f = max(f, m.callee.n.len - (n.len - a))
         formal = m.callee.n[f].sym
         m.firstMismatch.kind = kTypeMismatch
         if containsOrIncl(marker, formal.position) and container.isNil:

tests/misc/trfc405.nim

@@ -1,3 +1,6 @@
+
+{.experimental: "flexibleOptionalParams".}
+
 # https://github.com/nim-lang/RFCs/issues/405
 
 template main =