include generic bodies in allowMetaTypes (#23968)

fixes #19848

Not sure why this wasn't the case already. The `if cl.allowMetaTypes:
return` line below for `tyFromExpr` [was added 10 years
ago](d5798b43de).
Hopefully it was just negligence?

(cherry picked from commit 1befb8d4a3)

compiler/semtypinst.nim

@@ -578,6 +578,7 @@ proc replaceTypeVarsTAux(cl: var TReplTypeVars, t: PType): PType =
       result.kind = tyUserTypeClassInst
 
   of tyGenericBody:
+    if cl.allowMetaTypes: return
     localError(
       cl.c.config,
       cl.info,
@@ -651,7 +652,7 @@ proc replaceTypeVarsTAux(cl: var TReplTypeVars, t: PType): PType =
 
       for i in 0..<result.len:
         if result[i] != nil:
-          if result[i].kind == tyGenericBody:
+          if result[i].kind == tyGenericBody and not cl.allowMetaTypes:
             localError(cl.c.config, if t.sym != nil: t.sym.info else: cl.info,
               "cannot instantiate '" &
               typeToString(result[i], preferDesc) &

tests/generics/t19848.nim

@@ -0,0 +1,16 @@
+discard """
+  output: '''
+todo
+'''
+"""
+
+type
+  Maybe[T] = object
+  List[T] = object
+
+proc dump[M: Maybe](a: List[M]) =
+  echo "todo"
+
+var a: List[Maybe[int]]
+  
+dump(a)