From ba516c8eb574a113129f2684dc3101a3adc3ff6a Mon Sep 17 00:00:00 2001
From: metagn <metagngn@gmail.com>
Date: Tue, 20 Aug 2024 17:20:35 +0300
Subject: [PATCH] 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](https://github.com/nim-lang/Nim/commit/d5798b43dec547f372eb49d5a8848a9970b12260).
Hopefully it was just negligence?

(cherry picked from commit 1befb8d4a3559b4611921e851a1fcc64c1b9a17d)
---
 compiler/semtypinst.nim   |  3 ++-
 tests/generics/t19848.nim | 16 ++++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)
 create mode 100644 tests/generics/t19848.nim

diff --git a/compiler/semtypinst.nim b/compiler/semtypinst.nim
index 61a74a9710..2bd769ef6a 100644
--- a/compiler/semtypinst.nim
+++ b/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) &
diff --git a/tests/generics/t19848.nim b/tests/generics/t19848.nim
new file mode 100644
index 0000000000..f80f0e298a
--- /dev/null
+++ b/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)