From dc1bf1e482e37175268fea787423e6fe177792f3 Mon Sep 17 00:00:00 2001
From: "Andrey R (cooldome)" <ariabushenko@gmail.com>
Date: Sun, 15 Nov 2020 12:21:55 +0000
Subject: [PATCH] fix #15959

---
 compiler/semexprs.nim   | 3 +++
 compiler/semtypinst.nim | 7 +++++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/compiler/semexprs.nim b/compiler/semexprs.nim
index b2f0207e7caa..a60c1669e1e1 100644
--- a/compiler/semexprs.nim
+++ b/compiler/semexprs.nim
@@ -1513,6 +1513,9 @@ proc semSubscript(c: PContext, n: PNode, flags: TExprFlags): PNode =
       result = n
     else:
       result = nil
+  of tyGenericParam:
+    result = n
+    result.typ = makeTypeFromExpr(c, n.copyTree)
   else:
     let s = if n[0].kind == nkSym: n[0].sym
             elif n[0].kind in nkSymChoices: n[0][0].sym
diff --git a/compiler/semtypinst.nim b/compiler/semtypinst.nim
index 931581d12a09..540b9c45e043 100644
--- a/compiler/semtypinst.nim
+++ b/compiler/semtypinst.nim
@@ -538,14 +538,17 @@ proc replaceTypeVarsTAux(cl: var TReplTypeVars, t: PType): PType =
     assert t.n.typ != t
     var n = prepareNode(cl, t.n)
     if n.kind != nkEmpty:
-      n = cl.c.semConstExpr(cl.c, n)
-    if n.typ.kind == tyTypeDesc:
+      n = cl.c.semExpr(cl.c, n)
+    case n.typ.kind
+    of ConcreteTypes: result = n.typ
+    of tyTypeDesc:
       # XXX: sometimes, chained typedescs enter here.
       # It may be worth investigating why this is happening,
       # because it may cause other bugs elsewhere.
       result = n.typ.skipTypes({tyTypeDesc})
       # result = n.typ.base
     else:
+      n = cl.c.semConstExpr(cl.c, n)
       if n.typ.kind != tyStatic:
         # XXX: In the future, semConstExpr should
         # return tyStatic values to let anyone make