fix undeclared identifier in templates in generics (#24069)

fixes #13979

Fixes templates in generics that use identifiers that aren't defined
yet, giving an early `undeclared identifier` error, by just marking
template bodies as in a mixin context in `semgnrc`.

2 files changed, 11 insertions, 1 deletions

diff --git a/compiler/semgnrc.nim b/compiler/semgnrc.nim
index e3a8daf99..16bb7f094 100644
--- a/compiler/semgnrc.nim
+++ b/compiler/semgnrc.nim
@@ -613,7 +613,8 @@ proc semGenericStmt(c: PContext, n: PNode,
       else:
         body = getBody(c.graph, s)
     else: body = n[bodyPos]
-    n[bodyPos] = semGenericStmtScope(c, body, flags, ctx)
+    let bodyFlags = if n.kind == nkTemplateDef: flags + {withinMixin} else: flags
+    n[bodyPos] = semGenericStmtScope(c, body, bodyFlags, ctx)
     closeScope(c)
   of nkPragma, nkPragmaExpr: discard
   of nkExprColonExpr, nkExprEqExpr:
diff --git a/tests/generics/tnestedtemplate.nim b/tests/generics/tnestedtemplate.nim
new file mode 100644
index 000000000..22d0a2d3c
--- /dev/null
+++ b/tests/generics/tnestedtemplate.nim
@@ -0,0 +1,9 @@
+block: # issue #13979
+  var s: seq[int]
+  proc filterScanline[T](input: openArray[T]) =
+    template currPix: untyped = input[i]
+    for i in 0..<input.len:
+      s.add currPix
+  let pix = [1, 2, 3]
+  filterScanline(pix)
+  doAssert s == @[1, 2, 3]