Add nkFastAsgn into semExpr (#20939)

* Add nkFastAsgn into case statement

* Add test case

(cherry picked from commit d26b1232ee)

compiler/semexprs.nim

@@ -3005,7 +3005,7 @@ proc semExpr(c: PContext, n: PNode, flags: TExprFlags = {}): PNode =
     # the node is left intact for now
     discard
   of nkStaticExpr: result = semStaticExpr(c, n[0])
-  of nkAsgn: result = semAsgn(c, n)
+  of nkAsgn, nkFastAsgn: result = semAsgn(c, n)
   of nkBlockStmt, nkBlockExpr: result = semBlock(c, n, flags)
   of nkStmtList, nkStmtListExpr: result = semStmtList(c, n, flags)
   of nkRaiseStmt: result = semRaise(c, n)

tests/magics/t20938.nim

@@ -0,0 +1,12 @@
+discard """
+  cmd: "nim c --mm:refc $file"
+  action: "compile"
+"""
+
+template foo(x: typed) =
+  discard x
+
+foo:
+  var x = "hello"
+  x.shallowCopy("test")
+  true