fixes #3804

compiler/vm.nim

@@ -123,7 +123,7 @@ template move(a, b: expr) {.immediate, dirty.} = system.shallowCopy(a, b)
 # XXX fix minor 'shallowCopy' overloading bug in compiler
 
 proc createStrKeepNode(x: var TFullReg; keepNode=true) =
-  if x.node.isNil:
+  if x.node.isNil or not keepNode:
     x.node = newNode(nkStrLit)
   elif x.node.kind == nkNilLit and keepNode:
     when defined(useNodeIds):

compiler/vmgen.nim

@@ -102,6 +102,10 @@ proc gABC(ctx: PCtx; n: PNode; opc: TOpcode; a, b, c: TRegister = 0) =
   let ins = (opc.uint32 or (a.uint32 shl 8'u32) or
                            (b.uint32 shl 16'u32) or
                            (c.uint32 shl 24'u32)).TInstr
+  when false:
+    if ctx.code.len == 72:
+      writeStackTrace()
+      echo "generating ", opc
   ctx.code.add(ins)
   ctx.debug.add(n.info)
 

tests/vm/twrong_concat.nim

@@ -0,0 +1,28 @@
+discard """
+  output: '''success'''
+"""
+
+# bug #3804
+
+#import sequtils
+
+type AnObj = ref object
+  field: string
+
+#proc aBug(objs: seq[AnObj]) {.compileTime.} =
+#  discard objs.mapIt(it.field & " bug")
+
+proc sameBug(objs: seq[AnObj]) {.compileTime.} =
+  var strSeq = newSeq[string](objs.len)
+  strSeq[0] = objs[0].field & " bug"
+
+static:
+  var objs: seq[AnObj] = @[]
+  objs.add(AnObj(field: "hello"))
+
+  sameBug(objs)
+  # sameBug(objs)
+  echo objs[0].field
+  assert(objs[0].field == "hello") # fails, because (objs[0].field == "hello bug") - mutated!
+
+echo "success"