fixes #5269

compiler/vmgen.nim

@@ -1259,6 +1259,13 @@ proc isTemp(c: PCtx; dest: TDest): bool =
 template needsAdditionalCopy(n): untyped =
   not c.isTemp(dest) and not fitsRegister(n.typ)
 
+proc genAdditionalCopy(c: PCtx; n: PNode; opc: TOpcode;
+                       dest, idx, value: TRegister) =
+  var cc = c.getTemp(n.typ)
+  c.gABC(n, whichAsgnOpc(n), cc, value, 0)
+  c.gABC(n, opc, dest, idx, cc)
+  c.freeTemp(cc)
+
 proc preventFalseAlias(c: PCtx; n: PNode; opc: TOpcode;
                        dest, idx, value: TRegister) =
   # opcLdObj et al really means "load address". We sometimes have to create a
@@ -1266,10 +1273,7 @@ proc preventFalseAlias(c: PCtx; n: PNode; opc: TOpcode;
   # mylocal = a.b  # needs a copy of the data!
   assert n.typ != nil
   if needsAdditionalCopy(n):
-    var cc = c.getTemp(n.typ)
-    c.gABC(n, whichAsgnOpc(n), cc, value, 0)
-    c.gABC(n, opc, dest, idx, cc)
-    c.freeTemp(cc)
+    genAdditionalCopy(c, n, opc, dest, idx, value)
   else:
     c.gABC(n, opc, dest, idx, value)
 
@@ -1352,7 +1356,7 @@ proc genGlobalInit(c: PCtx; n: PNode; s: PSym) =
   c.gABx(n, opcLdGlobal, dest, s.position)
   if s.ast != nil:
     let tmp = c.genx(s.ast)
-    c.preventFalseAlias(n, opcWrDeref, dest, 0, tmp)
+    c.genAdditionalCopy(n, opcWrDeref, dest, 0, tmp)
     c.freeTemp(dest)
     c.freeTemp(tmp)
 
@@ -1525,7 +1529,7 @@ proc genVarSection(c: PCtx; n: PNode) =
         if a.sons[2].kind != nkEmpty:
           let tmp = c.genx(a.sons[0], {gfAddrOf})
           let val = c.genx(a.sons[2])
-          c.preventFalseAlias(a.sons[2], opcWrDeref, tmp, 0, val)
+          c.genAdditionalCopy(a.sons[2], opcWrDeref, tmp, 0, val)
           c.freeTemp(val)
           c.freeTemp(tmp)
       else:

tests/vm/tcopy_global_var.nim

@@ -0,0 +1,30 @@
+discard """
+  nimout: "static done"
+"""
+
+# bug #5269
+
+proc assertEq[T](arg0, arg1: T): void =
+  assert arg0 == arg1, $arg0 & " == " & $arg1
+
+type
+  MyType = object
+    str: string
+    a: int
+
+block:
+  var localValue = MyType(str: "Original strning, (OK)", a: 0)
+  var valueCopy = localValue
+  valueCopy.a = 123
+  valueCopy.str = "Modified strning, (not OK when in localValue)"
+  assertEq(localValue.str, "Original strning, (OK)")
+  assertEq(localValue.a,   0)
+
+static:
+  var localValue = MyType(str: "Original strning, (OK)", a: 0)
+  var valueCopy = localValue
+  valueCopy.a = 123
+  valueCopy.str = "Modified strning, (not OK when in localValue)"
+  assertEq(localValue.str, "Original strning, (OK)")
+  assertEq(localValue.a,   0)
+  echo "static done"