From 9c85f4fd074661fcdd7fa233f2c152d4b560b32e Mon Sep 17 00:00:00 2001
From: metagn <metagngn@gmail.com>
Date: Fri, 11 Oct 2024 11:36:40 +0300
Subject: [PATCH] fix deref/addr pair deleting assignment location in C++
 (#24280)

fixes #24274

The code in the `if` branch replaces the current destination `d` with a
new one. But the location `d` can be an assignment location, in which
case the provided expression isn't generated. To fix this, don't trigger
this code for when the location already exists. An alternative would be
to call `putIntoDest` in this case as is done below.
---
 compiler/ccgexprs.nim           |  5 ++++-
 tests/ccgbugs/tvararrayiter.nim | 21 +++++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)
 create mode 100644 tests/ccgbugs/tvararrayiter.nim

diff --git a/compiler/ccgexprs.nim b/compiler/ccgexprs.nim
index 545d43ae83..a344816ba8 100644
--- a/compiler/ccgexprs.nim
+++ b/compiler/ccgexprs.nim
@@ -758,7 +758,10 @@ proc genDeref(p: BProc, e: PNode, d: var TLoc) =
     if typ.kind in {tyUserTypeClass, tyUserTypeClassInst} and typ.isResolvedUserTypeClass:
       typ = typ.last
     typ = typ.skipTypes(abstractInstOwned)
-    if typ.kind in {tyVar} and tfVarIsPtr notin typ.flags and p.module.compileToCpp and e[0].kind == nkHiddenAddr:
+    if typ.kind in {tyVar} and tfVarIsPtr notin typ.flags and
+        p.module.compileToCpp and e[0].kind == nkHiddenAddr and
+        # don't override existing location:
+        d.k == locNone:
       d = initLocExprSingleUse(p, e[0][0])
       return
     else:
diff --git a/tests/ccgbugs/tvararrayiter.nim b/tests/ccgbugs/tvararrayiter.nim
new file mode 100644
index 0000000000..b1f8c2087e
--- /dev/null
+++ b/tests/ccgbugs/tvararrayiter.nim
@@ -0,0 +1,21 @@
+discard """
+  targets: "c cpp"
+"""
+
+block: # issue #24274
+  iterator foo[T](x: var T): var T =
+    yield x
+
+  var x: array[3, char]
+  for a in foo(x):
+    let b = a
+  
+  var y: array[3, char] = ['a', 'b', 'c']
+  for a in foo(y):
+    let b = a
+    doAssert a[0] == 'a'
+    doAssert a[1] == 'b'
+    doAssert a[2] == 'c'
+    doAssert b[0] == 'a'
+    doAssert b[1] == 'b'
+    doAssert b[2] == 'c'