fix #25993; In-place object construction zeroes destination before evaluating self-referencing field values (#25994)

tests/ccgbugs/tobjconstr_self_alias.nim

@@ -0,0 +1,85 @@
+discard """
+  matrix: "--mm:refc; --mm:arc; --mm:orc"
+  output: '''42
+55
+42
+42
+42
+42'''
+"""
+
+# bug #25993 : an object constructor assigned to a location zeroed the
+# destination before evaluating a field value that reads from inside that same
+# destination, so `tp.h = H(a: tp.h.a)` produced `a == 0`.
+
+type
+  Inner = object
+    a: int
+    b: int
+  Mid = object
+    inner: Inner
+    x: int
+  RefT = ref object
+    h: Inner
+    other: Inner
+    m: Mid
+
+# --------------------------------------------------------------------------
+# bug demonstrations: each printed 0 before the fix
+# --------------------------------------------------------------------------
+
+proc refDotField(v: int) =
+  # dest `t.h` is a field of a ref; value reads `t.h.a` (nested in dest)
+  let t = RefT()
+  t.h.a = v
+  t.h = Inner(a: t.h.a)
+  echo t.h.a
+
+proc nestedConstr(v: int) =
+  # dest `t.m`; nested constructor value reads `t.m.inner.a` (nested in dest)
+  let t = RefT()
+  t.m.inner.a = v
+  t.m = Mid(inner: Inner(a: t.m.inner.a), x: 0)
+  echo t.m.inner.a
+
+proc refDeepField(v: int) =
+  # dest `t.m.inner`; value reads `t.m.inner.a` (nested in dest)
+  let t = RefT()
+  t.m.inner.a = v
+  t.m.inner = Inner(a: t.m.inner.a)
+  echo t.m.inner.a
+
+var gT: RefT
+
+proc readsField(t: RefT): int = t.h.a
+
+proc viaCall(v: int) =
+  # read of dest hidden behind a call whose argument is the root ref
+  let t = RefT()
+  t.h.a = v
+  t.h = Inner(a: readsField(t))
+  echo t.h.a
+
+proc viaClosureGlobal(v: int) =
+  # read of dest hidden behind a closure reaching it through a global
+  let t = RefT()
+  t.h.a = v
+  gT = t
+  let cl = proc(): int = gT.h.a
+  t.h = Inner(a: cl())
+  echo t.h.a
+
+proc viaClosureCapture(v: int) =
+  # read of dest hidden behind a closure that captures the root ref
+  let t = RefT()
+  t.h.a = v
+  let cl = proc(): int = t.h.a
+  t.h = Inner(a: cl())
+  echo t.h.a
+
+refDotField(42)
+nestedConstr(55)
+refDeepField(42)
+viaCall(42)
+viaClosureGlobal(42)
+viaClosureCapture(42)