From 11ef897c1a0a6fb68510e2eb7b1246c42454b43f Mon Sep 17 00:00:00 2001
From: ringabout <43030857+ringabout@users.noreply.github.com>
Date: Fri, 17 Jul 2026 17:39:48 +0800
Subject: [PATCH] fixes #26015; Multiple definition error when using
 codegenDecl regression

---
 compiler/cgen.nim                    | 18 ++++++++++++++----
 tests/ccgbugs/mcodegendeclglobal.nim |  4 ++++
 tests/ccgbugs/tcodegendeclglobal.nim | 12 ++++++++++++
 3 files changed, 30 insertions(+), 4 deletions(-)
 create mode 100644 tests/ccgbugs/mcodegendeclglobal.nim
 create mode 100644 tests/ccgbugs/tcodegendeclglobal.nim

diff --git a/compiler/cgen.nim b/compiler/cgen.nim
index 52c8726485..7339980bc5 100644
--- a/compiler/cgen.nim
+++ b/compiler/cgen.nim
@@ -1854,10 +1854,20 @@ proc genVarPrototype(m: BModule, n: PNode) =
         typ = ptrType(typ)
       if lfDynamicLib in sym.loc.flags:
         typ = ptrType(typ)
-      m.s[cfsVars].addVar(m, sym,
-        name = sym.loc.snippet,
-        typ = typ,
-        visibility = vis)
+      if sfCodegenDecl in sym.flags:
+        # `codegenDecl` customizes the definition, but a reference from a
+        # different module still needs a plain declaration. Applying the
+        # custom format here can turn the declaration into a tentative
+        # definition (and thus cause duplicate symbols at link time).
+        m.s[cfsVars].addDeclWithVisibility(vis):
+          m.s[cfsVars].addVar(kind = Local,
+            name = sym.loc.snippet,
+            typ = typ)
+      else:
+        m.s[cfsVars].addVar(m, sym,
+          name = sym.loc.snippet,
+          typ = typ,
+          visibility = vis)
       if m.hcrOn:
         m.initProc.procSec(cpsLocals).add('\t')
         m.initProc.procSec(cpsLocals).addAssignment(sym.loc.snippet,
diff --git a/tests/ccgbugs/mcodegendeclglobal.nim b/tests/ccgbugs/mcodegendeclglobal.nim
new file mode 100644
index 0000000000..1f2ab904f4
--- /dev/null
+++ b/tests/ccgbugs/mcodegendeclglobal.nim
@@ -0,0 +1,4 @@
+var codegenDeclGlobal* {.codegenDecl: "$# /* custom declaration */ $#".} = 123
+
+proc readCodegenDeclGlobal*(): int {.inline.} =
+  codegenDeclGlobal
diff --git a/tests/ccgbugs/tcodegendeclglobal.nim b/tests/ccgbugs/tcodegendeclglobal.nim
new file mode 100644
index 0000000000..c8c4f6b7d4
--- /dev/null
+++ b/tests/ccgbugs/tcodegendeclglobal.nim
@@ -0,0 +1,12 @@
+discard """
+  output: '''
+123
+123
+'''
+  targets: "c cpp"
+"""
+
+import ./mcodegendeclglobal
+
+echo codegenDeclGlobal
+echo readCodegenDeclGlobal()