From 2d1412a2eae9abc7c5827fac5bf028f71599790a Mon Sep 17 00:00:00 2001
From: ringabout <43030857+ringabout@users.noreply.github.com>
Date: Sun, 23 Aug 2026 18:37:15 +0800
Subject: [PATCH] fixes #26015; Multiple definition error when using
 codegenDecl regression (#26018)
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fixes #26015

Fixes imported global variables with codegenDecl being emitted as
definitions instead of extern declarations.

A variable’s codegenDecl format should customize its definition in the
owning module. Other modules referencing the variable must emit a normal
declaration:

```c
extern NI variable;
```

After the variable-declaration builder refactor, genVarPrototype passed
Extern visibility to addVar. However, the sfCodegenDecl branch returned
before applying that visibility. This caused importing modules to emit
another tentative definition, resulting in duplicate-symbol linker
errors.

The fix restores the previous distinction between the custom definition
and cross-module prototypes. It also adds C and C++ regression coverage
for both direct access and access through an inline procedure.

follows up https://github.com/nim-lang/Nim/pull/24423
---
 compiler/cgen.nim                    | 14 ++++++++++----
 tests/ccgbugs/mcodegendeclglobal.nim |  4 ++++
 tests/ccgbugs/tcodegendeclglobal.nim | 13 +++++++++++++
 3 files changed, 27 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 4cdd86347a..8566c62632 100644
--- a/compiler/cgen.nim
+++ b/compiler/cgen.nim
@@ -1854,10 +1854,16 @@ 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:
+        m.s[cfsVars].addDeclWithVisibility(vis):
+          m.s[cfsVars].addVar(m, sym,
+            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..b630d948c7
--- /dev/null
+++ b/tests/ccgbugs/tcodegendeclglobal.nim
@@ -0,0 +1,13 @@
+discard """
+  output: '''
+123
+123
+'''
+  ccodecheck: "'extern NI /* custom declaration */ codegenDeclGlobal'"
+  targets: "c cpp"
+"""
+
+import ./mcodegendeclglobal
+
+echo codegenDeclGlobal
+echo readCodegenDeclGlobal()