fixes #26015; Multiple definition error when using codegenDecl regression

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,

tests/ccgbugs/mcodegendeclglobal.nim

@@ -0,0 +1,4 @@
+var codegenDeclGlobal* {.codegenDecl: "$# /* custom declaration */ $#".} = 123
+
+proc readCodegenDeclGlobal*(): int {.inline.} =
+  codegenDeclGlobal

tests/ccgbugs/tcodegendeclglobal.nim

@@ -0,0 +1,12 @@
+discard """
+  output: '''
+123
+123
+'''
+  targets: "c cpp"
+"""
+
+import ./mcodegendeclglobal
+
+echo codegenDeclGlobal
+echo readCodegenDeclGlobal()