From ee2b480da629f23ced29a244ca706e4dd650f235 Mon Sep 17 00:00:00 2001
From: ringabout <43030857+ringabout@users.noreply.github.com>
Date: Fri, 19 Sep 2025 02:50:46 +0800
Subject: [PATCH] makes `DuplicateModuleImport` back to an error (#25178)

fixes #24998

Basically it retraces back to the situation before
https://github.com/nim-lang/Nim/pull/18366 and
https://github.com/nim-lang/Nim/pull/18362, i.e.

```nim
import fuzz/a
import fuzz/a
```

```nim
import fuzz/a
from buzz/a
```

```nim
import fuzz/a except nil
from fuzz/a import addInt
```

All of these cases are now flagged as invalid and triggers a
redefinition error, i.e., each module name importing is treated as
consistent as the symbol definition

kinda annoying for importing/exporting with `when conditions` though

ref https://github.com/nim-lang/Nim/issues/18762
https://github.com/nim-lang/Nim/issues/20907

```nim
from std/strutils import toLower
when not defined(js):
  from std/strutils import toUpper
```

(cherry picked from commit 87ee9c84cb8cba3a404af235755c80037aefea6b)
---
 changelog.md             | 2 ++
 compiler/condsyms.nim    | 2 ++
 compiler/lookups.nim     | 2 +-
 compiler/nim.cfg         | 2 ++
 compiler/suggest.nim     | 2 +-
 lib/pure/terminal.nim    | 7 +------
 lib/system/alloc.nim     | 1 -
 lib/system/strmantle.nim | 4 +---
 8 files changed, 10 insertions(+), 12 deletions(-)

diff --git a/changelog.md b/changelog.md
index cf2d4cb170..959f105669 100644
--- a/changelog.md
+++ b/changelog.md
@@ -25,6 +25,8 @@ errors.
 
 - `std/parsesql` has been moved to a nimble package, use `nimble` or `atlas` to install it.
 
+- With `-d:nimPreviewDuplicateModuleError`, importing two modules that share the same name becomes a compile-time error. This includes importing the same module more than once. Use `import foo as foo1` (or other aliases) to avoid collisions.
+
 ## Standard library additions and changes
 
 [//]: # "Additions:"
diff --git a/compiler/condsyms.nim b/compiler/condsyms.nim
index 81cc40f3f6..8bfcb2dac8 100644
--- a/compiler/condsyms.nim
+++ b/compiler/condsyms.nim
@@ -171,3 +171,5 @@ proc initDefines*(symbols: StringTableRef) =
   defineSymbol("nimHasJsNoLambdaLifting")
   defineSymbol("nimHasDefaultFloatRoundtrip")
   defineSymbol("nimHasXorSet")
+
+  defineSymbol("nimHasPreviewDuplicateModuleError")
diff --git a/compiler/lookups.nim b/compiler/lookups.nim
index 34f65973cf..acaad9d9b4 100644
--- a/compiler/lookups.nim
+++ b/compiler/lookups.nim
@@ -388,7 +388,7 @@ proc addDeclAt*(c: PContext; scope: PScope, sym: PSym, info: TLineInfo) =
   if sym.name.id == ord(wUnderscore): return
   let conflict = scope.addUniqueSym(sym)
   if conflict != nil:
-    if sym.kind == skModule and conflict.kind == skModule:
+    if sym.kind == skModule and conflict.kind == skModule and not c.config.isDefined("nimPreviewDuplicateModuleError"):
       # e.g.: import foo; import foo
       # xxx we could refine this by issuing a different hint for the case
       # where a duplicate import happens inside an include.
diff --git a/compiler/nim.cfg b/compiler/nim.cfg
index 0cc8c476ec..9dab29eeed 100644
--- a/compiler/nim.cfg
+++ b/compiler/nim.cfg
@@ -12,6 +12,8 @@ define:nimPreviewNonVarDestructor
 define:nimPreviewCheckedClose
 define:nimPreviewAsmSemSymbol
 define:nimPreviewCStringComparisons
+define:nimPreviewDuplicateModuleError
+
 threads:off
 
 #import:"$projectpath/testability"
diff --git a/compiler/suggest.nim b/compiler/suggest.nim
index 1317fb2e48..3953936eb6 100644
--- a/compiler/suggest.nim
+++ b/compiler/suggest.nim
@@ -35,7 +35,7 @@
 import prefixmatches, suggestsymdb
 from wordrecg import wDeprecated, wError, wAddr, wYield
 
-import std/[algorithm, sets, parseutils, tables, os]
+import std/[algorithm, sets, parseutils, os]
 
 when defined(nimsuggest):
   import pathutils # importer
diff --git a/lib/pure/terminal.nim b/lib/pure/terminal.nim
index 91f0910585..895f658e4d 100644
--- a/lib/pure/terminal.nim
+++ b/lib/pure/terminal.nim
@@ -100,7 +100,7 @@ const
   stylePrefix = "\e["
 
 when defined(windows):
-  import std/[winlean, os]
+  import std/os
 
   const
     DUPLICATE_SAME_ACCESS = 2
@@ -926,8 +926,6 @@ when defined(windows):
     stdout.write "\n"
 
 else:
-  import std/termios
-
   proc readPasswordFromStdin*(prompt: string, password: var string):
                             bool {.tags: [ReadIOEffect, WriteIOEffect].} =
     password.setLen(0)
@@ -981,9 +979,6 @@ proc isTrueColorSupported*(): bool =
   ## Returns true if a terminal supports true color.
   return getTerminal().trueColorIsSupported
 
-when defined(windows):
-  import std/os
-
 proc enableTrueColors*() =
   ## Enables true color.
   var term = getTerminal()
diff --git a/lib/system/alloc.nim b/lib/system/alloc.nim
index 63a0970536..fcb7ccb0c8 100644
--- a/lib/system/alloc.nim
+++ b/lib/system/alloc.nim
@@ -12,7 +12,6 @@
 
 include osalloc
 import std/private/syslocks
-import std/sysatomics
 
 template track(op, address, size) =
   when defined(memTracker):
diff --git a/lib/system/strmantle.nim b/lib/system/strmantle.nim
index 694ef64257..92a8653f2a 100644
--- a/lib/system/strmantle.nim
+++ b/lib/system/strmantle.nim
@@ -8,9 +8,7 @@
 #
 
 # Compilerprocs for strings that do not depend on the string implementation.
-
-import std/private/digitsutils
-
+import std/private/digitsutils as digitsutils2
 
 proc cmpStrings(a, b: string): int {.inline, compilerproc.} =
   let alen = a.len