implements #21620: allowing to import multiple modules with shared names (#21628)

compiler/importer.nim

@@ -228,6 +228,11 @@ proc importForwarded(c: PContext, n: PNode, exceptSet: IntSet; fromMod: PSym; im
     for i in 0..n.safeLen-1:
       importForwarded(c, n[i], exceptSet, fromMod, importSet)
 
+proc addUnique[T](x: var seq[T], y: sink T) {.noSideEffect.} =
+  for i in 0..high(x):
+    if x[i] == y: return
+  x.add y
+    
 proc importModuleAs(c: PContext; n: PNode, realModule: PSym, importHidden: bool): PSym =
   result = realModule
   template createModuleAliasImpl(ident): untyped =
@@ -245,6 +250,7 @@ proc importModuleAs(c: PContext; n: PNode, realModule: PSym, importHidden: bool)
     result.options.incl optImportHidden
   c.unusedImports.add((result, n.info))
   c.importModuleMap[result.id] = realModule.id
+  c.importModuleLookup.mgetOrPut(realModule.name.id, @[]).addUnique realModule.id
 
 proc transformImportAs(c: PContext; n: PNode): tuple[node: PNode, importHidden: bool] =
   var ret: typeof(result)

compiler/lookups.nim

@@ -8,7 +8,7 @@
 #
 
 # This module implements lookup helpers.
-import std/[algorithm, strutils]
+import std/[algorithm, strutils, tables]
 
 when defined(nimPreviewSlimSystem):
   import std/assertions
@@ -343,14 +343,15 @@ proc addDeclAt*(c: PContext; scope: PScope, sym: PSym, info: TLineInfo) =
   if sym.name.s == "_": return
   let conflict = scope.addUniqueSym(sym)
   if conflict != nil:
-    if sym.kind == skModule and conflict.kind == skModule and
-                        sym.position == conflict.position:
+    if sym.kind == skModule and conflict.kind == skModule:      
       # 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.
-      localError(c.config, info, hintDuplicateModuleImport,
-        "duplicate import of '$1'; previous import here: $2" %
-        [sym.name.s, c.config $ conflict.info])
+      # where a duplicate import happens inside an include.      
+      if c.importModuleMap[sym.id] == c.importModuleMap[conflict.id]:
+        #only hints if the conflict is the actual module not just a shared name
+        localError(c.config, info, hintDuplicateModuleImport,
+          "duplicate import of '$1'; previous import here: $2" %
+          [sym.name.s, c.config $ conflict.info])
     else:
       wrongRedefinition(c, info, sym.name.s, conflict.info, errGenerated)
 
@@ -631,7 +632,11 @@ proc qualifiedLookUp*(c: PContext, n: PNode, flags: set[TLookupFlag]): PSym =
         if m == c.module:
           result = strTableGet(c.topLevelScope.symbols, ident).skipAlias(n, c.config)
         else:
-          result = someSym(c.graph, m, ident).skipAlias(n, c.config)
+          if c.importModuleLookup.getOrDefault(m.name.id).len > 1:
+            var amb: bool
+            result = errorUseQualifier(c, n.info, m, amb)
+          else:
+            result = someSym(c.graph, m, ident).skipAlias(n, c.config)
         if result == nil and checkUndeclared in flags:
           result = errorUndeclaredIdentifierHint(c, n[1], ident)
       elif n[1].kind == nkSym:

compiler/semdata.nim

@@ -166,6 +166,7 @@ type
     lastTLineInfo*: TLineInfo
     sideEffects*: Table[int, seq[(TLineInfo, PSym)]] # symbol.id index
     inUncheckedAssignSection*: int
+    importModuleLookup*: Table[int, seq[int]] # (module.ident.id, [module.id])
 
 template config*(c: PContext): ConfigRef = c.graph.config
 

tests/import/t21496.nim

@@ -1,5 +1,5 @@
 discard """
-  errormsg: "redefinition of 'm21496'; previous declaration here: t21496.nim(5, 12)"
+  errormsg: "ambiguous identifier: 'm21496'"
 """
 
 import fizz/m21496, buzz/m21496