[Issue 23904] Spurious "struct [...] already exists"

May 08, 2023

Posted by Vladimir Panteleev

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Vladimir Panteleev

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https://issues.dlang.org/show_bug.cgi?id=23904

--- Comment #1 from Vladimir Panteleev <dlang-bugzilla@thecybershadow.net> ---
BTW, it compiles in 2.080.1. I did not mark it as a regression because, even though the release compiler does not produce any errors, the debug build of that version trips an assertion:

core.exception.AssertError@dmd/dsymbolsem.d(4208): Assertion failure
----------------
dmd() [0x5869ff]
dmd(_ZN22DsymbolSemanticVisitor5visitEP17StructDeclaration+0x8ea) [0x582552]
dmd(_ZN17StructDeclaration6acceptEP7Visitor+0x1d) [0x55d31d]
dmd(_Z15dsymbolSemanticP7DsymbolP5Scope+0x3d) [0x5779d5]
dmd(_ZN16TemplateInstance13expandMembersEP5Scope+0xdd) [0x596735]
dmd(_ZN16TemplateInstance16tryExpandMembersEP5Scope+0x5c) [0x5967a4]
dmd(void
dmd.dsymbolsem.templateInstanceSemantic(dmd.dtemplate.TemplateInstance,
dmd.dscope.Scope*,
dmd.root.array.Array!(dmd.expression.Expression).Array*)+0x8d8) [0x585b50]
dmd(_ZN22DsymbolSemanticVisitor5visitEP16TemplateInstance+0x16) [0x57da96]
dmd(_ZN16TemplateInstance6acceptEP7Visitor+0x1d) [0x596845]
dmd(_Z15dsymbolSemanticP7DsymbolP5Scope+0x3d) [0x5779d5]
dmd(_ZN25ExpressionSemanticVisitor5visitEP8ScopeExp+0x3c6) [0x5af336]
dmd(_ZN8ScopeExp6acceptEP7Visitor+0x1d) [0x5a36fd]
dmd(_Z18expressionSemanticP10ExpressionP5Scope+0x43) [0x5c38fb]
dmd(_ZN25ExpressionSemanticVisitor5visitEP7CallExp+0x49e) [0x5b1cae]
dmd(_ZN7CallExp6acceptEP7Visitor+0x1d) [0x5a6b7d]
dmd(_Z18expressionSemanticP10ExpressionP5Scope+0x43) [0x5c38fb]
dmd(_ZN24StatementSemanticVisitor5visitEP12ExpStatement+0x50) [0x63d278]
dmd(_ZN12ExpStatement6acceptEP7Visitor+0x1d) [0x622d1d]
dmd(_Z17statementSemanticP9StatementP5Scope+0x43) [0x63d19b]
dmd(_ZN24StatementSemanticVisitor5visitEP17CompoundStatement+0xe8) [0x63d518]
dmd(_ZN17CompoundStatement6acceptEP7Visitor+0x1d) [0x62344d]
dmd(_Z17statementSemanticP9StatementP5Scope+0x43) [0x63d19b]
dmd(_ZN16Semantic3Visitor5visitEP15FuncDeclaration+0x1174) [0x6504dc]
dmd(_ZN15FuncDeclaration6acceptEP7Visitor+0x1d) [0x5c9fd5]
dmd(_Z9semantic3P7DsymbolP5Scope+0x3d) [0x64ef7d]
dmd(_ZN16Semantic3Visitor5visitEP16TemplateInstance+0x14f) [0x64f0f7]
dmd(_ZN16TemplateInstance6acceptEP7Visitor+0x1d) [0x596845]
dmd(_Z9semantic3P7DsymbolP5Scope+0x3d) [0x64ef7d]
dmd(_ZN16TemplateInstance12trySemantic3EP5Scope+0x5c) [0x59680c]
dmd(void
dmd.dsymbolsem.templateInstanceSemantic(dmd.dtemplate.TemplateInstance,
dmd.dscope.Scope*,
dmd.root.array.Array!(dmd.expression.Expression).Array*)+0xcc3) [0x585f3b]
dmd(void dmd.dtemplate.functionResolve(dmd.declaration.Match*,
dmd.dsymbol.Dsymbol, dmd.globals.Loc, dmd.dscope.Scope*,
dmd.root.array.Array!(dmd.root.rootobject.RootObject).Array*, dmd.mtype.Type,
dmd.root.array.Array!(dmd.expression.Expression).Array*, const(char)**)+0x148)
[0x58b760]
dmd(_Z15resolveFuncCallRK3LocP5ScopeP7DsymbolP5ArrayIP10RootObjectEP4TypePS6_IP10ExpressionEi+0xd1)
[0x5ca761]
dmd(_ZN25ExpressionSemanticVisitor5visitEP7CallExp+0x2504) [0x5b3d14]
dmd(_ZN7CallExp6acceptEP7Visitor+0x1d) [0x5a6b7d]
dmd(_Z18expressionSemanticP10ExpressionP5Scope+0x43) [0x5c38fb]
dmd(_ZN24StatementSemanticVisitor5visitEP12ExpStatement+0x50) [0x63d278]
dmd(_ZN12ExpStatement6acceptEP7Visitor+0x1d) [0x622d1d]
dmd(_Z17statementSemanticP9StatementP5Scope+0x43) [0x63d19b]
dmd(_ZN24StatementSemanticVisitor5visitEP17CompoundStatement+0xe8) [0x63d518]
dmd(_ZN17CompoundStatement6acceptEP7Visitor+0x1d) [0x62344d]
dmd(_Z17statementSemanticP9StatementP5Scope+0x43) [0x63d19b]
dmd(_ZN16Semantic3Visitor5visitEP15FuncDeclaration+0x1174) [0x6504dc]
dmd(_ZN15FuncDeclaration6acceptEP7Visitor+0x1d) [0x5c9fd5]
dmd(_Z9semantic3P7DsymbolP5Scope+0x3d) [0x64ef7d]
dmd(_ZN16Semantic3Visitor5visitEP16TemplateInstance+0x14f) [0x64f0f7]
dmd(_ZN16TemplateInstance6acceptEP7Visitor+0x1d) [0x596845]
dmd(_Z9semantic3P7DsymbolP5Scope+0x3d) [0x64ef7d]
dmd(_ZN16TemplateInstance12trySemantic3EP5Scope+0x5c) [0x59680c]
dmd(void
dmd.dsymbolsem.templateInstanceSemantic(dmd.dtemplate.TemplateInstance,
dmd.dscope.Scope*,
dmd.root.array.Array!(dmd.expression.Expression).Array*)+0xac9) [0x585d41]
dmd(void dmd.dtemplate.functionResolve(dmd.declaration.Match*,
dmd.dsymbol.Dsymbol, dmd.globals.Loc, dmd.dscope.Scope*,
dmd.root.array.Array!(dmd.root.rootobject.RootObject).Array*, dmd.mtype.Type,
dmd.root.array.Array!(dmd.expression.Expression).Array*, const(char)**)+0x148)
[0x58b760]
dmd(_Z15resolveFuncCallRK3LocP5ScopeP7DsymbolP5ArrayIP10RootObjectEP4TypePS6_IP10ExpressionEi+0xd1)
[0x5ca761]
dmd(_ZN25ExpressionSemanticVisitor5visitEP7CallExp+0x2504) [0x5b3d14]
dmd(_ZN7CallExp6acceptEP7Visitor+0x1d) [0x5a6b7d]
dmd(_Z18expressionSemanticP10ExpressionP5Scope+0x43) [0x5c38fb]
dmd(_ZN24StatementSemanticVisitor5visitEP12ExpStatement+0x50) [0x63d278]
dmd(_ZN12ExpStatement6acceptEP7Visitor+0x1d) [0x622d1d]
dmd(_Z17statementSemanticP9StatementP5Scope+0x43) [0x63d19b]
dmd(_ZN24StatementSemanticVisitor5visitEP17CompoundStatement+0xe8) [0x63d518]
dmd(_ZN17CompoundStatement6acceptEP7Visitor+0x1d) [0x62344d]
dmd(_Z17statementSemanticP9StatementP5Scope+0x43) [0x63d19b]
dmd(_ZN16Semantic3Visitor5visitEP15FuncDeclaration+0x1174) [0x6504dc]
dmd(_ZN15FuncDeclaration6acceptEP7Visitor+0x1d) [0x5c9fd5]
dmd(_Z9semantic3P7DsymbolP5Scope+0x3d) [0x64ef7d]
dmd(_ZN16Semantic3Visitor5visitEP6Module+0x6a) [0x64f31a]
dmd(_ZN6Module6acceptEP7Visitor+0x1d) [0x553055]
dmd(_Z9semantic3P7DsymbolP5Scope+0x3d) [0x64ef7d]
dmd(int dmd.mars.tryMain(ulong, const(char)**)+0x2151) [0x5f1491]
dmd(_Dmain+0x27) [0x5f238f]
dmd(_D2rt6dmain211_d_run_mainUiPPaPUAAaZiZ6runAllMFZ9__lambda1MFZv+0x1f)
[0x73626b]
dmd(void rt.dmain2._d_run_main(int, char**, extern (C) int
function(char[][])*).tryExec(scope void delegate())+0x2a) [0x7361c6]
dmd(void rt.dmain2._d_run_main(int, char**, extern (C) int
function(char[][])*).runAll()+0x2b) [0x736227]
dmd(void rt.dmain2._d_run_main(int, char**, extern (C) int
function(char[][])*).tryExec(scope void delegate())+0x2a) [0x7361c6]
dmd(_d_run_main+0x1d2) [0x736146]
dmd(main+0x14) [0x5f6074]

--

https://issues.dlang.org/show_bug.cgi?id=23904

RazvanN <razvan.nitu1305@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |razvan.nitu1305@gmail.com

--- Comment #2 from RazvanN <razvan.nitu1305@gmail.com> ---
Further reduced:
======================
  template f1(alias fun)
  {
      struct UnusedButConflicting
      {
      }

      auto f1()
      {
      }
  }


  template f2(T)
  {
      alias fun = {};

      auto f2(auto ref T)
      {
          f1!fun();
      }
  }

  void main()
  {
      int a;
      f2(5);
      f2(a);
  }
==========================

The problem here comes from the way dmd stores types. It essentially creates a decoration by getting the mangled name of the type. In this case f2 creates different template instances depending on whether f2 is called with an lvalue or an rvalue (due to the presence of auto ref). Having 2 template instances, this leads to having 2 f1 template instances due to `fun` being declared twice in 2 different scopes, although the type is the same.

When semantic is performed on the unused struct the compiler tries to get a unique identifier for the struct type by generating the mangled type. However, the mangling is performing only by taking into account types. Since for both template instances the type of the alias parameter is the same is the same (although the value is different), the mangling ends up being the same, however, the declarations are different, hence the error.

It seems that using the type for alias template parameters to generate the mangling for such declarations is not a correct solution.

--

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