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Compiler bug when combining object variants and case switch #19833

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AurelienSaussay opened this issue May 26, 2022 · 2 comments · May be fixed by #23516
Open

Compiler bug when combining object variants and case switch #19833

AurelienSaussay opened this issue May 26, 2022 · 2 comments · May be fixed by #23516
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refc refc issues

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@AurelienSaussay
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AurelienSaussay commented May 26, 2022 •
edited

I am using a custom AST for a mathematical modeling application. This led me to a compiler bug involving object variants and case switch.

The bug can be reproduced from the following minimal example below. The C code output from this example doesn't compile. Interestingly, it does compile if I remove if n.lag < 0: s.append(&"({$n.lag})")

Example

import strformat

type
  NodeKind* = enum
    nkBinary,
    nkVariable,
    nkExpression
  CompiledNode* = ref CompileNodeObj
  CompileNodeObj = object
    case kind*: NodeKind
    of nkExpression:
      node*: CompiledNode
    of nkBinary:
      op*: string
      lhs*: CompiledNode
      rhs*: CompiledNode
    of nkVariable:
      root*: string
      subscripts*: seq[string]
      lag*: int
      
proc append(s: var string, s2: string): var string {.discardable.} = s.add(s2); return s

proc tmpl*(s: var string, n: CompiledNode): var string {.discardable.} =
  case n.kind:
  of nkBinary: s.tmpl(n.lhs).append(&" {n.op} ").tmpl(n.rhs)
  of nkExpression: s.tmpl(n.node)
  of nkVariable: 
    s.append(n.root)
    if n.lag < 0: s.append(&"({$n.lag})")
  return s

var n = CompiledNode(kind: nkBinary, op: "+", lhs: CompiledNode(kind: nkVariable, root: "a"), rhs: CompiledNode(kind: nkVariable, root: "b"))
var s: string
echo s.tmpl(n)

Current Output

@mminimal.nim.c:495:30: error: use of undeclared identifier 'T4_'; did you mean 'T6_'?
                unsureAsgnRef((void**) (&(*T4_)), copyString((*T6_)));
                                           ^~~
                                           T6_
@mminimal.nim.c:490:19: note: 'T6_' declared here
                NimStringDesc** T6_;
                                ^
1 error generated.

Additional information

Inspecting mminimal.nim.c reveals that the C code that fails to compile was emitted by the corresponding Nim line of nkExpression: s.tmpl(n.node).

$ nim -v
Nim Compiler Version 1.6.4 [MacOSX: amd64]
(also fails on WIndows 10: x86-64)
@metagn
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metagn commented May 29, 2022

Works when using discard on the tmpl call in nkExpression.

@shirleyquirk
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shirleyquirk commented Dec 9, 2023 •
edited

works with --mm:orc, so in 2.0 and devel. still doesn't work with --mm:refc

@ringabout ringabout added the refc refc issues label Dec 15, 2023
ringabout added a commit that referenced this issue Apr 18, 2024
@ringabout
fixes #19833 #19833; transform discardable into discard statement
e4ac3de
@ringabout ringabout linked a pull request Apr 18, 2024 that will close this issue
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fixes #19833 #19833; transform a discardable expression into a discard statement nim-lang/Nim
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@AurelienSaussay @metagn @shirleyquirk @ringabout