compiler: cryptic error when the track expression is empty for the control flow @for

[pkozlowski-opensource]

Which @angular/* package(s) are the source of the bug?

compiler

Is this a regression?

No

Description

Writing the following in a template: @for (item of items; track ) will result in the cryptic compilation error:

✘ [ERROR] Angular compilation emit failed. [plugin angular-compiler]

  Error: AssertionError: Unsupported reification of ir.Expression kind: EmptyExpr
      at reifyIrExpression (file:///Users/pkozlowski/trash/ng-track-by/node_modules/@angular/compiler/fesm2022/compiler.mjs:22452:19)
      at transformExpressionsInExpression (file:///Users/pkozlowski/trash/ng-track-by/node_modules/@angular/compiler/fesm2022/compiler.mjs:10647:12)
      at transformExpressionsInOp (file:///Users/pkozlowski/trash/ng-track-by/node_modules/@angular/compiler/fesm2022/compiler.mjs:10489:24)
      at reifyCreateOperations (file:///Users/pkozlowski/trash/ng-track-by/node_modules/@angular/compiler/fesm2022/compiler.mjs:22136:9)
      at Object.reify [as fn] (file:///Users/pkozlowski/trash/ng-track-by/node_modules/@angular/compiler/fesm2022/compiler.mjs:22105:9)
      at transform (file:///Users/pkozlowski/trash/ng-track-by/node_modules/@angular/compiler/fesm2022/compiler.mjs:24249:19)
      at compileComponentFromMetadata (file:///Users/pkozlowski/trash/ng-track-by/node_modules/@angular/compiler/fesm2022/compiler.mjs:30764:9)
      at ComponentDecoratorHandler.compileFull (file:///Users/pkozlowski/trash/ng-track-by/node_modules/@angular/compiler-cli/bundles/chunk-WYZJV3LZ.js:7628:17)
      at TraitCompiler.compile (file:///Users/pkozlowski/trash/ng-track-by/node_modules/@angular/compiler-cli/bundles/chunk-WYZJV3LZ.js:2618:38)
      at IvyCompilationVisitor.visitClassDeclaration (file:///Users/pkozlowski/trash/ng-track-by/node_modules/@angular/compiler-cli/bundles/chunk-WYZJV3LZ.js:2958:37)

For 17.2 and the previous template compilation pipeline we got a different, but equally cryptic error:

  TypeError: Cannot read properties of undefined (reading 'visit')
      at convertPureComponentScopeFunction (file:///home/projects/stackblitz-starters-vlsqkz/node_modules/@angular/compiler/fesm2022/compiler.mjs:7370:33)
      at TemplateDefinitionBuilder.createTrackByFunction (file:///home/projects/stackblitz-starters-vlsqkz/node_modules/@angular/compiler/fesm2022/compiler.mjs:29677:23)
      at TemplateDefinitionBuilder.visitForLoopBlock (file:///home/projects/stackblitz-starters-vlsqkz/node_modules/@angular/compiler/fesm2022/compiler.mjs:29546:109)
      at ForLoopBlock.visit (file:///home/projects/stackblitz-starters-vlsqkz/node_modules/@angular/compiler/fesm2022/compiler.mjs:4281:24)
      at visitAll$1 (file:///home/projects/stackblitz-starters-vlsqkz/node_modules/@angular/compiler/fesm2022/compiler.mjs:4467:34)
      at TemplateDefinitionBuilder.buildTemplateFunction (file:///home/projects/stackblitz-starters-vlsqkz/node_modules/@angular/compiler/fesm2022/compiler.mjs:28541:9)
      at Module.compileComponentFromMetadata (file:///home/projects/stackblitz-starters-vlsqkz/node_modules/@angular/compiler/fesm2022/compiler.mjs:30970:60)
      at ComponentDecoratorHandler.compileFull (file:///home/projects/stackblitz-starters-vlsqkz/node_modules/@angular/compiler-cli/bundles/chunk-LVVK56VK.js:7660:35)
      at TraitCompiler.compile (file:///home/projects/stackblitz-starters-vlsqkz/node_modules/@angular/compiler-cli/bundles/chunk-LVVK56VK.js:2673:38)
      at IvyCompilationVisitor.visitClassDeclaration (file:///home/projects/stackblitz-starters-vlsqkz/node_modules/@angular/compiler-cli/bundles/chunk-LVVK56VK.js:3013:37)

Instead, we should detect those situations earlier and report the same / similar error as missing track keyword.

Interestingly the same applies to the language service where the described situation is not reported.

Please provide a link to a minimal reproduction of the bug

https://stackblitz.com/edit/stackblitz-starters-vlsqkz?description=An%20angular-cli%20project%20based%20on%20@angular/animations,%20@angular/common,%20@angular/compiler,%20@angular/core,%20@angular/forms,%20@angular/platform-browser,%20@angular/platform-browser-dynamic,%20@angular/router,%20core-js,%20rxjs,%20tslib%20and%20zone.js&file=src%2Fmain.ts,package.json&template=node&title=Angular%20Starter

Please provide the exception or error you saw

No response

Please provide the environment you discovered this bug in (run ng version)

No response

Anything else?

No response

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}