Bug 21492 – betterC: TypeInfo is generated for code guarded by if(__ctfe)

Example code that demonstrates the issue, compiled with dmd and the -betterC flag DMD64 D Compiler v2.094.2 int bar(){ if(__ctfe){ int[] foo = [1]; // Error: TypeInfo cannot be used with -betterC } return 0; } The same code compiles with ldc without issue. My understanding of the difference between the two compilers is that ldc doesn’t generate code at all for `if`s that have compile-time known constant-expressions with no labels. I don’t know how dmd works, but presumably it is instead relying on the backend optimizing out the dead branch? In other words, the __ctfe is not treated as a special case by ldc. The same result is seen with this code, which compiles with ldc but fails with dmd: int bar(){ if(false){ int[] foo = [1]; } return 0; } I’m posting this as an issue as it means that betterC code that compiles with ldc won’t compile with dmd. betterC is under-specified, so I don’t know if ldc is being overly-permissive or if dmd is improperly detecting betterC violations when it attempts to generate code instead of during semantic analysis. This issue crops up if you want CTFE-able code that calculates something inefficiently at ctfe while it just calls some external function at runtime.

The error message has been change to “uses the GC”, but still fails. Dead branches like this should be pruned so that more things will work with betterC.

What D does is generate the code, then prune away all the false code blocks. Looks like ldc does it is better.

@WalterBright created dlang/dmd pull request #14830 "fix Issue 21492 - betterC: TypeInfo is generated for code guarded by …" fixing this issue: - fix Issue 21492 - betterC: TypeInfo is generated for code guarded by if(__ctfe) https://github.com/dlang/dmd/pull/14830

dlang/dmd pull request #14830 "fix Issue 21492 - betterC: TypeInfo is generated for code guarded by …" was merged into master: - f90b298ca578c00104b0121ea2ecfacdd76f9827 by Walter Bright: fix Issue 21492 - betterC: TypeInfo is generated for code guarded by if(__ctfe) https://github.com/dlang/dmd/pull/14830