Bug 19950 – access violation at compile time, possibly via template constraint and alias interaction

The following snippet causes an access violation at compile time on dmd v2.086, after a `test.d(2): Error: undefined identifier NotHere` error: --- struct Bar { alias Baz = NotHere; static foreach (name; 0 .. 1) { alias someAlias = set!(); } void set()(Baz) if (true) {} } --- Changing any meaningful structure in the snippet, such as: - removing the if guard in `set` (changing the condition seems fine) - changing the arguments in `set` - removing the static foreach (altering what it iterates over is fine, as long as it's not empty) - changing what `someAlias` aliases to makes compilation act as normal (failing but no access violation), so I assume this is a weird edge case with static foreach, if guards, and aliases. Information DMD asked me to send: --- DMD %.*s predefs DigitalMars Windows CRuntime_DigitalMars CppRuntime_DigitalMars LittleEndian D_Version2 all D_InlineAsm D_InlineAsm_X86 X86 Win32 assert D_ModuleInfo D_Exceptions D_TypeInfo D_HardFloat binary C:\D\dmd2\windows\bin\dmd.exe version v2.086.0 config C:\D\dmd2\windows\bin\sc.ini DFLAGS -IC:\D\dmd2\windows\bin\..\..\src\phobos -IC:\D\dmd2\windows\bin\..\..\src\druntime\import --- run.dlang.io says the bug has been present ever since static foreach was introduced, in dmd v2.076. Before that, the code obviously fails to parse.

Actually, it seems you can create an even tinier example: --- alias Foo = NotHere; alias Bar = baz!(); void baz()(Foo) if (true) {} --- Which compiles fine (with a normal error) from dmd v2.060 until v2.066, but in v2.067 and v2.068 it has this cryptic error: --- onlineapp.d(1): Error: undefined identifier NotHere dmd: template.c:930: MATCH TemplateDeclaration::matchWithInstance(Scope*, TemplateInstance*, Objects*, Expressions*, int): Assertion `fd->type->ty == Tfunction' failed. --- And from v2.069 onwards it has a segfault/access violation.

@Basile-z created dlang/dmd pull request #10195 "fix issue 19950 - access violation at compile time, possibly via template constraint and alias interaction" fixing this issue: - fix issue 19950 - access violation at compile time, possibly via template constraint and alias interaction https://github.com/dlang/dmd/pull/10195

dlang/dmd pull request #10195 "fix issue 19950 - access violation at compile time, possibly via template constraint and alias interaction" was merged into stable: - 9cd97ab03969186b1d45c95a18abf8d43b6c40a1 by Basile Burg: fix issue 19950 - access violation at compile time, possibly via template constraint and alias interaction https://github.com/dlang/dmd/pull/10195