[PATCH] D40381: Parse concept definition

Faisal Vali via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Sat Dec 23 09:10:19 PST 2017

faisalv added inline comments.

Comment at: lib/Sema/SemaTemplate.cpp:7735
+  ActOnDocumentableDecl(NewDecl);
+  CurContext->addDecl(NewDecl);
+  return NewDecl;
changyu wrote:
> faisalv wrote:
> > Why not use 'PushOnScopeChains' onto the enclosing scope?
> > Something along these lines:
> >    assert(S->isTemplateParamScope() && S->getParent() && 
> >     !S->getParent()->isTemplateParamScope() && "...");
> >    PushOnScopeChains(NewDecl, S->getParent(),/*AddToContext*/true);
> The condition
> ```
> S->isTemplateParamScope()
> ```
> fails in this case
> ```
> template<concept T> concept D1 = true; // expected-error {{expected template parameter}}
> ```
> `ParseTemplateDeclarationOrSpecialization` calls `ParseTemplateParameters` which eventually calls `ParseNonTypeTemplateParameter`. `ParseNonTypeTemplateParameter` prints the diag and fails but does not cause `ParseTemplateParameters` to fail (I'm not sure why).  `ParseTemplateDeclarationOrSpecialization` proceeds to parse the concept definition, and we get this
> ```
> /home/changyu/test.cpp:1:10: error: expected template parameter
> template<concept T> concept D1 = true; // expected-error {{expected template parameter}}
>          ^
> clang: /home/changyu/git/llvm/tools/clang/lib/Sema/SemaTemplate.cpp:7747: clang::Decl* clang::Sema::ActOnConceptDefinition(clang::Scope*, clang::MultiTemplateParamsArg, clang::IdentifierInfo*, clang::SourceLocation, clang::Expr*): Assertion `S->isTemplateParamScope() && "Not in template param scope?"' failed.
> ```
> What should we do?
I think this occurs because our template parameter list is incorrectly perceived as empty - i.e. once the error occurs - to continue processing errors clang assumes this case is an explicit-specialization and replaces the template parameter scope with the outer scope.  I think the thing to do here - which might also address the case where folks actually write 'template<> concept' is to actually check if template-parameter-list is empty - and emit a diagnostic about concepts can not be explicit specializations or some such ...

On a somewhat related note - i think the logic for signaling, handling and propagating failures of parsing the template parameter list might be a little broken (fixing which, might avoid triggering the assertion in template<concept> but not template<>).  I might submit a patch to fix that error handling-issue separately - but either way I think we should handle the explicit-specialization error?



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