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[Clang] Add warning message for C++17 alias template CTAD #133806

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Merged
merged 7 commits into from
Apr 3, 2025
Merged

[Clang] Add warning message for C++17 alias template CTAD #133806

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dd97898
Add warning message for C++17 alias template CTAD
GeorgeKA Mar 31, 2025
c48c4a6
Switched to using multiclasses for both messages
GeorgeKA Mar 31, 2025
01789a4
Adde release notes
GeorgeKA Apr 1, 2025
42a08fb
Merge branch 'main' into 125913-warning_msg_for_cxx17_alias_template_…
GeorgeKA Apr 1, 2025
5299a58
Merge branch 'llvm:main' into 125913-warning_msg_for_cxx17_alias_temp…
GeorgeKA Apr 2, 2025
df703ea
Switched to using DiagCompat for compatibility messages
GeorgeKA Apr 2, 2025
eec39e4
Merge branch 'main' into 125913-warning_msg_for_cxx17_alias_template_…
GeorgeKA Apr 2, 2025
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1 change: 1 addition & 0 deletions clang/docs/ReleaseNotes.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -398,6 +398,7 @@ Bug Fixes to C++ Support
- Clang no longer crashes when establishing subsumption between some constraint expressions. (#GH122581)
- Clang now issues an error when placement new is used to modify a const-qualified variable
in a ``constexpr`` function. (#GH131432)
- Clang now emits a warning when class template argument deduction for alias templates is used in C++17. (#GH133806)

Bug Fixes to AST Handling
^^^^^^^^^^^^^^^^^^^^^^^^^
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7 changes: 2 additions & 5 deletions clang/include/clang/Basic/DiagnosticSemaKinds.td
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Expand Up @@ -49,6 +49,8 @@ defm constexpr_ctor_missing_init : CXX20Compat<
defm adl_only_template_id : CXX20Compat<
"use of function template name with no prior declaration in function call "
"with explicit template arguments is">;
defm ctad_for_alias_templates
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@shafik shafik Apr 1, 2025 •
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@Sirraide while I get the "efficiency" here I am bit uncomfortable that as a reviewer it is no longer obvious what diagnostics I need to look for in tests and as we have seen here easy to overlook that we are not fully covering tests.

This feels very anti-ergonomic for code reviewers. Maybe worth discussing during wg meeting CC @AaronBallman

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@Sirraide Sirraide Apr 2, 2025

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This feels very anti-ergonomic for code reviewers.

I feel like that might mainly be because this is a new thing we just added. I personally don’t think it’s that big of a deal because the wording of compatibility warnings is very consistent as a result of this, so so long as it’s clear that this always generates something of the form ‘... incompatibile with C++ standards before C++XY’ and ‘... a C++XY extension’, you know what to look for.

Also, even if it adds another thing to be aware of, it also removes the need to check if the compatibility warning is actually written correctly and whether we’re checking for the right lang opt when we emit it etc. etc. (though of course that last part will only come into effect once #132348 is merged).

: CXX20Compat<"class template argument deduction for alias templates is">;

// C++23 compatibility with C++20 and earlier.
defm constexpr_static_var : CXX23Compat<
Expand Down Expand Up @@ -8444,11 +8446,6 @@ let CategoryName = "Lambda Issue" in {
def warn_cxx17_compat_lambda_def_ctor_assign : Warning<
"%select{default construction|assignment}0 of lambda is incompatible with "
"C++ standards before C++20">, InGroup<CXXPre20Compat>, DefaultIgnore;

// C++20 class template argument deduction for alias templates.
def warn_cxx17_compat_ctad_for_alias_templates : Warning<
"class template argument deduction for alias templates is incompatible with "
"C++ standards before C++20">, InGroup<CXXPre20Compat>, DefaultIgnore;
}

def err_return_in_captured_stmt : Error<
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4 changes: 3 additions & 1 deletion clang/lib/Sema/SemaInit.cpp
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Expand Up @@ -9920,7 +9920,9 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
if (auto *AliasTemplate = dyn_cast_or_null<TypeAliasTemplateDecl>(
TemplateName.getAsTemplateDecl())) {
Diag(Kind.getLocation(),
diag::warn_cxx17_compat_ctad_for_alias_templates);
getLangOpts().CPlusPlus20
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@erichkeane erichkeane Apr 1, 2025

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@Sirraide : Didn't we have a function you were making to get the right diagnostic here? Should/could we use that here?

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@Sirraide Sirraide Apr 2, 2025

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Yes we can use DiagCompat() here once #132348 is merged (which should happen in a few hours barring unforeseen circumstances). I’ll leave another comment once that has happened.

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@erichkeane erichkeane Apr 2, 2025

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Thanks! I'd vastly prefer we hold off on this patch until that is merged and rebase it with that. I hadn't realized that 132348 hadn't been merged yet.

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@Sirraide Sirraide Apr 2, 2025

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It has been merged since (I had to revert it once but I’ve already relanded it)

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@erichkeane erichkeane Apr 2, 2025

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Ah :) I was just blaming my bad memory. I just got back from a week off, so my brain isn't particularly caught up yet :D

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@Sirraide Sirraide Apr 2, 2025

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Ah :) I was just blaming my bad memory. I just got back from a week off, so my brain isn't particularly caught up yet :D

I merged it a few hours ago so you’re good ;Þ

? diag::compat_cxx20_ctad_for_alias_templates
: diag::compat_pre_cxx20_ctad_for_alias_templates);
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@shafik shafik Apr 1, 2025

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Do we check both of these branches in the test below? It does not look like it, we need to make sure we completely cover all new code in tests.

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@GeorgeKA GeorgeKA Apr 1, 2025

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These are the checks for each condition. Is there something additional you're looking for?

diag::compat_pre_cxx20_ctad_for_alias_templates:

llvm-project/clang/test/SemaCXX/cxx17-compat.cpp

Line 140 in 42a08fb

// expected-warning@-2 {{class template argument deduction for alias templates is a C++20 extension}}

diag::compat_cxx20_ctad_for_alias_templates:

llvm-project/clang/test/SemaCXX/cxx17-compat.cpp

Line 142 in 42a08fb

// expected-warning@-4 {{class template argument deduction for alias templates is incompatible with C++ standards before C++20}}

LookupTemplateDecl = AliasTemplate;
auto UnderlyingType = AliasTemplate->getTemplatedDecl()
->getUnderlyingType()
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2 changes: 1 addition & 1 deletion clang/test/SemaCXX/cxx17-compat.cpp
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Expand Up @@ -137,7 +137,7 @@ template<typename T> struct A { A(T); };
template<typename T> using B = A<T>;
B b = {1};
#if __cplusplus <= 201703L
// FIXME: diagnose as well
// expected-warning@-2 {{class template argument deduction for alias templates is a C++20 extension}}
#else
// expected-warning@-4 {{class template argument deduction for alias templates is incompatible with C++ standards before C++20}}
#endif
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4 changes: 2 additions & 2 deletions clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp
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Expand Up @@ -113,9 +113,9 @@ namespace dependent {
};
template<typename T> void f() {
typename T::X tx = 0;
typename T::Y ty = 0;
typename T::Y ty = 0; // expected-warning {{class template argument deduction for alias templates is a C++20 extension}}
}
template void f<B>();
template void f<B>(); // expected-note {{in instantiation of function template specialization 'dependent::f<dependent::B>' requested here}}

template<typename T> struct C { C(T); };
template<typename T> C(T) -> C<T>;
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