[Clang] Implement fix for DR2628

[royjacobson]

Summary

Implement suggested fix for DR2628. Couldn't update the DR docs because there hasn't been a DR index since it was filed, but the tests still run in CI.

Note: I only transfer the constructor constraints, not the struct constraints. I think that's OK because the struct constraints are the same
for all constructors so they don't affect the overload resolution, and if they deduce to something that doesn't pass the constraints
we catch it anyway. So (hopefully) that should be more efficient without sacrificing correctness.

Closes:
https://github.com/llvm/llvm-project/issues/57646
https://github.com/llvm/llvm-project/issues/43829

[erichkeane]

This seems right to me, thanks!

I'm currently pushing to try to make our diagnostics 'check' lines more readable... it doesn't gain a ton in this case, but more to try to push for consistency.

clang/test/CXX/drs/dr26xx.cpp

8

13

[royjacobson]

Add release note, slightly nicer test

clang/test/CXX/drs/dr26xx.cpp

13

That does look much nicer, thanks! I qualified it with DR2628 because the DR tests can get pretty large.

[royjacobson]

rebase on main

clang/docs/ReleaseNotes.rst

- Clang will now no longer treat a C 'overloadable' function without a prototype as
   a variadic function with the attribute.  This should make further diagnostics more
   clear.
 - Fixes to builtin template emulation of regular templates.
   `Issue 42102 <https://github.com/llvm/llvm-project/issues/42102>`_
   `Issue 51928 <https://github.com/llvm/llvm-project/issues/51928>`_
 - A SubstTemplateTypeParmType can now represent the pack index for a
   substitution from an expanded pack.
   `Issue 56099 <https://github.com/llvm/llvm-project/issues/56099>`_
+- Respect constructor constraints during class template argument deduction (CTAD).
+  This is the suggested resolution to CWG DR2628.
+  `Issue 57646 <https://github.com/llvm/llvm-project/issues/57646>`_
+  `Issue 43829 <https://github.com/llvm/llvm-project/issues/43829>`_
 
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Clang will now check compile-time determinable string literals as format strings.
   Fixes `Issue 55805: <https://github.com/llvm/llvm-project/issues/55805>`_.
 - ``-Wformat`` now recognizes ``%b`` for the ``printf``/``scanf`` family of
   functions and ``%B`` for the ``printf`` family of functions. Fixes

clang/lib/Sema/SemaTemplate.cpp

ArrayRef<ParmVarDecl *> Params =
         TInfo->getTypeLoc().castAs<FunctionProtoTypeLoc>().getParams();
 
     // Build the implicit deduction guide template.
     auto *Guide =
         CXXDeductionGuideDecl::Create(SemaRef.Context, DC, LocStart, ES, Name,
                                       TInfo->getType(), TInfo, LocEnd, Ctor);
     Guide->setImplicit();
     Guide->setParams(Params);
+    if (Ctor && Ctor->getTrailingRequiresClause())
+      Guide->setTrailingRequiresClause(Ctor->getTrailingRequiresClause());
 
     for (auto *Param : Params)
       Param->setDeclContext(Guide);
     for (auto *TD : MaterializedTypedefs)
       TD->setDeclContext(Guide);
 
     auto *GuideTemplate = FunctionTemplateDecl::Create(
         SemaRef.Context, DC, Loc, DeductionGuideName, TemplateParams, Guide);

clang/test/CXX/drs/dr26xx.cpp

+// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-unknown %s -verify
+
+namespace dr2628 { // dr2628: yes
+
+template <bool A = false, bool B = false>
+struct foo {
+  constexpr foo() requires (!A && !B) = delete; // #DR2628_CTOR
+  constexpr foo() requires (A || B) = delete;
+};
+
+void f() {
+  foo fooable; // expected-error {{call to deleted}}
+  // expected-note@#DR2628_CTOR {{marked deleted here}}
+}
+
+}