[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

7

12

[royjacobson]

Add release note, slightly nicer test

clang/test/CXX/drs/dr26xx.cpp

12

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

[royjacobson]

rebase on main

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; // expected-note {{marked deleted here}}
+  constexpr foo() requires (A || B) = delete;
+};
+
+void f() {
+  foo fooable; // expected-error {{call to deleted}}
+}
+
+}