[Clang][Sema] Don't set instantiated from function when rewriting operator<=> (#91339)

The following snippet causes a crash:
```
template<typename T>
struct A 
{
    bool operator<=>(const A&) const requires true = default;
};

bool f(A<int> a) 
{
    return a != A<int>();
}
```
This occurs because during the rewrite from `operator<=>` to
`operator==`, the "pattern" `operator<=>` function is set as the
instantiated from function for the newly created `operator==` function.
This is obviously incorrect, and this patch fixes it.

5 files changed, 41 insertions, 21 deletions

diff --git a/clang-tools-extra/clangd/unittests/FindTargetTests.cpp b/clang-tools-extra/clangd/unittests/FindTargetTests.cpp
index 9443785..0b2273f 100644
--- a/clang-tools-extra/clangd/unittests/FindTargetTests.cpp
+++ b/clang-tools-extra/clangd/unittests/FindTargetTests.cpp
@@ -642,10 +642,7 @@ TEST_F(TargetDeclTest, RewrittenBinaryOperator) {
     bool x = (Foo(1) [[!=]] Foo(2));
   )cpp";
   EXPECT_DECLS("CXXRewrittenBinaryOperator",
-               {"std::strong_ordering operator<=>(const Foo &) const = default",
-                Rel::TemplatePattern},
-               {"bool operator==(const Foo &) const noexcept = default",
-                Rel::TemplateInstantiation});
+               {"bool operator==(const Foo &) const noexcept = default"});
 }
 
 TEST_F(TargetDeclTest, FunctionTemplate) {
diff --git a/clang-tools-extra/clangd/unittests/HoverTests.cpp b/clang-tools-extra/clangd/unittests/HoverTests.cpp
index 28df24f..d9e97e5 100644
--- a/clang-tools-extra/clangd/unittests/HoverTests.cpp
+++ b/clang-tools-extra/clangd/unittests/HoverTests.cpp
@@ -3091,7 +3091,7 @@ TEST(Hover, All) {
             HI.NamespaceScope = "";
             HI.Definition =
                 "bool operator==(const Foo &) const noexcept = default";
-            HI.Documentation = "Foo spaceship";
+            HI.Documentation = "";
           }},
   };
 
@@ -3894,7 +3894,7 @@ TEST(Hover, SpaceshipTemplateNoCrash) {
   TU.ExtraArgs.push_back("-std=c++20");
   auto AST = TU.build();
   auto HI = getHover(AST, T.point(), format::getLLVMStyle(), nullptr);
-  EXPECT_EQ(HI->Documentation, "Foo bar baz");
+  EXPECT_EQ(HI->Documentation, "");
 }
 
 TEST(Hover, ForwardStructNoCrash) {
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index c4a9501..c8ef2e8 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -693,6 +693,8 @@ Bug Fixes to C++ Support
   and (#GH88832).
 - Clang now defers all substitution into the exception specification of a function template specialization
   until the noexcept-specifier is instantiated.
+- Fix a crash when an implicitly declared ``operator==`` function with a trailing requires-clause has its
+  constraints compared to that of another declaration.
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index d544cfa..fde2d92 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -2269,16 +2269,18 @@ Decl *TemplateDeclInstantiator::VisitFunctionDecl(
                             TemplateArgumentList::CreateCopy(SemaRef.Context,
                                                              Innermost),
                                                 /*InsertPos=*/nullptr);
-  } else if (isFriend && D->isThisDeclarationADefinition()) {
-    // Do not connect the friend to the template unless it's actually a
-    // definition. We don't want non-template functions to be marked as being
-    // template instantiations.
-    Function->setInstantiationOfMemberFunction(D, TSK_ImplicitInstantiation);
-  } else if (!isFriend) {
-    // If this is not a function template, and this is not a friend (that is,
-    // this is a locally declared function), save the instantiation relationship
-    // for the purposes of constraint instantiation.
-    Function->setInstantiatedFromDecl(D);
+  } else if (FunctionRewriteKind == RewriteKind::None) {
+    if (isFriend && D->isThisDeclarationADefinition()) {
+      // Do not connect the friend to the template unless it's actually a
+      // definition. We don't want non-template functions to be marked as being
+      // template instantiations.
+      Function->setInstantiationOfMemberFunction(D, TSK_ImplicitInstantiation);
+    } else if (!isFriend) {
+      // If this is not a function template, and this is not a friend (that is,
+      // this is a locally declared function), save the instantiation
+      // relationship for the purposes of constraint instantiation.
+      Function->setInstantiatedFromDecl(D);
+    }
   }
 
   if (isFriend) {
@@ -2669,7 +2671,7 @@ Decl *TemplateDeclInstantiator::VisitCXXMethodDecl(
                          TemplateArgumentList::CreateCopy(SemaRef.Context,
                                                           Innermost),
                                               /*InsertPos=*/nullptr);
-  } else if (!isFriend) {
+  } else if (!isFriend && FunctionRewriteKind == RewriteKind::None) {
     // Record that this is an instantiation of a member function.
     Method->setInstantiationOfMemberFunction(D, TSK_ImplicitInstantiation);
   }
diff --git a/clang/test/CXX/class/class.compare/class.compare.default/p4.cpp b/clang/test/CXX/class/class.compare/class.compare.default/p4.cpp
index 534c3b3..53a8bfc 100644
--- a/clang/test/CXX/class/class.compare/class.compare.default/p4.cpp
+++ b/clang/test/CXX/class/class.compare/class.compare.default/p4.cpp
@@ -18,14 +18,22 @@ namespace std {
 
 namespace N {
   struct A {
-    friend constexpr std::strong_ordering operator<=>(const A&, const A&) = default;
+    friend constexpr std::strong_ordering operator<=>(const A&, const A&) = default; // expected-note 2{{declared here}}
   };
 
-  constexpr bool (*test_a_not_found)(const A&, const A&) = &operator==; // expected-error {{undeclared}}
+  constexpr std::strong_ordering (*test_a_threeway_not_found)(const A&, const A&) = &operator<=>; // expected-error {{undeclared}}
+
+  constexpr std::strong_ordering operator<=>(const A&, const A&) noexcept;
+  constexpr std::strong_ordering (*test_a_threeway)(const A&, const A&) = &operator<=>;
+  static_assert(!(*test_a_threeway)(A(), A())); // expected-error {{static assertion expression is not an integral constant expression}}
+                                               // expected-note@-1 {{undefined function 'operator<=>' cannot be used in a constant expression}}
+
+  constexpr bool (*test_a_equal_not_found)(const A&, const A&) = &operator==; // expected-error {{undeclared}}
 
   constexpr bool operator==(const A&, const A&) noexcept;
-  constexpr bool (*test_a)(const A&, const A&) noexcept = &operator==;
-  static_assert((*test_a)(A(), A()));
+  constexpr bool (*test_a_equal)(const A&, const A&) noexcept = &operator==;
+  static_assert((*test_a_equal)(A(), A())); // expected-error {{static assertion expression is not an integral constant expression}}
+                                            // expected-note@-1 {{undefined function 'operator==' cannot be used in a constant expression}}
 }
 
 struct B1 {
@@ -161,3 +169,14 @@ struct non_constexpr_type {
 
 my_struct<non_constexpr_type> obj; // cxx2a-note {{in instantiation of template class 'GH61238::my_struct<GH61238::non_constexpr_type>' requested here}}
 }
+
+namespace Constrained {
+  template<typename T>
+  struct A {
+    std::strong_ordering operator<=>(const A&) const requires true = default;
+  };
+
+  bool f(A<int> a) {
+    return a != A<int>();
+  }
+}