[clang] [Clang][Sema] Fix crash when diagnosing near-match for 'constexpr' redeclaration in C++11 (PR #92452)

[Krystian Stasiowski via cfe-commits]

https://github.com/sdkrystian created https://github.com/llvm/llvm-project/pull/92452

Clang crashes when diagnosing the following invalid redeclaration in C++11:
```cpp
struct A {
  void f();
};

constexpr void A::f() { } // crash here
```
This happens because `DiagnoseInvalidRedeclaration` tries to create a fix-it to remove `const` from the out-of-line declaration of `f`, but there is no `SourceLocation` for the `const` qualifier (it's implicitly `const` due to `constexpr`) and an assert in `FunctionTypeInfo::getConstQualifierLoc` fails. 

This patch changes the accessors for _cv-qualifier-seq_ `SourceLocations` in `FunctionTypeInfo` to return an invalid `SourceLocation` if no _cv-qualifier-seq_ is present, and changes `DiagnoseInvalidRedeclaration` to only suggest the removal of the `const` qualifier when it was explicitly specified. 

>From 27fab1ec54259941e3ded174de18cd99aa89bf7e Mon Sep 17 00:00:00 2001
From: Krystian Stasiowski <sdkrystian at gmail.com>
Date: Thu, 16 May 2024 16:42:27 -0400
Subject: [PATCH] [Clang][Sema] Fix crash when diagnosing near-match for
 'constexpr' redeclaration in C++11

---
 clang/docs/ReleaseNotes.rst                    |  2 ++
 clang/include/clang/Sema/DeclSpec.h            | 12 ++++++------
 clang/lib/Sema/SemaDecl.cpp                    | 18 +++++++++---------
 .../CXX/dcl.dcl/dcl.spec/dcl.constexpr/p1.cpp  | 11 +++++++++++
 4 files changed, 28 insertions(+), 15 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 2d2928e418623..411a5f752899d 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -724,6 +724,8 @@ Bug Fixes to C++ Support
   templates during partial ordering when deducing template arguments from a function declaration or when
   taking the address of a function template.
 - Fix a bug with checking constrained non-type template parameters for equivalence. Fixes (#GH77377).
+- Fix a C++11 crash when a non-const non-static member function is defined out-of-line with
+  the ``constexpr`` specifier.
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/include/clang/Sema/DeclSpec.h b/clang/include/clang/Sema/DeclSpec.h
index 23bc780e04979..44d96db54b5f0 100644
--- a/clang/include/clang/Sema/DeclSpec.h
+++ b/clang/include/clang/Sema/DeclSpec.h
@@ -1527,20 +1527,20 @@ struct DeclaratorChunk {
 
     /// Retrieve the location of the 'const' qualifier.
     SourceLocation getConstQualifierLoc() const {
-      assert(MethodQualifiers);
-      return MethodQualifiers->getConstSpecLoc();
+      return MethodQualifiers ? MethodQualifiers->getConstSpecLoc()
+                              : SourceLocation();
     }
 
     /// Retrieve the location of the 'volatile' qualifier.
     SourceLocation getVolatileQualifierLoc() const {
-      assert(MethodQualifiers);
-      return MethodQualifiers->getVolatileSpecLoc();
+      return MethodQualifiers ? MethodQualifiers->getVolatileSpecLoc()
+                              : SourceLocation();
     }
 
     /// Retrieve the location of the 'restrict' qualifier.
     SourceLocation getRestrictQualifierLoc() const {
-      assert(MethodQualifiers);
-      return MethodQualifiers->getRestrictSpecLoc();
+      return MethodQualifiers ? MethodQualifiers->getRestrictSpecLoc()
+                              : SourceLocation();
     }
 
     /// Retrieve the location of the 'mutable' qualifier, if any.
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 0dbdf923df95a..22749dc4799bc 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -9203,15 +9203,15 @@ static NamedDecl *DiagnoseInvalidRedeclaration(
         << Idx << FDParam->getType()
         << NewFD->getParamDecl(Idx - 1)->getType();
     } else if (FDisConst != NewFDisConst) {
-      SemaRef.Diag(FD->getLocation(), diag::note_member_def_close_const_match)
-          << NewFDisConst << FD->getSourceRange().getEnd()
-          << (NewFDisConst
-                  ? FixItHint::CreateRemoval(ExtraArgs.D.getFunctionTypeInfo()
-                                                 .getConstQualifierLoc())
-                  : FixItHint::CreateInsertion(ExtraArgs.D.getFunctionTypeInfo()
-                                                   .getRParenLoc()
-                                                   .getLocWithOffset(1),
-                                               " const"));
+      auto DB = SemaRef.Diag(FD->getLocation(),
+                             diag::note_member_def_close_const_match)
+                << NewFDisConst << FD->getSourceRange().getEnd();
+      if (const auto &FTI = ExtraArgs.D.getFunctionTypeInfo(); !NewFDisConst)
+        DB << FixItHint::CreateInsertion(FTI.getRParenLoc().getLocWithOffset(1),
+                                         " const");
+      else if (SourceLocation ConstLoc = FTI.getConstQualifierLoc();
+               ConstLoc.isValid())
+        DB << FixItHint::CreateRemoval(ConstLoc);
     } else
       SemaRef.Diag(FD->getLocation(),
                    IsMember ? diag::note_member_def_close_match
diff --git a/clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p1.cpp b/clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p1.cpp
index a28a5f91c4775..788e93b56bb38 100644
--- a/clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p1.cpp
+++ b/clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p1.cpp
@@ -154,3 +154,14 @@ namespace {
   // FIXME: We should diagnose this prior to C++17.
   const int &r = A::n;
 }
+
+#if __cplusplus < 201402L
+namespace ImplicitConstexprDef {
+  struct A {
+    void f(); // expected-note {{member declaration does not match because it is not const qualified}}
+  };
+
+  constexpr void A::f() { } // expected-warning {{'constexpr' non-static member function will not be implicitly 'const' in C++14; add 'const' to avoid a change in behavior}}
+                            // expected-error at -1 {{out-of-line definition of 'f' does not match any declaration in 'ImplicitConstexprDef::A'}}
+}
+#endif