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

Clang crashes when diagnosing the following invalid redeclaration in
C++11:
```
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 `DiagnoseInvalidRedeclaration` to only suggest the removal of the
`const` qualifier when it was explicitly specified in the _cv-qualifier-seq_ of the declaration.

Author: sdkrystian

clang/docs/ReleaseNotes.rst

@@ -742,6 +742,8 @@ Bug Fixes to C++ Support
 - Fix a bug with checking constrained non-type template parameters for equivalence. Fixes (#GH77377).
 - Fix a bug where the last argument was not considered when considering the most viable function for
   explicit object argument member functions. Fixes (#GH92188).
+- Fix a C++11 crash when a non-const non-static member function is defined out-of-line with
+  the ``constexpr`` specifier. Fixes (#GH61004).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDecl.cpp

@@ -9217,19 +9217,20 @@ 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"));
-    } else
+      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 (FTI.hasMethodTypeQualifiers() &&
+               FTI.getConstQualifierLoc().isValid())
+        DB << FixItHint::CreateRemoval(FTI.getConstQualifierLoc());
+    } else {
       SemaRef.Diag(FD->getLocation(),
                    IsMember ? diag::note_member_def_close_match
                             : diag::note_local_decl_close_match);
+    }
   }
   return nullptr;
 }

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@-1 {{out-of-line definition of 'f' does not match any declaration in 'ImplicitConstexprDef::A'}}
+}
+#endif