[C++20][Modules] Correct an assert for modules-ts.

[iains]

Summary

When adding the support for modules partitions we added an assert that the
actual status of Global Module Fragments matches the state machine that is
driven by the module; keyword.

That does not apply to the modules-ts case, where there is an implicit GMF.

[iains]

somewhat on the trivial side.

[urnathan]

Is this because of history that ModulesTS option != p1103 modules? I thought we wanted to make the former become the latter (i.e. ModuleTS is the same as CPlusPlusModules) This seems to be moving in the wrong direction.

[iains]

In D122394#3405338, @urnathan wrote:

Is this because of history that ModulesTS option != p1103 modules? I thought we wanted to make the former become the latter (i.e. ModuleTS is the same as CPlusPlusModules) This seems to be moving in the wrong direction.

This is because we are not removing the fmodules-ts flag (nor. at this point, the code that it changes) - as I understand things. So I have now to fix (next patch) the problem that we currently generate wrong code for static vars and lambdas in modules since the linkage is defaulting to the modules-ts "module internal" model.

So, unless we shift priorities such that we take a detour to remove the modules-ts support, I need to make this change to avoid the assert firing for -fmodules-ts (it seems we have limited testing - since nothing fired until I tried to make other changes form my patchset).

[urnathan]

Understood. To document the approach:

1. clearly separate -fmodules-ts semantics (which are vague in places, but whatever)
2. implement c++20 modules semantics

3a) switch -fmodules-ts option to behave as #2 and delete any now-unreachable TS-specific code
3b) deprecate -fmodules-ts option and then remove.

clang/lib/Sema/SemaModule.cpp

Sema::ActOnModuleDecl(SourceLocation StartLoc, SourceLocation ModuleLoc,
   }
 
   // Find the global module fragment we're adopting into this module, if any.
   Module *GlobalModuleFragment = nullptr;
   if (!ModuleScopes.empty() &&
       ModuleScopes.back().Module->Kind == Module::GlobalModuleFragment)
     GlobalModuleFragment = ModuleScopes.back().Module;
 
-  assert((!getLangOpts().CPlusPlusModules ||
+  assert((!getLangOpts().CPlusPlusModules || getLangOpts().ModulesTS ||
           SeenGMF == (bool)GlobalModuleFragment) &&
          "mismatched global module state");
 
   // In C++20, the module-declaration must be the first declaration if there
   // is no global module fragment.
   if (getLangOpts().CPlusPlusModules && !IsFirstDecl && !SeenGMF) {
     Diag(ModuleLoc, diag::err_module_decl_not_at_start);
     SourceLocation BeginLoc =