[C++20] [Modules] Only diagnose the non-inline external variable definitions in header units

[ChuanqiXu]

Summary

Address part of https://github.com/llvm/llvm-project/issues/60079.

Since the the declaration of a non-inline static data member in its class definition is not a definition. The following form:

class A {
public:
    static const int value = 43; 
};

should be fine to appear in a header unit. From the perspective of implementation, it looks like we simply forgot to check if the variable is a definition...

[ChuanqiXu]

Remove redundancy.

[iains]

thanks, LGTM.

clang/lib/Sema/SemaDecl.cpp

if (RequireNonAbstractType(VDecl->getLocation(), VDecl->getType(),
                                diag::err_abstract_type_in_decl,
                                AbstractVariableType))
       VDecl->setInvalidDecl();
   }
 
   // C++ [module.import/6] external definitions are not permitted in header
   // units.
   if (getLangOpts().CPlusPlusModules && currentModuleIsHeaderUnit() &&
+      VDecl->isThisDeclarationADefinition() &&
       VDecl->getFormalLinkage() == Linkage::ExternalLinkage &&
       !VDecl->isInline()) {
     Diag(VDecl->getLocation(), diag::err_extern_def_in_header_unit);
     VDecl->setInvalidDecl();
   }
 
   // If adding the initializer will turn this declaration into a definition,
   // and we already have a definition for this variable, diagnose or otherwise

clang/test/CXX/module/module.import/p6.cpp

 inline int ok_inline_var = 1;
 
 static int ok_static_var;
 
 int ok_var_decl;
 
 int bad_var_definition = 3;  // expected-error {{non-inline external definitions are not permitted in C++ header units}}
 
+class A {
+public:
+    // This is a declaration instead of definition.
+    static const int value = 43; 
+};
+