[Clang] Do not emit nodiscard warnings for the base expr of static member access (llvm#131450)

For an expression `nodiscard_function().static_member(), the nodiscard
warnings added by llvm#120223, are not useful or actionable, and are
disruptive to some library implementations; we just remove them.

Fixes llvm#131410

(cherry picked from commit 9a1e390)

Author: cor3ntin

clang/include/clang/Sema/Sema.h

@@ -10671,11 +10671,6 @@ class Sema final : public SemaBase {
                            SourceLocation EndLoc);
   void ActOnForEachDeclStmt(DeclGroupPtrTy Decl);
 
-  /// DiagnoseDiscardedExprMarkedNodiscard - Given an expression that is
-  /// semantically a discarded-value expression, diagnose if any [[nodiscard]]
-  /// value has been discarded.
-  void DiagnoseDiscardedExprMarkedNodiscard(const Expr *E);
-
   /// DiagnoseUnusedExprResult - If the statement passed in is an expression
   /// whose result is unused, warn.
   void DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID);

clang/lib/Sema/SemaExprMember.cpp

@@ -1136,7 +1136,6 @@ Sema::BuildMemberReferenceExpr(Expr *BaseExpr, QualType BaseExprType,
     if (Converted.isInvalid())
       return true;
     BaseExpr = Converted.get();
-    DiagnoseDiscardedExprMarkedNodiscard(BaseExpr);
     return false;
   };
   auto ConvertBaseExprToGLValue = [&] {

clang/lib/Sema/SemaStmt.cpp

@@ -413,10 +413,6 @@ void DiagnoseUnused(Sema &S, const Expr *E, std::optional<unsigned> DiagID) {
 }
 } // namespace
 
-void Sema::DiagnoseDiscardedExprMarkedNodiscard(const Expr *E) {
-  DiagnoseUnused(*this, E, std::nullopt);
-}
-
 void Sema::DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID) {
   if (const LabelStmt *Label = dyn_cast_if_present<LabelStmt>(S))
     S = Label->getSubStmt();

clang/test/CXX/dcl.dcl/dcl.attr/dcl.attr.nodiscard/p2.cpp

@@ -164,19 +164,21 @@ struct X {
 
 [[nodiscard]] X get_X();
 // cxx11-warning@-1 {{use of the 'nodiscard' attribute is a C++17 extension}}
+[[nodiscard]] X* get_Ptr();
+// cxx11-warning@-1 {{use of the 'nodiscard' attribute is a C++17 extension}}
 void f() {
+  get_X(); // expected-warning{{ignoring return value of function declared with 'nodiscard' attribute}}
+  (void) get_X();
   (void) get_X().variant_member;
   (void) get_X().anonymous_struct_member;
   (void) get_X().data_member;
   (void) get_X().static_data_member;
-  // expected-warning@-1 {{ignoring return value of function declared with 'nodiscard' attribute}}
   (void) get_X().unscoped_enum;
-  // expected-warning@-1 {{ignoring return value of function declared with 'nodiscard' attribute}}
   (void) get_X().scoped_enum;
-  // expected-warning@-1 {{ignoring return value of function declared with 'nodiscard' attribute}}
   (void) get_X().implicit_object_member_function();
   (void) get_X().static_member_function();
-  // expected-warning@-1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  (void) get_Ptr()->implicit_object_member_function();
+  (void) get_Ptr()->static_member_function();
 #if __cplusplus >= 202302L
   (void) get_X().explicit_object_member_function();
 #endif

clang/test/SemaCXX/ms-property.cpp

@@ -2,6 +2,7 @@
 // RUN: %clang_cc1 -triple=x86_64-pc-win32 -fms-compatibility -emit-pch -o %t -verify %s
 // RUN: %clang_cc1 -triple=x86_64-pc-win32 -fms-compatibility -include-pch %t %s -ast-print -o - | FileCheck %s
 // RUN: %clang_cc1 -fdeclspec -fsyntax-only -verify %s -std=c++23
+// expected-no-diagnostics
 
 #ifndef HEADER
 #define HEADER
@@ -103,7 +104,6 @@ struct X {
 void f() {
   (void) get_x().imp;
   (void) get_x().st;
-  // expected-warning@-1 {{ignoring return value of function declared with 'nodiscard' attribute}}
 #if __cplusplus >= 202302L
   (void) get_x().exp;
 #endif