[Sema] Add FixIt when a C++ out-of-line method has extra/missing const

[hokein]

clang/test/FixIt/member-mismatch.cpp

9

nit: using annotation [[@LINE+1]]:15 can make the test more robust.

clang/lib/Sema/SemaDecl.cpp

if (unsigned Idx = NearMatch->second) {
       SourceLocation Loc = FDParam->getTypeSpecStartLoc();
       if (Loc.isInvalid()) Loc = FD->getLocation();
       SemaRef.Diag(Loc, IsMember ? diag::note_member_def_close_param_match
                                  : diag::note_local_decl_close_param_match)
         << 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 << FD->getSourceRange().getEnd()
+          << (NewFDisConst
+                  ? FixItHint::CreateRemoval(ExtraArgs.D.getFunctionTypeInfo()
+                                                 .getConstQualifierLoc())
+                  : FixItHint::CreateInsertion(ExtraArgs.D.getFunctionTypeInfo()
+                                                   .getRParenLoc()
+                                                   .getLocWithOffset(1),
+                                               " const"));
     } else
       SemaRef.Diag(FD->getLocation(),
                    IsMember ? diag::note_member_def_close_match
                             : diag::note_local_decl_close_match);
   }
   return nullptr;
 }
 

clang/test/FixIt/member-mismatch.cpp

+// RUN: %clang_cc1 -verify %s
+// RUN: not %clang_cc1 -fdiagnostics-parseable-fixits %s 2>&1 | FileCheck %s
+
+class Foo {
+  int get() const; // expected-note {{because it is const qualified}}
+  void set(int); // expected-note {{because it is not const qualified}}
+};
+
+// CHECK: fix-it:"{{.*}}":{10:15-10:15}:" const"
+int Foo::get() {} // expected-error {{does not match any declaration}}
+// CHECK: fix-it:"{{.*}}":{12:20-12:26}:""
+void Foo::set(int) const {} // expected-error {{does not match any declaration}}