Treat signed size_t literals to be of type ptrdiff_t

Author: YexuanXiao

clang/lib/Sema/SemaDeclCXX.cpp

@@ -16716,7 +16716,7 @@ static bool CheckOperatorNewDeclaration(Sema &SemaRef, FunctionDecl *FnDecl) {
     return true;
 
   CanQualType SizeTy =
-      SemaRef.Context.getCanonicalType(SemaRef.Context.getSizeType());
+    SemaRef.Context.getCanonicalType(SemaRef.Context.getSizeType());
 
   // C++ [basic.stc.dynamic.allocation]p1:
   //  The return type shall be void*. The first parameter shall have type

clang/lib/Sema/SemaExpr.cpp

@@ -4025,8 +4025,9 @@ ExprResult Sema::ActOnNumericConstant(const Token &Tok, Scope *UDLScope) {
 
         // Does it fit in size_t?
         if (ResultVal.isIntN(SizeTSize)) {
-          // Does it fit in ptrdiff_t?
+          // Does it fit in signed size_t?
           if (!Literal.isUnsigned && ResultVal[SizeTSize - 1] == 0)
+            // treat it as ptrdiff_t to distinguish it from long/long long
             Ty = Context.getPointerDiffType();
           else if (AllowUnsigned)
             Ty = Context.getSizeType();