[flang][runtime] Make NCOPIES= argument of REPEAT a signed integer, & check it

[klausler]

Summary

NCOPIES= is currently a std::size_t in the API. If a negative value is
used, the memory allocation will fail. Change it to be a signed integer,
and crash with a message instead if it be negative.

flang/include/flang/Runtime/character.h

 //===-- include/flang/Runtime/character.h -----------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // Defines API between compiled code and the CHARACTER
 // support functions in the runtime library.
 
 #ifndef FORTRAN_RUNTIME_CHARACTER_H_
 #define FORTRAN_RUNTIME_CHARACTER_H_
 #include "flang/Runtime/entry-names.h"
 #include <cstddef>
+#include <cstdint>
 
 namespace Fortran::runtime {
 
 class Descriptor;
 
 template <typename CHAR>
 int CharacterScalarCompare(
     const CHAR *x, const CHAR *y, std::size_t xChars, std::size_t yChars);

 void RTNAME(Adjustr)(Descriptor &result, const Descriptor &,
     const char *sourceFile = nullptr, int sourceLine = 0);
 std::size_t RTNAME(LenTrim1)(const char *, std::size_t);
 std::size_t RTNAME(LenTrim2)(const char16_t *, std::size_t);
 std::size_t RTNAME(LenTrim4)(const char32_t *, std::size_t);
 void RTNAME(LenTrim)(Descriptor &result, const Descriptor &, int kind,
     const char *sourceFile = nullptr, int sourceLine = 0);
 void RTNAME(Repeat)(Descriptor &result, const Descriptor &string,
-    std::size_t ncopies, const char *sourceFile = nullptr, int sourceLine = 0);
+    std::int64_t ncopies, const char *sourceFile = nullptr, int sourceLine = 0);
 void RTNAME(Trim)(Descriptor &result, const Descriptor &string,
     const char *sourceFile = nullptr, int sourceLine = 0);
 
 void RTNAME(CharacterMax)(Descriptor &accumulator, const Descriptor &x,
     const char *sourceFile = nullptr, int sourceLine = 0);
 void RTNAME(CharacterMin)(Descriptor &accumulator, const Descriptor &x,
     const char *sourceFile = nullptr, int sourceLine = 0);
 

flang/runtime/character.cpp

case CFI_type_char32_t:
     break;
   default:
     terminator.Crash(
         "SCAN: bad string type code %d", static_cast<int>(string.raw().type));
   }
 }
 
 void RTNAME(Repeat)(Descriptor &result, const Descriptor &string,
-    std::size_t ncopies, const char *sourceFile, int sourceLine) {
+    std::int64_t ncopies, const char *sourceFile, int sourceLine) {
   Terminator terminator{sourceFile, sourceLine};
+  if (ncopies < 0) {
+    terminator.Crash(
+        "REPEAT has negative NCOPIES=%jd", static_cast<std::intmax_t>(ncopies));
+  }
   std::size_t origBytes{string.ElementBytes()};
   result.Establish(string.type(), origBytes * ncopies, nullptr, 0, nullptr,
       CFI_attribute_allocatable);
   if (result.Allocate() != CFI_SUCCESS) {
     terminator.Crash("REPEAT could not allocate storage for result");
   }
   const char *from{string.OffsetElement()};
   for (char *to{result.OffsetElement()}; ncopies-- > 0; to += origBytes) {