[llvm-bugs] [Bug 35409] New: -Wconversion missing warning on truncation

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=35409

            Bug ID: 35409
           Summary: -Wconversion missing warning on truncation
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: lebedev.ri at gmail.com
                CC: llvm-bugs at lists.llvm.org

With Serus's help in IRC, the following issue was discovered:

#include <stdint.h>
#include <stdio.h>
#include <limits>

template<typename T>
T mul(T a, T b) {
  printf("%u * %u", a, b);
  T c = a * b;
  printf(" = %u\n", c);
  return c;
}

int main(int argc, char *[]) {
    mul(static_cast<unsigned char>(argc), std::numeric_limits<unsigned
char>::max());
    return 0;
}


GCC output:
$ g++ -std=c++14 -O2 -Wall -Wextra -Wconversion -pedantic -pthread main.cpp &&
./a.out a b
main.cpp: In instantiation of 'T mul(T, T) [with T = unsigned char]':
main.cpp:14:84:   required from here
main.cpp:8:11: warning: conversion to 'unsigned char' from 'int' may alter its
value [-Wconversion]
   T c = a * b;
         ~~^~~
3 * 255 = 253

Clang output:
clang++ -std=c++14 -O2 -Wall -Weverything -Wno-c++98-compat -pedantic -pthread
main.cpp && ./a.out a b
3 * 255 = 253


An alternative variant of the testcase, on which clang does warn:
#include <stdint.h>
#include <stdio.h>
#include <limits>

template<typename T>
T mul(T a, T b) {
  printf("%u * %u", a, b);
  auto c = a * b;
  T d = c;
  printf(" = %u\n", c);
  return d;
}

int main(int argc, char *[]) {
    mul(static_cast<unsigned char>(argc), std::numeric_limits<unsigned
char>::max());
    return 0;
}


clang++ -std=c++14 -O2 -Wall -Weverything -Wno-c++98-compat -pedantic -pthread
main.cpp && ./a.out a b
main.cpp:9:9: warning: implicit conversion loses integer precision: 'int' to
'unsigned char' [-Wconversion]
  T d = c;
    ~   ^
main.cpp:15:5: note: in instantiation of function template specialization
'mul<unsigned char>' requested here
    mul(static_cast<unsigned char>(argc), std::numeric_limits<unsigned
char>::max());
    ^
1 warning generated.
3 * 255 = 765

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