[PATCH] D155064: [clang][SemaCXX] Diagnose tautological uses of consteval if and is_constant_evaluated

[Hans Wennborg via Phabricator via cfe-commits]

hans added a comment.

We saw a new warning in Chromium after this, and I wasn't sure if that's intentional:

  #include <limits>
  
  template <typename T> float foo(T input) {
    if (sizeof(T) > 2) {
      return 42;
    } else {
      constexpr float inverseMax = 1.0f / std::numeric_limits<T>::max();
      return input * inverseMax;
    }
  }
  
  float f() {
    return foo(1);
  }
  $ build/bin/clang -c /tmp/a.cc
  /tmp/a.cc:7:41: warning: implicit conversion from 'int' to 'float' changes value from 2147483647 to 2147483648 [-Wimplicit-const-int-float-conversion]
      7 |     constexpr float inverseMax = 1.0f / std::numeric_limits<T>::max();
        |                                       ~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /tmp/a.cc:13:10: note: in instantiation of function template specialization 'foo<int>' requested here
     13 |   return foo(1);
        |          ^
  1 warning generated.

Note that the branch with the conversion can be determined to not be taken based on the template argument.

Using `if constexpr` suppresses the warning, but I'm not sure if that should really be necessary?

(Our tracking bug is https://crbug.com/1487142)


Repository:
  rG LLVM Github Monorepo

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