[cfe-users] clang-tidy simplify-boolean-expr check

[Richard]

Hi,

I've contributed a check to clang-tidy that simplifies
statements involving boolean literals.  The name of the check is
simplify-boolean-expr.  It is not enabled by default, so you must request
this check to get fixes applied to your code by clang-tidy.

I believe this check is sufficiently robust that it should make no changes
to your code that change the semantics of the code.  I've run this on
the clang and LLVM code bases and addressed issues and made improvements
to correct and such problems encountered there.  However, I would love
for this check to be run on more code bases to check for any additional
problems I might have missed.  You can provide feedback directly to me
by email or via the clang bug tracker.  <https://llvm.org/bugs/>

This check looks for boolean expressions involving boolean constants and
simplifies them to use the appropriate boolean expression directly.

Examples:
`if (b == true)`                           becomes `if (b)`
`if (b == false)`                          becomes `if (!b)`
`if (b && true)`                           becomes `if (b)`
`if (b && false)`                          becomes `if (false)`
`if (b || true)`                           becomes `if (true)`
`if (b || false)`                          becomes `if (b)`
`e ? true : false`                         becomes `e`
`e ? false : true`                         becomes `!e`
`if (true) t(); else f();`                 becomes `t();`
`if (false) t(); else f();`                becomes `f();`
`if (e) return true; else return false;`   becomes `return e;`
`if (e) return false; else return true;`   becomes `return !e;`
`if (e) b = true; else b = false;`         becomes `b = e;`
`if (e) b = false; else b = true;`         becomes `b = !e;`
`if (e) return true; return false;`        becomes `return e;`
`if (e) return false; return true;`        becomes `return !e;`

The resulting expression `e` is modified as follows:
1. Unnecessary parentheses around the expression are removed.
2. Negated applications of `!` are eliminated.
3. Negated applications of comparison operators are changed to use the
   opposite condition.
4. Implicit conversions of pointer to `bool` are replaced with explicit
   comparisons to `nullptr`.
5. Implicit casts to `bool` are replaced with explicit casts to `bool`.
6. Object expressions with `explicit operator bool` conversion operators
   are replaced with explicit casts to `bool`.

Examples:
1. The ternary assignment `bool b = (i < 0) ? true : false;` has redundant
   parentheses and becomes `bool b = i < 0;`.

2. The conditional return `if (!b) return false; return true;` has an
   implied double negation and becomes `return b;`.

3. The conditional return `if (i < 0) return false; return true;` becomes
   `return i >= 0;`.
   The conditional return `if (i != 0) return false; return true;` becomes
   `return i == 0;`.

4. The conditional return `if (p) return true; return false;` has an
   implicit conversion of a pointer to `bool` and becomes
   `return p != nullptr;`.
   The ternary assignment `bool b = (i & 1) ? true : false;` has an implicit
   conversion of `i & 1` to `bool` and becomes
   `bool b = static_cast<bool>(i & 1);`.

5. The conditional return `if (i & 1) return true; else return false;` has
   an implicit conversion of an integer quantity `i & 1` to `bool` and becomes
   `return static_cast<bool>(i & 1);`

6. Given `struct X { explicit operator bool(); };`, and an instance `x` of
   `struct X`, the conditional return `if (x) return true; return false;`
   becomes `return static_cast<bool>(x);`

When a conditional boolean return or assignment appears at the end of a
chain of `if`, `else if` statements, the conditional statement is left
unchanged unless the option `ChainedConditionalReturn` or
`ChainedConditionalAssignment`, respectively, is specified as non-zero.
The default value for both options is zero.
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