87cbea6cdc
When using
set(var "Example")
if (${var})
CMake will first resolve the if-argument to
if (Example)
And what then happens depends on the existance of a variable with the
name "Example":
1. If instead of "Example", a truth constant is used ("TRUE", "FALSE",
"ON", "OFF", "", "-NOTFOUND" ...), it is prioritized
(https://cmake.org/cmake/help/latest/command/if.html#constant)
2. If a variable with the name "Example" exists: Use the truthiness of
its content
(https://cmake.org/cmake/help/latest/command/if.html#variable)
3. Otherwise, it is false-y
That is, the the result of the conditional does not only depend on the
content of `var`, but also some other variable. This is usually
unintended and leads to problems such as addressed with #154537. The
only case where this is intended is when passing an expression to be
evaluated such as with `pythonize_bool`, `append_if` and
`libomp_append`. In all other cases, using `${var}` without quotes is a
pattern to be avoided.
Remark:
If `${var}` is not one of the values considered a [truthiness
constant](https://cmake.org/cmake/help/latest/command/if.html#constant),
the result of `if (var)` and `if ("${var}")` is different:
* `if (var)` is true-ish
(https://cmake.org/cmake/help/latest/command/if.html#variable)
* `if ("Example")` and therefore `if ("${var}")` are false-y
(https://cmake.org/cmake/help/latest/command/if.html#string)
In this PR I am preferring `if (var)` over `if ("${var}")` because has
less clutter, resembles Python's behaviour, and problably what most
users are expecting, even though `if (${var})` in most cases would
evaluate to false-y because a variable does not exist. This ambiguity
does not exist for STREQUAL and MATCHES.
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