Warn for wrong for nested loops

[dlangBugzillaToGithub]

bearophile_hugs reported this on 2013-09-12T12:39:34Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=11018

Description

A blog post that shows some of the warnings of the Clang compiler:

http://blog.llvm.org/2013/09/clang-warnings.html

It contains:
<<

for (int i = 0; i < size_; ++i) {
  for (int j = 1; j < size_; ++i) {
    ...
  }
}


This double nested loop gives bubble sort its n^2 running time.  Rather, in this case, an infinite running time.  Note the increment in both of the loops happen on i, even in the inner loop.  j is never touched, either here or inside the loop.  -Wloop-analysis will give a warning when all the variables inside a for loop conditional does not change during the loop iteration. Only in Clang.
>>

D has foreach that avoids most of similar bugs:


void main() {
    enum int size_ = 5;
    foreach (i; 0 .. size_) {
        foreach (i; 0 .. size_) {
        }
    }
}


test.d(4): Error: is shadowing declaration test.main.i

But in D you can't always use foreach (unless you also use iota()), sometimes you have to use for loops (like when the increment is not 1):


void main() {
    enum size_ = 5;
    for (int i = 0; i < size_; i += 2) {
        for (int j = 1; j < size_; i += 2) {}
    }
}


Perhaps it's a good idea for the D compiler to warn for such wrong loops, as Clang does.