[clang-format][PR41964] Fix crash with SIGFPE when TabWidth is set to 0 and line starts with tab

[MyDeveloperDay]

Summary

clang-format 8.0 crashes with SIGFPE (floating point exception) when formatting following file:
app.cpp:
void a() {
//line starts with '\t'
}

$ clang-format -style='{TabWidth: 0}' app.cpp

[klimek]

clang/lib/Format/FormatTokenLexer.cpp

660

Shouldn't that be (Style.TabWidth ? Column % Style.TabWidth)?
Otherwise can't that still crash when Column != 0 and TabWidth is 0?

[MyDeveloperDay]

clang/lib/Format/FormatTokenLexer.cpp

660

let me add some tests...

[MyDeveloperDay]

I should have known it was more involved.

v % 0 and v / 0 are both undefined behavior

[klimek]

LG. That makes lots of sense now :) Thanks!

clang/lib/Format/FormatTokenLexer.cpp

for (int i = 0, e = Text.size(); i != e; ++i) {
       case '\f':
       case '\v':
         Column = 0;
         break;
       case ' ':
         ++Column;
         break;
       case '\t':
-        Column += Style.TabWidth - Column % Style.TabWidth;
+        Column += Style.TabWidth - (Column ? Column % Style.TabWidth : 0);
         break;
       case '\\':
         if (i + 1 == e || (Text[i + 1] != '\r' && Text[i + 1] != '\n'))
           FormatTok->Type = TT_ImplicitStringLiteral;
         break;
       default:
         FormatTok->Type = TT_ImplicitStringLiteral;
         break;