[Webkit-unassigned] [Bug 205889] New: Remove LayoutUnit::operator(const float&) to avoid bad results when the LHS is accidentally made an int

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=205889

            Bug ID: 205889
           Summary: Remove LayoutUnit::operator(const float&) to avoid bad
                    results when the LHS is accidentally made an int
           Product: WebKit
           Version: WebKit Local Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebCore Misc.
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: dbates at webkit.org
                CC: zalan at apple.com

In the patch for bug #205563 I moved some code:

[[
...
LayoutUnit logicalWidth = snappedSelectionRect.width();
if (snappedSelectionRect.x() > logicalRight())
    logicalWidth = 0;
else if (snappedSelectionRect.maxX() > logicalRight())
    logicalWidth = logicalRight() - snappedSelectionRect.x();
...
]]

And in the process changed:

LayoutUnit logicalWidth = snappedSelectionRect.width();

to

auto logicalWidth = snappedSelectionRect.width();

As a result logicalWidth was now an int. This led to bad things when "snappedSelectionRect.maxX() > logicalRight()" evaluates to true because we would then compute "logicalRight() - snappedSelectionRect.x()" (which would be a float since logicalRight() is a float) and then integer truncate to assign to logicalWidth.

This bug would have been prevented if LayoutUnit::operator(const float&) did not exist because it would have forced the code to have been written (pre-my move):

[[
...
LayoutUnit logicalWidth = snappedSelectionRect.width();
if (snappedSelectionRect.x() > logicalRight())
    logicalWidth = 0;
else if (snappedSelectionRect.maxX() > logicalRight())
    logicalWidth = LayoutUnit { logicalRight() - snappedSelectionRect.x() };
...
]]

(Notice the explicit constructor call to LayoutUnit in the second branch).

And so if I had then changed LayoutUnit => auto it would have compile-time failed and I would have seen my mistake because the code would now be trying to illogically assign a LayoutUnit to an int.

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