`vec4(1.0, 1.0, 1.0, 1.0)` is a valid type constructor where element type needs to be deduced from the type of the arguments.
<rdar://problem/105480155>
Pull request: https://github.com/WebKit/WebKit/pull/10120
Committed 260330@main (e6c8fa17cce4): <https://commits.webkit.org/260330@main> Reviewed commits have been landed. Closing PR #10120 and removing active labels.