The -Wdouble-promotion warning was added to clang in r251588 (fixing PR15109). The added test, however, only covered a subset of the cases where implicit conversion can occur (assignment and function return). This patch extends the coverage to include argument passing and a binary operator (multiply).
The patch also includes coverage for *= and the conditional (ternary) operator. However, warnings are not produced for all the *= cases, and no warnings are generated for the conditional operator. These have been marked as FIXME and I will raise bugs for these separately.
Note, I've added George Burgess IV as reviewer as he committed the original change and Carl Norum (the author) doesn't seem to be registered.