Many gtest users aren't aware that a polymorphic matcher is not actually a `testing::Matcher<>` and thus doesn't know how to describe itself. Clarify this.
Also adds a recipe on how to define a composite matcher. In particular, explain how the composite matcher can describe itself in terms of the description(s) of its sub-matcher(s).
PiperOrigin-RevId: 735840759
Change-Id: I26cff6179349aa739fc7fdd528a2f5308d18c189
`std::abs()` doesn't work on custom types. While one can use the 3-argument version of `DistanceFrom()` to specify how to compute the distance, it's not as convenient as defining `abs()` for the custom type once in the type's namespace and then use the 2-argument version.
PiperOrigin-RevId: 735741409
Change-Id: If8fb668455eb963a2ccf089f7467c64965a2e7a6
We have a bunch of matchers for asserting that a value is near the target value, e.g.
`DoubleNear()` and `FloatNear()`. These matchers only work for specific types (`double` and `float`). They are not flexible enough to support other types that have the notion of a "distance" (e.g. N-dimensional points and vectors, which are commonly used in ML).
In this diff, we generalize the idea to a `DistanceFrom(target, get_distance, m)` matcher that works on arbitrary types that have the "distance" concept (the `get_distance` argument is optional and can be omitted for types that support `-`, and `std::abs()`). What it does:
1. compute the distance between the value and the target using `get_distance(value, target)`; if `get_distance` is omitted, compute the distance as `std::abs(value - target)`.
2. match the distance against matcher `m`; if the match succeeds, the `DistanceFrom()` match succeeds.
Examples:
```
// 0.5's distance from 0.6 should be <= 0.2.
EXPECT_THAT(0.5, DistanceFrom(0.6, Le(0.2)));
Vector2D v1(3.0, 4.0), v2(3.2, 6.0);
// v1's distance from v2, as computed by EuclideanDistance(v1, v2),
// should be >= 1.0.
EXPECT_THAT(v1, DistanceFrom(v2, EuclideanDistance, Ge(1.0)));
```
PiperOrigin-RevId: 734593292
Change-Id: Id6bb7074dc4aa4d8abd78b57ad2426637e590de5
to fail the test program if no test case is linked in
(a common programmer mistake).
PiperOrigin-RevId: 730571311
Change-Id: I1dab04adfe35581274d0b4ec79a017014d50e1ea
Zhanyong Wan