Error with EXPECT_EQ for sum of double or float

From https://testing.googleblog.com/2008/10/tott-floating-point-comparison.html

When comparing floating-point values, checking for equality might lead to unexpected results. Rounding errors can lead to a result that is close to the expected one, but not equal. As a consequence, an assertion might fail when checking for equality of two floating-point quantities even if the program is implemented correctly.

The Google C++ Testing Framework provides functions for comparing two floating-point quantities up to a given precision.

ASSERT_FLOAT_EQ(expected, actual);
ASSERT_DOUBLE_EQ(expected, actual);

EXPECT_FLOAT_EQ(expected, actual);
EXPECT_DOUBLE_EQ(expected, actual);

In your case,

TEST(simpleSum, sumOfFloat)
{
    EXPECT_DOUBLE_EQ(4.56, sum(0.56, 4.0));
}

See documentation for Floating Point Comparison

EXPECT_EQ uses exact match. But you cannot match two floating numbers exactly. (at least with ease.)

You can use EXPECT_FLOAT_EQ or EXPECT_DOUBLE_EQ. (with heuristic bounds) Also, you may use EXPECT_NEAR with specific bounds.

Use EXPECT_NEAR or the DoubleEq matcher instead. Floating point operations can lead to rounding errors which makes the results ever so slightly different.