OpenCV 3.4.9 / 4.2.0:

arg1 - uint8 matrix with 3 channels:

• Mat m__1_2_3(Size(2, 2), CV_8UC3, Scalar(1, 2, 3));

arg2:

• 5
• Scalar(5) (identical to Scalar(5, 0, 0, 0))
• Scalar(5, 1e-6, 0)
• Scalar::all(5) (identical to Scalar(5, 5, 5, 5))

Result variants:

1. [6, 2, 3] - modified 1st channel only
2. [6, 7, 8] - all channels are modified

Analyse:

• results with Scalar(5, 1e-6, 0)(arg3) and Scalar::all(5)(arg4) are consistent and adequate
• some other cases should be be deprecated / changed / removed?
• case with numeric scalar (5) may have designed to use with complex numbers (emulation through matrices with 2 channels) where 5 is interpreted as a real number.
• current implicit conversion of 5 into Scalar (no explicit bug/feature) converts this into Scalar(5, 0, 0, 0). Extra overloads may help to avoid implicit conversions (but may break user code).

Related unclear stuff:

• multi-channel Mat::eye() without element initializer.
• missing explicit for Vec and Scalar constructors: cv::Matx33f tmp = true compiles #11956
• almost zero tests for MatExpr stuff