From GLSL spec that describes shift operators:

The shift operators (<<) and (>>). For both operators, the operands must be signed or unsigned integers or integer vectors. One operand can be signed while the other is unsigned. In all cases, the resulting type will be the same type as the left operand. If the first operand is a scalar, the second operand has to be a scalar as well. If the first operand is a vector, the second operand must be a scalar or a vector with the same size as the first operand, and the result is computed component-wise. The result is undefined if the right operand is negative, or greater than or equal to the number of bits in the left expressionâ€™s base type. The value of E1 << E2 is E1 (interpreted as a bit pattern) left-shifted by E2 bits. The value of E1 >> E2 is E1 right-shifted by E2 bit positions. If E1 is a signed integer, the right-shift will extend the sign bit. If E1 is an unsigned integer, the right-shift will zero-extend.

I check other binary operators (+, -, â€¦ %) descriptions, these all include the same pattern â€“ â€śif two operands donâ€™t mismatch, then will be an implicit conversionâ€ť, only the bit shift operator doesnâ€™t have this.

The spec says the only valid condition is that the type is either integer scalar or integer vector. In some cases, such as two operands are both scalars but one is signed and the other is unsigned or one is int64_t and the other is int, how the shift operator works, and will it perform the implicit conversion in the right side operands?