Why is this an issue?

Multiple waveforms are not synthesizable. Only the first waveform is taken into account for synthesis leading to a mismatch between synthesis and simulation. Thus, multiple waveforms should not be used in synthesizable code.

How to fix it

Code examples

Noncompliant code example

a <= '1' AFTER clk, '0' AFTER 4 * clk_prd;

Compliant solution

a <= '0' AFTER 4 * clk_prd;