Fix #14783 / #14784 FN unusedVariable with templated type (default c… #20066

Job	Run time
build_make (ubuntu:24.04)	2m 53s
build_cmake (ubuntu:24.04, true, true)	2m 47s
build_make (ubuntu:25.10)	3m 31s
build_cmake (alpine:3.23, false, false)	3m 45s
build_cmake (ubuntu:25.10, true, true)	39m 19s
build_make (alpine:3.23)	5m 50s
	58m 5s

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