This recipe is very specific.  Small deviations have caused this not to
trigger for no reason that I can determine.

emacs -Q matchertags.hpp

M-x c-toggle-parse-state-debug RET
C-v C-n C-n C-n C-n C-n C-n C-n        // Should be on std::conditional line
M-f M-f M-f M-f C-f                    // Should be on ::value
M-d ( )

This causes the following output:

c-parse-state inconsistency at 774: using cache: (412 (368 . 391) 293), from scratch: ((706 . 708) 412 (368 . 391) 293)
Old state:
(setq c-state-cache ’(412 (368 . 391) 293)  c-state-cache-good-pos 708  c-state-nonlit-pos-cache nil  c-state-nonlit-pos-cache-limit 802  c-state-semi-nonlit-pos-cache nil  c-state-semi-nonlit-pos-cache-limit 802  c-state-brace-pair-desert ’(412 . 803)  c-state-point-min 1  c-state-point-min-lit-type nil  c-state-point-min-lit-start nil  c-state-min-scan-pos 1  c-state-old-cpp-beg nil  c-state-old-cpp-end nil  c-parse-state-point 711)
c-parse-state inconsistency at 778: using cache: (412 (368 . 391) 293), from scratch: ((706 . 708) 412 (368 . 391) 293)
Old state:
(setq c-state-cache ’(412 (368 . 391) 293)  c-state-cache-good-pos 708  c-state-nonlit-pos-cache nil  c-state-nonlit-pos-cache-limit 802  c-state-semi-nonlit-pos-cache nil  c-state-semi-nonlit-pos-cache-limit 802  c-state-brace-pair-desert ’(412 . 803)  c-state-point-min 1  c-state-point-min-lit-type nil  c-state-point-min-lit-start nil  c-state-min-scan-pos 1  c-state-old-cpp-beg nil  c-state-old-cpp-end nil  c-parse-state-point 774)