'Undecidable grammar' is a very poor choice of words. A truly undecidable grammar is such that there exists no parser for the grammar that will terminate on all possible inputs. What they likely mean is that C++ grammar is not context-free, but even this is somewhat a matter of taste: Where to draw the line between syntax and semantics? Any compiler will admit only a proper subset of those programs that pass the parser stage without syntax errors and only a proper subset of those programs actually run without errors, thus no language is truly context-free or even decidable (barring perhaps some esoteric languages).
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