The C++ templates example is kind of a meme, and Bjarne has pushed back on the idea that it was "accidental."
Sure, they didn't set out saying "we want to design a Turing-complete generics system" (because what kind of problem statement is that?), but templates were explicitly designed to be as general-purpose as possible. Turing-completeness was the result of that design goal:
"...I had aimed for generalty (and gotten Turing completeness modulo translation limits). I opposed restrictions to C++ immediately when Erwin Unruh presented what is widely believed to be the first template metaprogram to the ISO Standards committee's evolution working group."
Sure, they didn't set out saying "we want to design a Turing-complete generics system" (because what kind of problem statement is that?), but templates were explicitly designed to be as general-purpose as possible. Turing-completeness was the result of that design goal:
"...I had aimed for generalty (and gotten Turing completeness modulo translation limits). I opposed restrictions to C++ immediately when Erwin Unruh presented what is widely believed to be the first template metaprogram to the ISO Standards committee's evolution working group."
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