c++ - Why was the restriction on the comma operator being in a constant expression removed in C++11? -

recently when answering question realized comma operator allowed in constant expression in c++11 long expression surrounded (), example:

int a[ (1, 2) ] ; 

pre c++11 forbidden use comma operator in constant expression, draft pre c++11 standard section 5.19 constant expressions says (emphasis mine):

[...]in particular, except in sizeof expressions, functions, class objects, pointers, or references shall not used, , assignment, increment, decrement, function-call, or comma operators shall not used.

why comma operator not allowed in constant expression pre c++11 , why restriction lifted?

we can find answer in std-discussion group comma operator in constant-expression thread gabriel dos reis says:

for c++11 proposed allow because restriction appeared arbitrary , reasons i've heard rationale ban appear unconvincing , specious me.

and richard smith earlier in thread notes of uses of comma operator in constant expression in both c++11 , c++14:

i disagree argument , conclusion. in c++11, comma operator useful within constexpr functions, because aren't allowed multiple statements:

template<typename t>   constexpr t my_array<t>::at(size_type n) {   return (n < size() || throw "n large"), (*this)[n];   } 

in c++14, it's useful in of cases it's useful outside of constant expressions:

constexpr void do_stuff(int x[]) {   (int = 0, j = 100; != j; ++i, --j)     x[i] = x[j];   } 

more philosophically, shouldn't ban things constant expressions because we're not imaginative enough find cases they're genuinely useful. constant expressions should not semi-random sublanguage of c++, missing random features, extent can avoid that. these days, top-level commas prohibited because constant-expressions tend occur in contexts comma mean else.

note argued c++11 example not correct since the expression containing comma operator should in () example gives essence of argument. argument based on grammar section 5.19 constant expressions:

constant-expression:    conditional-expression 

we can not comma operator conditional-expression can primary-expression gets ( expression ) , can comma operator expression.

t.c. points out may not case since relevant section seems vague on point.