Why was 1

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The relevant issue is CWG 1457, where the justification is that the change allows 1 << 31 to be used in constant expressions:

The current wording of 5.8 [expr.shift] paragraph 2 makes it undefined
behavior to create the most-negative integer of a given type by
left-shifting a (signed) 1 into the sign bit, even though this is not
uncommonly done and works correctly on the majority of
(twos-complement) architectures:

…if E1 has a signed type and non-negative value, and E1 * 2E2 is
representable in the result type, then that is the resulting value;
otherwise, the behavior is undefined.

As a result, this technique
cannot be used in a constant expression, which will break a
significant amount of code.

Constant expressions can’t contain undefined behavior, which means that using an expression containing UB in a context requiring a constant expression makes the program ill-formed. libstdc++’s numeric_limits::min, for example, once failed to compile in clang due to this.

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