Bug 16055 – double.stringof is not precise

The D front-end uses two fewer digits than needed for lossless conversion of floating-point values to strings: --- module example; import std.conv, std.format, std.math, std.meta, std.stdio; void main() { foreach (F; AliasSeq!(real, double, float)) { writeln(F.stringof); const allDig = cast(int)ceil(log(pow(2.0L, F.mant_dig - 1)) / log(10.0L) + 1); assert(allDig == (F.dig + 2)); writefln("\tallDig: %s", allDig); enum F mostDecDig = 1 + F.epsilon; // 16 for double: FAIL assert(to!F(format("%." ~ to!string(F.dig) ~ "g", mostDecDig)) != mostDecDig); // 17 for double: FAIL assert(to!F(format("%." ~ to!string(F.dig + 1) ~ "g", mostDecDig)) != mostDecDig); // 18 for double: PASS enum goodSpec = "%." ~ to!string(F.dig + 2) ~ "g"; static if (!is(F == real)) // to!real(string) is not precise! assert(to!F(format(goodSpec, mostDecDig)) == mostDecDig); // This is a very subtle issue; even the D frontend gets it wrong! assert(mostDecDig != mixin(mostDecDig.stringof)); writefln("\t" ~ goodSpec ~ " != %s", mostDecDig, mostDecDig.stringof); } } --- DPaste: https://dpaste.dzfl.pl/1bd14e5c3f83#line-25

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