Bug 22334 – TypeInfo is used in inexplicable places

This code doesn't work with `-betterC`: ------------------------------------------- void _ctfeAppend(T)(ref T[] store, T dat) { static if (is(T U: const(U))) { U[] buf; buf.length = 1; buf[0] = dat; store = cast(T[])buf; } } void put(R, E)(R, E){} struct Data{ string text;} Data[] getData(string str) { if (__ctfe) { Data[] ret; Data tmp; tmp.text._ctfeAppend(str[0]); ret._ctfeAppend(tmp); return ret; } return null; } void foo(Data dat, DstRange)(DstRange dst) { put(dst, dat); } extern(C) void main() { static immutable dat = getData("a")[0]; char[] buf; foo!dat(buf); } ------------------------------------------- Even though the above code does not use TypeInfo at first glance, the compiler says "Error: `TypeInfo` cannot be used with -betterC". By the way, in CTFE, there should be no restriction on using TypeInfo, even if it is betterC. The issue related to the relaxation of the CTFE+BetterC restrictions relates to the following: https://issues.dlang.org/show_bug.cgi?id=18472

Assigning to an array's length may reallocate, and allocation still needs TypeInfo. You could try to get away with a lambda (not tested): alias _ctfeAppend = (store, dat) { alias T = typeof(dat); static if (is(T U: const(U))) { pragma(msg, U); U[] buf; buf.length = 1; buf[0] = dat; store = cast(T[])buf; } }; However, the lambda hack is unusable for anything slightly less trivial (no overloading, if-constraints, variadic parameters, etc.)

(In reply to Max Samukha from comment #1) > Assigning to an array's length may reallocate, and allocation still needs > TypeInfo. You could try to get away with a lambda (not tested): > > alias _ctfeAppend = (store, dat) > { > alias T = typeof(dat); > > static if (is(T U: const(U))) > { > pragma(msg, U); > U[] buf; > buf.length = 1; > buf[0] = dat; > store = cast(T[])buf; > } > }; > > However, the lambda hack is unusable for anything slightly less trivial (no > overloading, if-constraints, variadic parameters, etc.) Apparently, lambda does not solve the problem either... And I doubt that reallocate is the cause, as the following code works: ------------------------------------------- void _ctfeAppend(T)(ref T[] store, T dat) { static if (is(T U: const(U))) { U[] buf; buf.length = 1; buf[0] = dat; store = cast(T[])buf; } } string[] getData() { if (__ctfe) { string[] dat; dat._ctfeAppend("aaa"); return dat; } return null; } extern(C) void main() { static immutable dat = getData()[0]; static assert(dat == "aaa"); } -------------------------------------------

The error now gives a location to the offending expression. Allowing TypeInfo needing operations in CTFE only is covered by Issue 18472 like you mentioned. *** This issue has been marked as a duplicate of issue 21477 ***