https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101681
--- Comment #2 from Steven Sun <StevenSun2021 at hotmail dot com> --- The root cause for this is that the compiler forbids constant folding when involving PMF of an incomplete class. https://gcc.gnu.org/git?p=gcc.git;a=blob;f=gcc/cp/expr.c;h=d16d1896f2ddd08264b389b02b9640cca332ec13;hb=refs/heads/master#l42 (gcc/cp/expr.c) > 42 /* We can't lower this until the class is complete. */ > 43 if (!COMPLETE_TYPE_P (DECL_CONTEXT (member))) > 44 return cst; If we comment this `if`, the constant folding will succeed at (gcc/cp/expr.c) > 67 expand_ptrmemfunc_cst (cst, &delta, &pfn); > 68 cst = build_ptrmemfunc1 (type, delta, pfn); solving everything.
