The following testcase is rejected because, for this line: bool b = X() ?: false;
arg2 is missing and arg1 is a TARGET_EXPR. A TARGET_EXPR is a class prvalue so we wrap it in a SAVE_EXPR. Later when building 'this' we call build_this (SAVE_EXPR <TARGET_EXPR <...>>) which triggers lvalue_error: 5856 cp_lvalue_kind kind = lvalue_kind (arg); 5857 if (kind == clk_none) 5858 { 5859 if (complain & tf_error) 5860 lvalue_error (input_location, lv_addressof); because all SAVE_EXPRs are non-lvalue. Since a) cp_build_addr_expr_1 can process xvalues and class prvalues, b) TARGET_EXPRs are only evaluated once (gimplify_target_expr), I thought we could do the following. The testcase ensures that with the omitted operand we only construct X once. Bootstrapped/regtested on x86_64-linux, ok for trunk? 2018-06-21 Marek Polacek <pola...@redhat.com> PR c++/86184 * call.c (build_conditional_expr_1): Don't wrap TARGET_EXPRs in a SAVE_EXPR. * g++.dg/ext/cond3.C: New test. --- gcc/cp/call.c +++ gcc/cp/call.c @@ -4806,6 +4806,10 @@ build_conditional_expr_1 (location_t loc, tree arg1, tree arg2, tree arg3, /* Make sure that lvalues remain lvalues. See g++.oliva/ext1.C. */ if (lvalue_p (arg1)) arg2 = arg1 = cp_stabilize_reference (arg1); + else if (TREE_CODE (arg1) == TARGET_EXPR) + /* TARGET_EXPRs are only expanded once, don't wrap it in a SAVE_EXPR, + rendering it clk_none of clk_class. */ + arg2 = arg1; else arg2 = arg1 = cp_save_expr (arg1); } --- gcc/testsuite/g++.dg/ext/cond3.C +++ gcc/testsuite/g++.dg/ext/cond3.C @@ -0,0 +1,20 @@ +// PR c++/86184 +// { dg-do run } +// { dg-options "" } + +int j; +struct X { + X() { j++; } + operator bool() { return true; } +}; + +/* Only create X once. */ +bool b = X() ?: false; +bool b2 = X() ? X() : false; + +int +main () +{ + if (j != 3) + __builtin_abort (); +}