Bug 12541 – templated __traits(compiles, ...) on value symbols causes a compilation error - under certain conditions

now this is quite intricate, please bear with me. under certain conditions (see below), compilation fails with an unsubstantiated error message when trying to assign the outcome of a __traits(compiles, ...) reflection via template. in the two following code examples (compile separately) 'test' is the test symbol that shall be validated, i.e., tested via __traits(compiles, ...). this shall happen indirectly through the template 'valid'. the two examples demonstrate two slightly different conditions under which compilation fails with error message "variable [...] had semantic errors when compiling". this buggy behavior occurs only if the test symbol in question is a value (enum, const, inmutable). test symbols that are functions, structs, classes or empty enums don't cause any problems. this is quite a show-stopper for the meta / code reflection / code generation stuff i am working on. it probably contains the root cause for many other issues that are very difficult to reduce. it is actually an example for a heisenbug: if you add a pragma(msg, ...) at the right place for inspection (see example/condition 2), the bug will disappear! === CODE EXAMPLE / CONDITION 1 === /++ bug condition 1: if first call to __traits(compiles, ...) is via template, then compilation fails for test symbols that are enum, const and immutable values but: o always works for test symbols that are funcs, structs, classes and empty enums ++/ enum fails = valid!(test); // Error: variable fails had semantic errors when compiling pragma(msg, "fails: "~fails.stringof); // true // always works (as expected): enum ok = __traits(compiles, test); pragma(msg, "ok: "~ok.stringof); // true enum test = 13; // fails // immutable test = 42;// fails // const test = 42; // fails //BUT: // enum test; // works // void test(){} // works // struct test{} // works // class test{} // works // always works (as expected): enum ok2 = valid!(test); pragma(msg, "ok2: "~ok2.stringof); // true template valid(alias sym){ enum valid = __traits(compiles, sym); } // no difference if this template is declared first void main(){} === CODE EXAMPLE / CONDITION 2 === /++ bug condition 2: if first call to __traits(compiles, ...) happens from module level and is not inquiring about the test symbol, then compilation fails for test symbols that are enum, const and immutable values. but: o will compile if first call is __traits(compiles, test) o always works for test symbols that are funcs, structs, classes and empty enums ++/ enum ok = __traits(compiles, main); // 'enum fails' below will not compile! // pragma(msg, __traits(compiles, test) ); // 'enum fails' below will compile now // enum ok = __traits(compiles, test); // 'enum fails' below will compile now // pragma(msg, "ok: "~ok.stringof); // true enum fails = valid!(test); // Error: variable fails had semantic errors when compiling pragma(msg, "fails: "~fails.stringof); // true enum test = 13; // enum test; // no compilation errors if test symbol is not a value // fails only for enum, const and immutable values // (but always works for functions, structs, classes and empty enums) // always works (as expected): enum ok2 = valid!(test); pragma(msg, "ok2: "~ok2.stringof); // true template valid(alias sym){ enum valid = __traits(compiles, sym); } // no difference if this template is declared first void main(){}

this is a result of issue 12553 which covers the bug more generally *** This issue has been marked as a duplicate of issue 12553 ***