C++ PATCH for c++/89836 - bool constant expression and explicit conversions

[Jason Merrill]

On 3/27/19 3:08 PM, Marek Polacek wrote:
> I noticed that this test doesn't compile because build_converted_constant_expr
> failed to consider explicit conversion functions.  That's wrong: while Core
> Issue 1981 specifies that explicit conversion functions are not considered for
> contextual conversions, they are considered in contextual conversions to bool,
> as defined in Core 2039.
> 
> Fixed by adding a new wrapper that uses LOOKUP_NORMAL instead of
> LOOKUP_IMPLICIT.
> 
> Besides [dcl.fct.spec], these: [except.spec], [dcl.dcl]/6 (static_assert),
> [stmt.if] (constexpr if) all talk about "a contextually converted constant
> expression of type bool", so it would seem to make sense to use
> build_converted_constant_bool_expr for them also.  E.g. use that instead of
> perform_implicit_conversion_flags in build_noexcept_spec.  But that doesn't
> work for this test:
> 
> int e();
> int fn() noexcept(e);
> 
> because build_converted_constant_expr would issue a conversion error.  We're
> converting "e" to "bool".  We have a ck_lvalue conversion from "int f()" to
> "int (*f)()" and then a ck_std conversion from "int (*f)()" to bool.  Those
> should work fine but we issue an error for the ck_std conversion.

Converting a pointer to bool is a "boolean conversion", which is not 
allowed under the rules for a converted constant expression 
([expr.const]p7).  So we should reject that testcase.

> Also build_converted_constant_expr doesn't have the processing_template_decl
> handling that perform_implicit_conversion_flags does, which (I believe) lead me
> to changing check_narrowing to use maybe_constant_value instead of
> fold_non_dependent_expr.

As I was saying elsewhere, I think that change was probably a mistake, 
and that we should have fixed that bug by changing the caller instead.

> Anyway, this patch should be safe.
> 
> Bootstrapped/regtested on x86_64-linux, ok for trunk?

OK.

Jason