These turn up frequently in real-world code, for example when converting
a mutable global `Cpp.std.string_view` to a `Cpp.std.string`. Only
reject a non-constant call if the callee is `consteval`, not if it's
`constexpr`.
This makes it possible to do const eval when calling a constexpr C++
function with params and return types other than 32/64-bit integers.
Most of the new logic is in `MaybeModifyCppThunkCallForConstEval`, which
is called by `MakeConstantForCall`. This checks if the callee is a C++
thunk (using a new `SpecialFunctionKind::CppThunk` variant), and if so
it:
* Changes the callee from the C++ thunk to the thunk's callee
* Remaps parameters that are passed by pointer to the thunk to the
underlying value
* Drops the return value parameter, if present
Initialize the `Diag` field `EvalResult` to get notes from clang when
`EvaluateAsConstantExpr` fails, then emit them using clang's diagnostic
infrastructure.
Also set valid source locations in a couple places, otherwise clang's
diagnostics code crashes.
This moves the LValue path code from macros.cpp to constant.cpp, so that
it can be called from `MapAPValueToConstant`. TODO messages are updated
accordingly to avoid referring to macros. Added a constexpr pointer test
to `constexpr.carbon` to show the result of this change.
For integral and float types, `TryEvaluateMacroToConstant` now calls
`MapAPValueToConstant` to directly convert from an APValue, rather than
converting the `APValue` to an expression and importing it with
`MapConstant`.
`MapConstant` is still used, but only for string literals and nullptrs.
Since it's only used by `TryEvaluateMacroToConstant`, moved it to
`macros.cpp` and removed the code for other types of expressions.
Richard Smith
Nicholas Bishop
Geoff Romer