2670 - improvements to generics

Eliminated a few holes, gained more clarity on the shape of others.

Maybe I was sleep-deprived, but this was really hard until I wrote a few
unit tests directly on replace_type_ingredient.

Still one flaw remaining: the type-checker isn't smart enough to handle
'merge' for all the new cases. Tests pass since we don't use those
features outside C++ tests yet.

1 files changed, 2 insertions, 0 deletions

diff --git a/030container.cc b/030container.cc
index cd9d3255..a7ae2984 100644
--- a/030container.cc
+++ b/030container.cc
@@ -808,6 +808,8 @@ void check_merge_call(const vector<reagent>& ingredients, const reagent& product
       default: {
         if (!types_coercible(container, ingredients.at(ingredient_index))) {
           raise_error << maybe(caller.name) << "incorrect type of ingredient " << ingredient_index << " in '" << inst.to_string() << "'\n" << end();
+          cerr << "  expected " << debug_string(container) << '\n';
+          cerr << "  got " << debug_string(ingredients.at(ingredient_index)) << '\n';
           return;
         }
         ++ingredient_index;