From 5fd82fd322f17cf247f9c3bbb6f1337973abb4df Mon Sep 17 00:00:00 2001
From: Maxwell Jeffress <maxwelljeffress@proton.me>
Date: Mon, 3 Aug 2026 20:03:54 +1000
Subject: [PATCH] Function return type checking

---
 src/typechecker/type.hpp        |  5 +-
 src/typechecker/typechecker.cpp | 82 +++++++++++++++++++++++++++++++--
 src/typechecker/typechecker.hpp |  1 +
 3 files changed, 84 insertions(+), 4 deletions(-)

diff --git a/src/typechecker/type.hpp b/src/typechecker/type.hpp
index 282ecde..84b46ff 100644
--- a/src/typechecker/type.hpp
+++ b/src/typechecker/type.hpp
@@ -3,10 +3,12 @@
 #include <functional>
 #include <memory>
 #include <optional>
+#include <unordered_map>
 #include <unordered_set>
 #include <map>
 #include <string>
 #include <variant>
+#include <vector>
 
 namespace Solstice {
     enum class BaseType {
@@ -137,8 +139,9 @@ namespace Solstice {
     };
 
     struct Function {
-        PossibleType returnTypes;
         std::vector<PossibleType> argumentTypes;
+        std::unordered_map<std::vector<Type>, Type> returnTypes;
+        std::vector<std::vector<Type>> invalidCombinations;
         Function() = default;
     };
 
diff --git a/src/typechecker/typechecker.cpp b/src/typechecker/typechecker.cpp
index 4117abd..16a820e 100644
--- a/src/typechecker/typechecker.cpp
+++ b/src/typechecker/typechecker.cpp
@@ -1,5 +1,6 @@
 #include "typechecker.hpp"
 #include "type.hpp"
+#include <functional>
 #include <stdexcept>
 #include <iostream>
 
@@ -153,6 +154,45 @@ namespace Solstice {
             variables[*id].unknownType = true;
         }
     }
+    static inline std::vector<std::vector<Type>> doCartesianProductOnTypeSets(const std::vector<PossibleType>& typeSets) {
+        if (typeSets.empty()) return {};
+        for (const auto& s : typeSets) {
+            if (s.possiblities.empty()) {
+                return {};
+            }
+        }
+        
+        std::vector<std::vector<Type>> result;
+        std::vector<std::unordered_set<Type>::const_iterator> iterators;
+        
+        // Initialize iterators to the start of each set
+        for (const auto& s : typeSets) {
+            iterators.push_back(s.possiblities.begin());
+        }
+        
+        while (true) {
+            // Collect current elements from each set
+            std::vector<Type> current;
+            for (const auto& it : iterators) {
+                current.push_back(*it);
+            }
+            result.push_back(current);
+            
+            // Increment like an odometer (rightmost digit first)
+            int idx = iterators.size() - 1;
+            while (idx >= 0) {
+                iterators[idx]++;
+                if (iterators[idx] != typeSets[idx].possiblities.end()) break;
+                iterators[idx] = typeSets[idx].possiblities.begin();
+                idx--;
+            }
+            
+            // If we wrapped the leftmost position, we're done
+            if (idx < 0) break;
+        }
+        
+        return result;
+    }
 
     void TypeChecker::checkFunctionBindNodeType(Node& node) {
         auto id = node.children[0].getIdentifier();
@@ -205,13 +245,38 @@ namespace Solstice {
 
         typeChecker.checkTypes();
 
-        // collect the (possibly still-polymorphic) narrowed argument types,
-        // and the inferred return type from the body's implicit return value
+        // collect the (possibly still-polymorphic) narrowed argument types
         Function fn;
         for (const auto& name : paramNames) {
             fn.argumentTypes.push_back(typeChecker.variables[name]);
         }
-        fn.returnTypes = node.children[2].ptype;
+
+        std::vector<std::vector<Type>> candidates = doCartesianProductOnTypeSets(fn.argumentTypes);
+
+        // re-check the body once per candidate with every parameter bound to
+        // a single concrete type. concrete (non-unknownType) variables always
+        // take the Narrowing::None path, so this reuses all the existing
+        // operator-checking logic and naturally rejects combinations that
+        // only looked valid in the marginal sets.
+        for (const auto& combo : candidates) {
+            TypeChecker concreteChecker(*this);
+            concreteChecker.setNode(node.children[2]);
+            concreteChecker.inFunction = true;
+            for (std::size_t i = 0; i < paramNames.size(); i++) {
+                concreteChecker.setVariable(paramNames[i], combo[i]);
+            }
+            try {
+                concreteChecker.checkTypes();
+                auto retType = node.children[2].ptype.getOnlyType();
+                if (retType.has_value()) {
+                    fn.returnTypes[combo] = *retType;
+                } else {
+                    fn.invalidCombinations.push_back(combo);
+                }
+            } catch (const std::exception&) {
+                fn.invalidCombinations.push_back(combo);
+            }
+        }
 
         functions[*id] = fn;
     }
@@ -245,6 +310,13 @@ namespace Solstice {
         auto& left = node.children[0];
         auto& right = node.children[1];
 
+        // this node may be getting checked again (e.g. once polymorphically,
+        // then once more per concrete argument combination in
+        // checkFunctionBindNodeType) - possiblities is only ever inserted
+        // into below, so it must be reset here or results from a previous
+        // pass over this same node would leak into this one
+        node.ptype.possiblities.clear();
+
         // narrow types if we need to
         switch (doesNodeChildrenNeedNarrowing(node)) {
             case Narrowing::Both: {
@@ -323,6 +395,10 @@ namespace Solstice {
 
     }
 
+    void TypeChecker::narrowFunctionCallNode(Node& node) {
+        
+    }
+
     void TypeChecker::checkSetNodeType(Node& node) {
         auto id = node.children[0].getIdentifier();
         if (!id.has_value()) {
diff --git a/src/typechecker/typechecker.hpp b/src/typechecker/typechecker.hpp
index 3f9e759..f94d1b9 100644
--- a/src/typechecker/typechecker.hpp
+++ b/src/typechecker/typechecker.hpp
@@ -47,6 +47,7 @@ namespace Solstice {
 
         Narrowing doesNodeChildrenNeedNarrowing(Node& node);
         void narrowBinaryNode(Node& node, std::unordered_map<TypePair, Type>& overloads);
+        void narrowFunctionCallNode(Node& node);
 
         void checkAddType(Node& node);
         void checkSubtractType(Node& node);