Most tests pass

Author: LPeter1997

src/Draco.Compiler/Internal/Solver/ConstraintSolver.cs

@@ -68,7 +68,7 @@ public Argument Arg(SyntaxNode? syntax, BindingTask<BoundLvalue> lvalue, Diagnos
     /// <param name="diagnostics">The bag to report diagnostics to.</param>
     public void Solve(DiagnosticBag diagnostics)
     {
-        while (this.ApplyRulesOnce(diagnostics)) { }
+        this.SolveUntilFixpoint(diagnostics);
 
         // Check for uninferred locals
         this.CheckForUninferredLocals(diagnostics);
@@ -77,6 +77,11 @@ public void Solve(DiagnosticBag diagnostics)
         this.CheckForIncompleteInference(diagnostics);
     }
 
+    private void SolveUntilFixpoint(DiagnosticBag diagnostics)
+    {
+        while (this.ApplyRulesOnce(diagnostics)) { }
+    }
+
     private void CheckForUninferredLocals(DiagnosticBag diagnostics)
     {
         foreach (var local in this.localVariables)

src/Draco.Compiler/Internal/Solver/ConstraintSolver_Rules.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Linq;
@@ -455,7 +456,38 @@ private static void FailOverload(Overload overload)
     /// </summary>
     private void FailRemainingRules()
     {
-        // TODO
-        throw new NotImplementedException();
+        var previousStoreSize = this.constraintStore.Count;
+        while (true)
+        {
+            // We unify type variables with the error type
+            foreach (var typeVar in this.typeVariables)
+            {
+                var unwrapped = typeVar.Substitution;
+                if (unwrapped is TypeVariable unwrappedTv) UnifyAsserted(unwrappedTv, WellKnownTypes.UninferredType);
+            }
+
+            var constraintsToRemove = new List<Constraint>();
+
+            // We can also solve all overload constraints by failing them instantly
+            foreach (var overload in this.constraintStore.Query<Overload>())
+            {
+                FailOverload(overload);
+                constraintsToRemove.Add(overload);
+            }
+
+            this.constraintStore.RemoveAll(constraintsToRemove);
+
+            // Assume this solves everything
+            this.SolveUntilFixpoint(DiagnosticBag.Empty);
+
+            // Check for exit condition
+            if (previousStoreSize == this.constraintStore.Count) break;
+            previousStoreSize = this.constraintStore.Count;
+        }
+
+        if (this.constraintStore.Count > 0)
+        {
+            throw new InvalidOperationException("fallback operation could not solve all constraints");
+        }
     }
 }

src/Draco.Compiler/Internal/Solver/ConstraintStore.cs

@@ -101,6 +101,19 @@ public IEnumerable<TConstraint> Query<TConstraint>(Func<TConstraint, bool>? pred
         }
     }
 
+    /// <summary>
+    /// Removes all constraints in the given sequence.
+    /// </summary>
+    /// <param name="constraints">The constraints to remove.</param>
+    public void RemoveAll(IEnumerable<Constraint> constraints)
+    {
+        foreach (var constraint in constraints)
+        {
+            if (!this.constraints.TryGetValue(constraint.GetType(), out var list)) ThrowConstraintNotFound(constraint);
+            if (!list.Remove(constraint)) ThrowConstraintNotFound(constraint);
+        }
+    }
+
     /// <summary>
     /// Removes all constraints in the given sequence.
     /// </summary>
@@ -112,7 +125,7 @@ public void RemoveAll<TConstraint>(IEnumerable<TConstraint> constraints)
         var list = this.GetConstraintList(typeof(TConstraint));
         foreach (var constraint in constraints)
         {
-            if (!list.Remove(constraint)) throw new InvalidOperationException($"Constraint {constraint} not found in the store.");
+            if (!list.Remove(constraint)) ThrowConstraintNotFound(constraint);
         }
     }
 
@@ -125,4 +138,8 @@ private List<Constraint> GetConstraintList(Type type)
         }
         return list;
     }
+
+    [DoesNotReturn]
+    private static void ThrowConstraintNotFound(Constraint constraint) =>
+        throw new InvalidOperationException($"Constraint {constraint} not found in the store.");
 }