Fix `extension` incorrectly interacting with `equality` and `type-coercion` constraints (#7578)

* fix problem

* cleanup comment

* format code

* make change more restrictive

* format code

* push logic update

* format code

* push test fix

* make test more general

---------

Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>

3 files changed, 12 insertions, 2 deletions

diff --git a/source/slang/slang-check-constraint.cpp b/source/slang/slang-check-constraint.cpp
index 7c55c440c..7c6f8929a 100644
--- a/source/slang/slang-check-constraint.cpp
+++ b/source/slang/slang-check-constraint.cpp
@@ -363,6 +363,7 @@ DeclRef<Decl> SemanticsVisitor::trySolveConstraintSystem(
         ValUnificationContext unificationContext;
         unificationContext.optionalConstraint =
             constraintDeclRef.getDecl()->hasModifier<OptionalConstraintModifier>();
+        unificationContext.equalityConstraint = constraintDeclRef.getDecl()->isEqualityConstraint;
         if (!TryUnifyTypes(
                 *system,
                 unificationContext,
@@ -492,6 +493,8 @@ DeclRef<Decl> SemanticsVisitor::trySolveConstraintSystem(
                 {
                     if (c.isOptional)
                         joinType = type;
+                    else if (c.isEquality)
+                        joinType = type;
                     else
                         // failure!
                         return DeclRef<Decl>();
@@ -970,6 +973,7 @@ bool SemanticsVisitor::TryUnifyTypeParam(
     constraint.val = type;
     constraint.isUsedAsLValue = type.isLeftValue;
     constraint.isOptional = unificationContext.optionalConstraint;
+    constraint.isEquality = unificationContext.equalityConstraint;
     constraints.constraints.add(constraint);
 
     return true;
diff --git a/source/slang/slang-check-decl.cpp b/source/slang/slang-check-decl.cpp
index 558834c34..5de83e24d 100644
--- a/source/slang/slang-check-decl.cpp
+++ b/source/slang/slang-check-decl.cpp
@@ -3398,9 +3398,9 @@ void SemanticsDeclHeaderVisitor::visitGenericDecl(GenericDecl* genericDecl)
             ensureDecl(valParam, DeclCheckState::ReadyForReference);
             valParam->parameterIndex = parameterIndex++;
         }
-        else if (auto constraint = as<GenericTypeConstraintDecl>(m))
+        else if (as<GenericTypeConstraintDecl>(m) || as<TypeCoercionConstraintDecl>(m))
         {
-            ensureDecl(constraint, DeclCheckState::ReadyForReference);
+            ensureDecl(m, DeclCheckState::ReadyForReference);
         }
     }
 }
diff --git a/source/slang/slang-check-impl.h b/source/slang/slang-check-impl.h
index 30e317401..4a6ccfe17 100644
--- a/source/slang/slang-check-impl.h
+++ b/source/slang/slang-check-impl.h
@@ -2305,6 +2305,11 @@ public:
         // if it is otherwise unconstrained, but doesn't take precedence over a constraint that is
         // not optional.
         bool isOptional = false;
+
+        // Is this constraint an equality? This tells us that "joining" types is meaningless, we
+        // know the result will be the sub type. If it is not, we will error once we start
+        // substituting types.
+        bool isEquality = false;
     };
 
     // A collection of constraints that will need to be satisfied (solved)
@@ -2658,6 +2663,7 @@ public:
     {
         Index indexInTypePack = 0;
         bool optionalConstraint = false;
+        bool equalityConstraint = false;
     };
 
     // Try to find a unification for two values