Fix the handling of equality constraints involving concrete types

Sources/FrontEnd/TypeChecking/TypeChecker.swift

@@ -2207,7 +2207,7 @@ struct TypeChecker {
 
     // Rule orientation depends on ordering.
     var v = buildTerm(a)
-    var u = b.isTypeParameter ? buildTerm(b) : v.appending(.concrete(b))
+    var u = buildTerm(b)
 
     if let t = TraitDecl.ID(e.decl) {
       v = v.substituting([.parameterType(program[t].receiver)], for: [.trait(t)])

Tests/HyloTests/TestCases/TypeChecking/ConditionalExtension3.hylo

@@ -0,0 +1,20 @@
+//- typeCheck expecting: .failure
+
+type Box<Contents: Regular> {
+  public var contents: Contents
+  public memberwise init
+}
+
+extension Box where Contents == Int {
+  public fun contains_zero() -> Bool {
+    contents == 0
+  }
+}
+
+public fun main() {
+  let b0 = Box(contents: true)
+  _ = b0.contains_zero() //! diagnostic reference to 'contains_zero' requires that 'Bool' be equal to 'Int'
+
+  let b1 = Box(contents: 1)
+  _ = b1.contains_zero()
+}