Add getClassType API to Definition[T <: Class]. (#3877)

This allows users to get a ClassType for use in other Property types,
which is especially useful when constructing aggregate Property types
of a specific Class.

(cherry picked from commit 8a02fb3)

core/src/main/scala/chisel3/properties/Class.scala

@@ -199,13 +199,33 @@ object Class {
     def getPropertyType: Property[ClassType] = {
       // Get the BaseModule for the Class this is a definition of.
       val baseModule = definition.getInnerDataContext.getOrElse(
-        throwException("Internal Error! Class instance did not have an associated BaseModule.")
+        throwException(
+          s"Internal Error! Class instance did not have an associated BaseModule for definition ${definition}."
+        )
       )
 
       // Get a Property[ClassType] type from the Class name.
       val classType = ClassType.unsafeGetClassTypeByName(baseModule.name)
       Property[classType.Type]()
     }
+
+    /** Get a ClassType type from a Definition[Class].
+      *
+      * This is useful when a ClassType type is needed for other Property types.
+      *
+      * This method is safe, and should be used over unsafeGetClassTypeByName when possible.
+      */
+    def getClassType: ClassType = {
+      // Get the BaseModule for the Class this is a definition of.
+      val baseModule = definition.getInnerDataContext.getOrElse(
+        throwException(
+          s"Internal Error! Class instance did not have an associated BaseModule for definition ${definition}."
+        )
+      )
+
+      // Get a ClassType from the Class name.
+      ClassType.unsafeGetClassTypeByName(baseModule.name)
+    }
   }
 
   implicit class ClassInstanceOps[T <: Class](instance: Instance[T]) {
@@ -218,7 +238,9 @@ object Class {
     def getPropertyReference: Property[ClassType] = {
       // Get the BaseModule from the Instance.
       val baseModule = instance.getInnerDataContext.getOrElse(
-        throwException("Internal Error! Class instance did not have an associated BaseModule.")
+        throwException(
+          s"Internal Error! Class instance did not have an associated BaseModule for instance ${instance}."
+        )
       )
 
       // Get a StaticObject for bookkeeping.

src/test/scala/chiselTests/properties/ClassSpec.scala

@@ -181,4 +181,14 @@ class ClassSpec extends ChiselFlatSpec with MatchesAndOmits {
       "propassign obj2.in, obj1.out"
     )()
   }
+
+  it should "support getting a ClassType from a Class Definition" in {
+    class Test extends Class {}
+
+    ChiselStage.emitCHIRRTL(new RawModule {
+      val definition = Definition(new Test)
+      val classType = definition.getClassType
+      classType.name should equal("Test")
+    })
+  }
 }