PS: Improve constant expression AST hierachy.

Author: MathiasVP

powershell/ql/lib/semmle/code/powershell/BaseConstantExpression.qll

@@ -1,3 +1,13 @@
 import powershell
 
-class BaseConstExpr extends @base_constant_expression, Expr { }
+/** The base class for constant expressions. */
+class BaseConstExpr extends @base_constant_expression, Expr {
+  /** Gets the type of this constant expression. */
+  string getType() { none() }
+
+  /** Gets a string literal of this constant expression. */
+  StringLiteral getValue() { none() }
+
+  /** Gets a string literal representing this constant expression. */
+  final override string toString() { result = this.getValue().toString() }
+}

powershell/ql/lib/semmle/code/powershell/ConstantExpression.qll

@@ -3,11 +3,9 @@ import powershell
 class ConstExpr extends @constant_expression, BaseConstExpr {
   override SourceLocation getLocation() { constant_expression_location(this, result) }
 
-  string getType() { constant_expression(this, result) }
+  override string getType() { constant_expression(this, result) }
 
-  StringLiteral getValue() { constant_expression_value(this, result) }
-
-  override string toString() { result = this.getValue().toString() }
+  override StringLiteral getValue() { constant_expression_value(this, result) }
 }
 
 private newtype TConstantValue =
@@ -97,7 +95,7 @@ class ConstString extends ConstantValue, TConstString {
     result = "\"" + this.asString().replaceAll("\"", "\\\"") + "\""
   }
 
-  final override ConstExpr getAnExpr() { result.getValue().getValue() = this.getValue() }
+  final override BaseConstExpr getAnExpr() { result.getValue().getValue() = this.getValue() }
 }
 
 /** A constant boolean value. */

powershell/ql/lib/semmle/code/powershell/StringConstantExpression.qll

@@ -1,10 +1,10 @@
 import powershell
 
+/** A string constant. */
 class StringConstExpr extends @string_constant_expression, BaseConstExpr {
-  StringLiteral getValue() { string_constant_expression(this, result) }
+  override StringLiteral getValue() { string_constant_expression(this, result) }
 
-  /** Get the full string literal with all its parts concatenated */
-  override string toString() { result = this.getValue().toString() }
+  override string getType() { result = "String" }
 
   override SourceLocation getLocation() { string_constant_expression_location(this, result) }
 }