Stricter type pattern matching rules (#463)

Vineflower's previous implementation of the type casting pattern
matching in switch statements was not strict enough, considering any
form of cast in the first expression of the case block a result of type
pattern matching.

This would incorrectly attempt to emit such type pattern matching source
code for switches that simply yielded different types.

This commit tightens the logic to consider a switch case a type pattern
matching one if the first expression is a simple cast of the switch
head's variable.

Author: lynxplay

src/org/jetbrains/java/decompiler/modules/decompiler/SwitchPatternMatchProcessor.java

@@ -187,7 +187,7 @@ private static boolean processStatement(SwitchStatement stat, Statement root) {
             if (assign.getLeft() instanceof VarExprent) {
               VarExprent var = (VarExprent) assign.getLeft();
 
-              if (assign.getRight() instanceof FunctionExprent && ((FunctionExprent) assign.getRight()).getFuncType() == FunctionExprent.FunctionType.CAST) {
+              if (isPatternMatchingCastAssignment(head, assign)) {
                 FunctionExprent cast = (FunctionExprent) assign.getRight();
 
                 List<Exprent> operands = new ArrayList<>();
@@ -547,4 +547,19 @@ private static boolean isSwitchPatternMatch(SwitchHeadExprent head) {
   public static boolean hasPatternMatch(RootStatement root) {
     return root.mt.getBytecodeVersion().hasSwitchPatternMatch() && DecompilerContext.getOption(IFernflowerPreferences.PATTERN_MATCHING);
   }
+
+  private static boolean isPatternMatchingCastAssignment(final SwitchHeadExprent switchHead, final AssignmentExprent assignment) {
+    if (!(assignment.getRight() instanceof final FunctionExprent functionExprent)) return false;
+    if (functionExprent.getFuncType() != FunctionType.CAST) return false;
+
+    final List<Exprent> lstOperands = functionExprent.getLstOperands();
+    // We expect the assignment to be a literal `n = (Type) m`.
+    // Any other operands are not allowed in simple pattern matching.
+    if (lstOperands.size() < 2) return false;
+    if (!(lstOperands.get(0) instanceof final VarExprent switchHeadRef)) return false;
+    if (!(lstOperands.get(1) instanceof final ConstExprent castTypeRef)) return false;
+    if (!switchHead.containsVar(switchHeadRef.getVarVersionPair())) return false; // Not the switch head var ref
+
+    return true;
+  }
 }