fix(pacmak): .NET bindings fail to compile with error CS8120

If a type union includes several candidates that are related to each
other (A extends B or A implements B), `jsii-pacmak` may generate type
checking clauses in a pattern matching `switch` statement in an order
such that the compiler identifies dead clauses, which is an error in C#.

This adds a provision to NOT emit such a clause so as to not cause the
error. It is worth mentioning that the error cannot be "opted out" of
via a `#pragma` directive like a warning would be, which is unfortunate.

Fixes #3759

packages/jsii-calc/lib/index.ts

@@ -25,3 +25,4 @@ export * as cdk16625 from './cdk16625';
 export * as jsii3656 from './jsii3656';
 
 export * as anonymous from './anonymous';
+export * as union from './union';

packages/jsii-calc/lib/union.ts

@@ -0,0 +1,21 @@
+import { IFriendly } from '@scope/jsii-calc-lib';
+
+export interface IResolvable {
+  resolve(): any;
+}
+
+export class Resolvable implements IResolvable {
+  private constructor() {}
+
+  public resolve(): any {
+    return false;
+  }
+}
+
+export class ConsumesUnion {
+  public static unionType(param: IResolvable | Resolvable | IFriendly) {
+    void param;
+  }
+
+  private constructor() {}
+}

packages/jsii-calc/test/assembly.jsii

@@ -421,6 +421,13 @@
         "line": 4
       },
       "symbolId": "lib/submodule/returns-param/index:"
+    },
+    "jsii-calc.union": {
+      "locationInModule": {
+        "filename": "lib/index.ts",
+        "line": 28
+      },
+      "symbolId": "lib/union:"
     }
   },
   "targets": {
@@ -18199,8 +18206,125 @@
       "name": "ReturnsSpecialParameter",
       "namespace": "submodule.returnsparam",
       "symbolId": "lib/submodule/returns-param/index:ReturnsSpecialParameter"
+    },
+    "jsii-calc.union.ConsumesUnion": {
+      "assembly": "jsii-calc",
+      "docs": {
+        "stability": "stable"
+      },
+      "fqn": "jsii-calc.union.ConsumesUnion",
+      "kind": "class",
+      "locationInModule": {
+        "filename": "lib/union.ts",
+        "line": 15
+      },
+      "methods": [
+        {
+          "docs": {
+            "stability": "stable"
+          },
+          "locationInModule": {
+            "filename": "lib/union.ts",
+            "line": 16
+          },
+          "name": "unionType",
+          "parameters": [
+            {
+              "name": "param",
+              "type": {
+                "union": {
+                  "types": [
+                    {
+                      "fqn": "jsii-calc.union.IResolvable"
+                    },
+                    {
+                      "fqn": "jsii-calc.union.Resolvable"
+                    },
+                    {
+                      "fqn": "@scope/jsii-calc-lib.IFriendly"
+                    }
+                  ]
+                }
+              }
+            }
+          ],
+          "static": true
+        }
+      ],
+      "name": "ConsumesUnion",
+      "namespace": "union",
+      "symbolId": "lib/union:ConsumesUnion"
+    },
+    "jsii-calc.union.IResolvable": {
+      "assembly": "jsii-calc",
+      "docs": {
+        "stability": "stable"
+      },
+      "fqn": "jsii-calc.union.IResolvable",
+      "kind": "interface",
+      "locationInModule": {
+        "filename": "lib/union.ts",
+        "line": 3
+      },
+      "methods": [
+        {
+          "abstract": true,
+          "docs": {
+            "stability": "stable"
+          },
+          "locationInModule": {
+            "filename": "lib/union.ts",
+            "line": 4
+          },
+          "name": "resolve",
+          "returns": {
+            "type": {
+              "primitive": "any"
+            }
+          }
+        }
+      ],
+      "name": "IResolvable",
+      "namespace": "union",
+      "symbolId": "lib/union:IResolvable"
+    },
+    "jsii-calc.union.Resolvable": {
+      "assembly": "jsii-calc",
+      "docs": {
+        "stability": "stable"
+      },
+      "fqn": "jsii-calc.union.Resolvable",
+      "interfaces": [
+        "jsii-calc.union.IResolvable"
+      ],
+      "kind": "class",
+      "locationInModule": {
+        "filename": "lib/union.ts",
+        "line": 7
+      },
+      "methods": [
+        {
+          "docs": {
+            "stability": "stable"
+          },
+          "locationInModule": {
+            "filename": "lib/union.ts",
+            "line": 10
+          },
+          "name": "resolve",
+          "overrides": "jsii-calc.union.IResolvable",
+          "returns": {
+            "type": {
+              "primitive": "any"
+            }
+          }
+        }
+      ],
+      "name": "Resolvable",
+      "namespace": "union",
+      "symbolId": "lib/union:Resolvable"
     }
   },
   "version": "3.20.120",
-  "fingerprint": "Ze43eowG9ImRufT3MQ8yO+bW8JzOQlZIYtFsjpc960E="
+  "fingerprint": "OaHwYmdPa8tbAJnlREahLGpRaNNBor2aoG04vsA/SvM="
 }

packages/jsii-pacmak/lib/targets/dotnet/runtime-type-checking.ts

@@ -172,6 +172,26 @@ abstract class Validation {
         code.openBlock(`switch (${expression})`);
         for (const type of types) {
           validTypes.push(resolver.toDotNetTypeName(type.spec!));
+
+          /**
+           * Filter to remove classes and interfaces from a set of type references that
+           * are implied by another entry in the set. Practically this is meant to remove
+           * types from a set if a parent type of it is also present in the set, keeping
+           * only the most generic declaration.
+           *
+           * This is useful because the TypeScript compiler and jsii do not guarantee that
+           * all entries in a type union are unrelated, but the C# compiler treats dead
+           * code as an error, and will refuse to compile (error CS8120) a patter-matching
+           * switch case if it cannot be matched (for example, if it matches on a child of
+           * a type that was previously matched on already).
+           */
+          if (
+            (type.type?.isClassType() || type.type?.isInterfaceType())
+            && types.some((other) => other !== type && other.type != null && type.type!.extends(other.type))
+          ) {
+            continue;
+          }
+
           const typeNames = [resolver.toDotNetType(type.spec!)];
           if (typeNames[0] === 'double') {
             // For doubles, we accept any numeric value, really...