[typechecker] Emit error diagnostic if the type in is-pattern does not conform to error and we're in a catch statement

Author: Suyash Srijan

lib/Sema/TypeCheckConstraints.cpp

@@ -4028,6 +4028,27 @@ CheckedCastKind TypeChecker::typeCheckCheckedCast(Type fromType,
     if (toType->isAnyObject() || fromType->isAnyObject())
       return CheckedCastKind::ValueCast;
 
+    // A checked cast from Swift.Error to a type that does not conform to
+    // Swift.Error cannot succeed.
+    //
+    // struct S {}
+    // enum FooError: Error { case bar }
+    // func foo() throws {
+    //   do {
+    //     throw FooError.bar
+    //   } catch is X {
+    //     print("Caught bar error")
+    //   }
+    // }
+    auto exceptionType = getExceptionType(dc, SourceLoc());
+    auto toTypeConformsToError =
+        conformsToProtocol(toType, Context.getErrorDecl(), dc,
+                           ConformanceCheckFlags::InExpression);
+    if (fromType->isEqual(exceptionType) && !toTypeConformsToError) {
+      diagnose(diagLoc, diag::type_does_not_conform, toType, exceptionType);
+      return failed();
+    }
+
     bool toRequiresClass;
     if (toType->isExistentialType())
       toRequiresClass = toType->getExistentialLayout().requiresClass();

test/stmt/errors.swift

@@ -165,3 +165,26 @@ func thirteen() {
     }
   }
 }
+
+// SR 6400
+
+enum SR_6400_E: Error {
+  case castError
+}
+
+struct SR_6400_S_1 {}
+struct SR_6400_S_2: Error {}
+
+func sr_6400() throws {
+  do {
+    throw SR_6400_E.castError
+  } catch is SR_6400_S_1 { // expected-error {{type 'SR_6400_S_1' does not conform to protocol 'Error'}} expected-warning {{cast from 'Error' to unrelated type 'SR_6400_S_1' always fails}}
+    print("Caught error")
+  }
+
+  do {
+    throw SR_6400_E.castError
+  } catch is SR_6400_S_2 {
+    print("Caught error") // Ok
+  }
+}