A switch's default clause falls through like any other clause

tslang/lib/TypeScript/MLIRGenImpl.h

@@ -4635,9 +4635,11 @@ class MLIRGenImpl
                 }
             }
 
-            // exit;
+            // exit; without a `break` it falls through into the next clause's body - from `default`
+            // too, wherever it stands. A branch left pending after the last clause keeps the merge
+            // block it was created with.
             auto branchOp = builder.create<mlir::cf::BranchOp>(location, mergeBlock);
-            if (!hasBreak && !isDefaultCase)
+            if (!hasBreak)
             {
                 pendingBranches.push_back(branchOp);
             }

tslang/test/tester/CMakeLists.txt

@@ -474,6 +474,7 @@ tslang_add_test(NAME test-compile-00-string-length-resize COMMAND test-runner "$
 tslang_add_test(NAME test-compile-00-array-length-grow-zeroed COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00array_length_grow_zeroed.ts")
 tslang_add_test(NAME test-compile-00-rest-args-pack-owned COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00rest_args_pack_owned.ts")
 tslang_add_test(NAME test-compile-00-switch-char-string-cases COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00switch_char_string_cases.ts")
+tslang_add_test(NAME test-compile-00-switch-default-fallthrough COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00switch_default_fallthrough.ts")
 tslang_add_test(NAME test-compile-00-generic-class-constructor-inference COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00generic_class_constructor_inference.ts")
 tslang_add_test(NAME test-compile-00-generator-done-value-zero COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00generator_done_value_zero.ts")
 tslang_add_test(NAME test-compile-00-any-equality-undefined COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00any_equality_undefined.ts")
@@ -991,6 +992,7 @@ tslang_add_test(NAME test-jit-00-string-length-resize COMMAND test-runner -jit "
 tslang_add_test(NAME test-jit-00-array-length-grow-zeroed COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00array_length_grow_zeroed.ts")
 tslang_add_test(NAME test-jit-00-rest-args-pack-owned COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00rest_args_pack_owned.ts")
 tslang_add_test(NAME test-jit-00-switch-char-string-cases COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00switch_char_string_cases.ts")
+tslang_add_test(NAME test-jit-00-switch-default-fallthrough COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00switch_default_fallthrough.ts")
 tslang_add_test(NAME test-jit-00-generic-class-constructor-inference COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00generic_class_constructor_inference.ts")
 tslang_add_test(NAME test-jit-00-generator-done-value-zero COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00generator_done_value_zero.ts")
 tslang_add_test(NAME test-jit-00-any-equality-undefined COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00any_equality_undefined.ts")
@@ -1986,6 +1988,7 @@ set(TSLANG_CORPUS
     00string_length_resize.ts
     00strings.ts
     00switch.ts
+    00switch_default_fallthrough.ts
     00switch_char_string_cases.ts
     00symbol.ts
     00throw_in_catch.ts

tslang/test/tester/tests/00switch_default_fallthrough.ts

@@ -0,0 +1,89 @@
+// A `default` clause without a `break` never fell through into the clauses after it: its body
+// jumped straight out of the switch, so `default: r += "q"; case 6: r += "x"` gave "q".
+
+function defaultFirst(x: number) {
+    let r = "";
+    switch (x) {
+        default:
+            r += "q";
+        // fallthrough
+        case 6:
+        case 7:
+            r += "x";
+        // fallthrough
+        case 8:
+            r += "y";
+            break;
+        case 10:
+            r += "z";
+            break;
+    }
+
+    return r;
+}
+
+function defaultInMiddle(x: number) {
+    let r = "";
+    switch (x) {
+        case 6:
+            r += "x";
+        // fallthrough
+        default:
+            r += "q";
+        // fallthrough
+        case 8:
+            r += "y";
+            break;
+        case 9:
+            r += "w";
+    }
+
+    return r;
+}
+
+function defaultLast(x: number) {
+    let r = "";
+    switch (x) {
+        case 1:
+            r += "a";
+        // fallthrough
+        default:
+            r += "d";
+    }
+
+    return r + ".";
+}
+
+function defaultWithBreak(x: number) {
+    let r = "";
+    switch (x) {
+        default:
+            r += "q";
+            break;
+        case 1:
+            r += "a";
+    }
+
+    return r;
+}
+
+function main() {
+    assert(defaultFirst(100) == "qxy", "defaultFirst(100)");
+    assert(defaultFirst(6) == "xy", "defaultFirst(6)");
+    assert(defaultFirst(7) == "xy", "defaultFirst(7)");
+    assert(defaultFirst(8) == "y", "defaultFirst(8)");
+    assert(defaultFirst(10) == "z", "defaultFirst(10)");
+
+    assert(defaultInMiddle(100) == "qy", "defaultInMiddle(100)");
+    assert(defaultInMiddle(6) == "xqy", "defaultInMiddle(6)");
+    assert(defaultInMiddle(8) == "y", "defaultInMiddle(8)");
+    assert(defaultInMiddle(9) == "w", "defaultInMiddle(9)");
+
+    assert(defaultLast(1) == "ad.", "defaultLast(1)");
+    assert(defaultLast(2) == "d.", "defaultLast(2)");
+
+    assert(defaultWithBreak(5) == "q", "defaultWithBreak(5)");
+    assert(defaultWithBreak(1) == "a", "defaultWithBreak(1)");
+
+    print("done.");
+}