An error a pass reports fails the compile, even when the pass succeeds

tslang/test/tester/CMakeLists.txt

@@ -2339,6 +2339,22 @@ foreach(call_arity_case ${call_arity_cases})
                                     FAIL_REGULAR_EXPRESSION "Stack dump|Assertion failed")
 endforeach()
 
+# An error reported by a pass that still returns success - a lowering pattern that cannot cast,
+# says so and carries on - fails the compile. It used to be printed and then ignored: the object
+# file was written, and under --emit=jit the program ran and crashed. Two tests because one
+# property cannot say both things: the message is the error we meant, and the exit code is not 0.
+set(lowering_error_file "${PROJECT_SOURCE_DIR}/test/tester/lowering-errors/cast_not_supported.ts")
+add_test(NAME test-compile-lowering-error-message
+         COMMAND $<TARGET_FILE:tslang> --emit=obj --no-default-lib -mm=none "${lowering_error_file}"
+                 -o "${CMAKE_CURRENT_BINARY_DIR}/lowering-error-message.obj")
+set_tests_properties(test-compile-lowering-error-message
+                     PROPERTIES PASS_REGULAR_EXPRESSION "cast from '!ts.array<!ts.number>' to '!ts.string' is not supported"
+                                FAIL_REGULAR_EXPRESSION "Stack dump|Assertion failed")
+add_test(NAME test-compile-lowering-error-exit
+         COMMAND $<TARGET_FILE:tslang> --emit=obj --no-default-lib -mm=none "${lowering_error_file}"
+                 -o "${CMAKE_CURRENT_BINARY_DIR}/lowering-error-exit.obj")
+set_tests_properties(test-compile-lowering-error-exit PROPERTIES WILL_FAIL TRUE)
+
 # @dllname and @linkname are one decorator under two names, and a symbol is bound once: each
 # case is a program the compiler must reject with the given message instead of binding
 # something else or making the name unique.

tslang/test/tester/lowering-errors/cast_not_supported.ts

@@ -0,0 +1,6 @@
+// Lowering has no cast from an array to a string, and says so with an error. That error has to
+// stop the compile: before it did, the program was emitted and run anyway and crashed.
+function main() {
+    let a: number[] | null = [1, 2];
+    print("a=" + a);
+}

tslang/tslang/transform.cpp

@@ -200,6 +200,14 @@ int runMLIRPasses(mlir::MLIRContext &context, llvm::SourceMgr &sourceMgr, mlir::
         result = 4;
     }
 
+    // A pass can report an error and still succeed - a lowering pattern that cannot cast says so
+    // and carries on with a value that is not one - so the run's result alone is not the answer.
+    // Without this the error was printed and the module compiled, linked and run anyway.
+    if (llvm::any_of(postponedMessages, [](auto &diag) { return diag->getSeverity() == mlir::DiagnosticSeverity::Error; }))
+    {
+        result = 4;
+    }
+
     SourceMgrDiagnosticHandlerEx sourceMgrHandler(sourceMgr, &context);
     printDiagnostics(sourceMgrHandler, postponedMessages, disableWarnings);
     return result;