Errors in a generated cast are reported where the cast is

tslang/lib/TypeScript/MLIRGenCast.cpp

@@ -2116,15 +2116,17 @@ namespace mlirgen
 
         auto src = ss.str();
 
-        {
-            MLIRLocationGuard vgLoc(overwriteLoc); 
-            overwriteLoc = location;
+        // The generated function's body is generated when the call below instantiates it, not when
+        // it is parsed, so the call site has to stay the location until then. Without it a
+        // diagnostic there took the generated text's offset as a position in the user's file - an
+        // error reported inside a comment on line 1.
+        MLIRLocationGuard vgLoc(overwriteLoc);
+        overwriteLoc = location;
 
-            if (mlir::failed(parsePartialStatements(src)))
-            {
-                assert(false);
-                return mlir::failure();
-            }
+        if (mlir::failed(parsePartialStatements(src)))
+        {
+            assert(false);
+            return mlir::failure();
         }
 
         auto funcResult = resolveIdentifier(location, funcName, genContext);
@@ -2308,15 +2310,14 @@ namespace mlirgen
 
                 auto src = ss.str();
 
-                {
-                    MLIRLocationGuard vgLoc(overwriteLoc); 
-                    overwriteLoc = location;
+                // kept until the call below has instantiated the function - see castPrimitiveTypeFromAny
+                MLIRLocationGuard vgLoc(overwriteLoc);
+                overwriteLoc = location;
 
-                    if (mlir::failed(parsePartialStatements(src)))
-                    {
-                        assert(false);
-                        return mlir::failure();
-                    }
+                if (mlir::failed(parsePartialStatements(src)))
+                {
+                    assert(false);
+                    return mlir::failure();
                 }
 
                 auto funcResult = resolveIdentifier(location, funcName, genContext);

tslang/test/tester/CMakeLists.txt

@@ -2613,6 +2613,17 @@ foreach(call_arity_case ${call_arity_cases})
                                     FAIL_REGULAR_EXPRESSION "Stack dump|Assertion failed")
 endforeach()
 
+# An error inside a cast function the compiler generates (here: null in a union cast to string) is
+# reported where the cast is - line 6 - and not at the generated text's offset taken as a position
+# in the user's file, which put it inside the comment on line 1.
+add_test(NAME test-compile-diagnostic-location-generated-cast
+         COMMAND $<TARGET_FILE:tslang> --emit=obj --no-default-lib -mm=none
+                 "${PROJECT_SOURCE_DIR}/test/tester/diagnostic-location/generated_cast.ts"
+                 -o "${CMAKE_CURRENT_BINARY_DIR}/diagnostic-location-generated-cast.obj")
+set_tests_properties(test-compile-diagnostic-location-generated-cast
+                     PROPERTIES PASS_REGULAR_EXPRESSION "generated_cast\\.ts:6:23: error: can't cast from 'null' to 'string'"
+                                FAIL_REGULAR_EXPRESSION "Stack dump|Assertion failed|generated_cast\\.ts:1:")
+
 # A call to the collector's API (GC_gcollect, or the weak-link functions DefaultLib's WeakRef uses)
 # is an error under every model but gc, which alone links the collector - it used to reach the
 # linker as an unresolved symbol. Under gc the same file compiles.

tslang/test/tester/diagnostic-location/generated_cast.ts

@@ -0,0 +1,10 @@
+// An error inside the cast function the compiler generates for a union was reported at the
+// generated text's offset taken as a position in this file - inside this comment - rather than
+// where the cast is.
+
+function text(x: number | string | null): string {
+    const s: string = x;
+    return s;
+}
+
+print(text("a"));