fix(validation): reject THIS.<member> without dereference

src/validation.rs

@@ -110,7 +110,15 @@ pub struct Validator<'a> {
 
 impl Validators for Validator<'_> {
     fn push_diagnostic(&mut self, diagnostic: Diagnostic) {
-        self.diagnostics.push(diagnostic);
+        // Skip exact duplicates: lowering passes occasionally clone an AST
+        // node into multiple positions in the post-lowered tree (e.g. the
+        // receiver of a method call ends up both in the call's operator
+        // chain and in its synthesised `self` argument), which causes the
+        // same statement-level diagnostic to be raised more than once for
+        // the same source location.
+        if !self.diagnostics.contains(&diagnostic) {
+            self.diagnostics.push(diagnostic);
+        }
     }
     fn take_diagnostics(&mut self) -> Vec<Diagnostic> {
         std::mem::take(&mut self.diagnostics)

src/validation/statement.rs

@@ -226,6 +226,14 @@ fn validate_reference_expression<T: AnnotationMap>(
                             .with_location(m.get_location())
                             .with_error_code("E119"),
                     );
+                } else if base.is_this() {
+                    // THIS is `POINTER TO <enclosing FB>`; a member access on it
+                    // requires an explicit dereference (`THIS^.member`).
+                    validator.push_diagnostic(
+                        Diagnostic::new("`THIS` must be dereferenced to access its members.")
+                            .with_location(m.get_location())
+                            .with_error_code("E120"),
+                    );
                 }
             }
 

src/validation/tests/this_keyword_validation_tests.rs

@@ -448,3 +448,34 @@ fn this_calling_functionblock_body_from_method_is_ok() {
     );
     assert!(diagnostics.is_empty());
 }
+
+#[test]
+fn this_member_access_without_deref_is_an_error() {
+    // Regression test for #1728: `THIS.<member>` (without `^`) used to slip
+    // through validation and panic in codegen with
+    // `Builder error: GEP pointee is not a struct`. It must surface as E120.
+    let diagnostics = parse_and_validate_buffered(
+        r#"
+        INTERFACE IAnimal
+            METHOD legs : DINT
+            END_METHOD
+        END_INTERFACE
+
+        FUNCTION_BLOCK fb
+            VAR
+                a : IAnimal;
+            END_VAR
+            METHOD m : DINT
+                THIS.a.legs();
+            END_METHOD
+        END_FUNCTION_BLOCK
+    "#,
+    );
+    assert_snapshot!(diagnostics, @r"
+    error[E120]: `THIS` must be dereferenced to access its members.
+       ┌─ <internal>:12:22
+       │
+    12 │                 THIS.a.legs();
+       │                      ^ `THIS` must be dereferenced to access its members.
+    ");
+}

tests/lit/single/oop/this_member_access_without_deref_errors.st

@@ -0,0 +1,24 @@
+// RUN: rm -f %T/%basename_t.out
+// RUN: not %COMPILE --error-format clang %s 2>&1 | %CHECK %s
+// RUN: test ! -e %T/%basename_t.out
+
+// Regression test for #1728: `THIS.<member>` without an explicit `^` deref
+// must be rejected by validation, not silently passed through to codegen
+// (where it used to crash with "GEP pointee is not a struct").
+
+// CHECK: {{.*}}this_member_access_without_deref_errors.st:{{[0-9]+}}:{{[0-9]+}}:{{.*}}error[E120]: `THIS` must be dereferenced to access its members.
+// CHECK: Compilation aborted due to critical errors
+
+INTERFACE IAnimal
+    METHOD legs : DINT
+    END_METHOD
+END_INTERFACE
+
+FUNCTION_BLOCK fb
+VAR
+    a : IAnimal;
+END_VAR
+METHOD m : DINT
+    THIS.a.legs();
+END_METHOD
+END_FUNCTION_BLOCK