Properly verify a variable name isn't in use

src/frontend/Typechecker.ml

@@ -110,8 +110,10 @@ let verify_name_fresh_var loc tenv name =
   if Utils.is_unnormalized_distribution name then
     Semantic_error.ident_has_unnormalized_suffix loc name |> error
   else if
-    Env.mem tenv name
-    && not (Stan_math_signatures.is_stan_math_function_name name)
+    List.exists (Env.find tenv name) ~f:(function
+      | {kind= `StanMath; _} ->
+          false (* user variables can shadow library names *)
+      | _ -> true )
   then Semantic_error.ident_in_use loc name |> error
 
 (** verify that the variable being declared is previous unused.

test/integration/bad/lang/reused_name.stan

@@ -0,0 +1,5 @@
+generated quantities {
+    vector[3] e;
+    // e is selected because it is also a Stan-math function 'e()'
+    vector[3] e;
+}

test/integration/bad/lang/stanc.expected

@@ -696,6 +696,17 @@ Expected an expression inside "[" and "]" but found a statement. Indexing should
   var[1 + i]
 not
   var[for (n in 1:N) ...]
+  $ ../../../../../install/default/bin/stanc reused_name.stan
+Semantic error in 'reused_name.stan', line 4, column 14 to column 15:
+   -------------------------------------------------
+     2:      vector[3] e;
+     3:      // e is selected because it is also a Stan-math function 'e()'
+     4:      vector[3] e;
+                       ^
+     5:  }
+   -------------------------------------------------
+
+Identifier 'e' is already in use.
   $ ../../../../../install/default/bin/stanc unterminated_comment.stan
 Syntax error in 'unterminated_comment.stan', line 4, column -1, lexing error:
    -------------------------------------------------