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Forward Parameter Mismatch

Velox reports this error when a procedure or function implementation is matched to an earlier forward declaration but their callable signatures are not equivalent.

Expected form​

function Normalise(const Value: string; var Changed: Boolean): string; forward;

function Normalise(const Value: string; var Changed: Boolean): string;
begin
  Changed := True;
  Result := Value;
end;

Example​

The parameter mode changes from var to an ordinary value parameter:

procedure UpdateValue(var Value: string); forward;

procedure UpdateValue(Value: string); // Forward Parameter Mismatch
begin
end;

Repeat the same mode and type:

procedure UpdateValue(var Value: string);
begin
end;

What the compiler compares​

The modified PascalScript compiler resolves the implementation to the forward routine and calls its signature comparison. TPSParametersDecl.Same checks:

  • the number of parameters;
  • the result type identity for a function;
  • each parameter's mode, such as ordinary, const, var or out; and
  • each parameter's resolved type identity, in declaration order.

Parameter names are not compared. Renaming Value to Text alone does not cause this diagnostic, although keeping names aligned makes the source easier to audit.

Type identity matters​

The comparison uses the compiler's resolved type objects, not a broad “convertible at runtime” rule. Two types that can be assigned between each other may still be different forward-signature types. Use the same declared type name and shape on both headings.

Correction procedure​

  1. Locate the preceding forward declaration for the routine.
  2. Compare procedure versus function and confirm the result type.
  3. Compare parameter count and order.
  4. Compare every parameter mode and resolved type.
  5. Make the implementation heading match the forward declaration exactly, then recompile.

Common causes​

  • Adding a parameter to the implementation but not the forward declaration.
  • Changing var to a value or const parameter.
  • Changing a type alias or function result on only one heading.
  • Reordering parameters.
  • Editing one copy of a declaration in included source while leaving another stale.
  • Forward declarations — declaring routines before their bodies.
  • Unsatisfied forward — a forward declaration with no implementation.
  • Invalid number of parameters — a call-site argument-count failure.

External references​