Clarify Duplicate Declaration error (#580)

* Add missing Forward Reference data model error

* Apply suggestions from code review

* Refactor PR to redefine Duplicate Declaration rules

* Drop Forward Reference error definition

* Update Duplicate Declaration error description

* Apply suggestions from code review

Co-authored-by: Addison Phillips <[email protected]>

---------

Co-authored-by: Addison Phillips <[email protected]>

Author: eemeli

spec/errors.md

@@ -129,10 +129,9 @@ or contains a _variable_ that does not directly or indirectly reference a _decla
 
 ### Duplicate Declaration
 
-A **_<dfn>Duplicate Declaration</dfn>_** error occurs when a _variable_ appears in two _declarations_.
-This includes when an _input-declaration_ binds a _variable_ that appears in a previous _declaration_,
-when a _local-declaration_ binds a _variable_ that appears in a previous _declaration_,
-or when a _local-declaration_ refers to its bound _variable_ in its _expression_.
+A **_<dfn>Duplicate Declaration</dfn>_** error occurs when a _variable_ is declared more than once.
+Note that an input _variable_ is implicitly declared when it is first used,
+so explicitly declaring it after such use is also an error.
 
 > Examples of invalid messages resulting in a _Duplicate Declaration_ error:
 >
@@ -149,6 +148,10 @@ or when a _local-declaration_ refers to its bound _variable_ in its _expression_
 > .local $var = {$ext :number maxFractionDigits=0}
 > {{Redeclaration of an input variable}}
 >
+> .input {$var :number minFractionDigits=$var2}
+> .input {$var2 :number}
+> {{Redeclaration of the implicit input variable $var2}}
+>
 > .local $var = {$ext :someFunction}
 > .local $var = {$error}
 > .local $var2 = {$var2 :error}

spec/syntax.md

@@ -175,14 +175,14 @@ local-declaration = local s variable [s] "=" [s] expression
 _Variables_, once declared, MUST NOT be redeclared. 
 A _message_ that does any of the following is not _valid_ and will produce a 
 _Duplicate Declaration_ error during processing:
-- An _input-declaration_ MUST NOT bind a _variable_ that appears as a _variable_ in a previous 
-  _declaration_.
-- A _local-declaration_ MUST NOT bind a _variable_ that appears as a _variable_ in a previous
-  _declaration_.
+- A _declaration_ MUST NOT bind a _variable_
+  that appears as a _variable_ anywhere within a previous _declaration_.
+- An _input-declaration_ MUST NOT bind a _variable_
+  that appears anywhere within the _annotation_ of its _variable-expression_.
 - A _local-declaration_ MUST NOT bind a _variable_ that appears in its _expression_.
 
 A _local-declaration_ MAY overwrite an external input value as long as the
-external input value does not appear in a _declaration_.
+external input value does not appear in a previous _declaration_.
 
 > [!Note]
 > These restrictions only apply to _declarations_.