parser: Detect function declaration (#580)

tools/isledecomp/isledecomp/parser/error.py

@@ -78,6 +78,11 @@ class ParserError(Enum):
     # they annotate are different.
     WRONG_STRING = 205
 
+    # ERROR: This lineref FUNCTION marker is next to a function declaration or
+    # forward reference. The correct place for the marker is where the function
+    # is implemented so we can match with the PDB.
+    NO_IMPLEMENTATION = 206
+
 
 @dataclass
 class ParserAlert:

tools/isledecomp/isledecomp/parser/parser.py

@@ -487,6 +487,9 @@ class DecompParser:
                 ):
                     self._function_starts_here()
                     self._function_done()
+                elif self.function_sig.endswith(");"):
+                    # Detect forward reference or declaration
+                    self._syntax_error(ParserError.NO_IMPLEMENTATION)
                 else:
                     self.state = ReaderState.WANT_CURLY