Give an error for expressions with incomplete struct/union types.

These are erroneous, in situations where the expression is used for its value. For function return types, this violates a constraint (C17 6.5.2.2 p1), so a diagnostic is required. We also now diagnose this issue for identifier expressions or unary * (indirection) expressions. These cases cause undefined behavior per C17 6.3.2.1 p2, so a diagnostic is not required, but it is nice to give one.

Expression.pas

@@ -2887,6 +2887,23 @@ var
    ldoDispose: boolean;                 {local copy of doDispose}
 
 
+   procedure CheckForIncompleteStructType;
+   
+   { Check if expressionType is an incomplete struct/union type. }
+   
+   var
+      tp: typePtr;                      {the type}
+   
+   begin
+   tp := expressionType;
+   while tp^.kind = definedType do
+      tp := tp^.dType;
+   if tp^.kind in [structType,unionType] then
+      if tp^.size = 0 then
+         Error(187);
+   end;
+
+
    function ExpressionKind (tree: tokenPtr): typeKind;
 
    { returns the type of an expression                           }
@@ -3516,6 +3533,7 @@ var
             ftype^.dispatcher);
       expressionType := ftype^.fType;
       lastWasConst := false;
+      CheckForIncompleteStructType;
       end; {else}
    end; {FunctionCall}      
 
@@ -3676,6 +3694,7 @@ case tree^.token.kind of
             LoadAddress(tree);                             
             if expressionType^.kind = pointerType then
                expressionType := expressionType^.ptype;
+            CheckForIncompleteStructType;
             end;
 
          enumConst: begin
@@ -4645,7 +4664,9 @@ case tree^.token.kind of
             ((lType^.kind in [functionType,arrayType,structType,unionType])
             or ((lType^.kind = definedType) and  {handle const struct/union}
                 (lType^.dType^.kind in [structType,unionType]))) then
-            Error(79);
+            Error(79)
+         else
+            CheckForIncompleteStructType;
          end {if}
       else
          Error(79);

Scanner.pas

@@ -802,6 +802,7 @@ if list or (numErr <> 0) then begin
         184: msg := @'segment exceeds bank size';
         185: msg := @'lint: unused variable: ';
         186: msg := @'lint: implicit conversion changes value of constant';
+        187: msg := @'expression has incomplete struct or union type';
          end; {case}
        if extraStr <> nil then begin
           extraStr^ := concat(msg^,extraStr^);