fix(parser): report missing THEN and ')' on the line that needs them

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

5 files changed, 15 insertions, 7 deletions

diff --git a/include/cps/Parser.h b/include/cps/Parser.h
index eff25a5..e710150 100644
--- a/include/cps/Parser.h
+++ b/include/cps/Parser.h
@@ -19,6 +19,7 @@ class Parser {
     Lexer &Lex;
     int CurTok = tok_eof;
     bool StartsLine = true;
+    int PrevTokLine = 0; // line of the token before CurTok
     bool ParsingCaseLabel = false;
     std::vector<unsigned> CaseBodyDepths;
     unsigned NumErrors = 0;
@@ -33,6 +34,9 @@ class Parser {
     };
 
     void getNextToken();
+    // Line for a diagnostic about a token missing before CurTok: the previous
+    // line when CurTok starts a new one.
+    int missingLine() const { return StartsLine ? PrevTokLine : Lex.getLine(); }
     int GetTokPrecedence();
 
     std::unique_ptr<ExprAST> ParseExpression();
diff --git a/lib/Parser/Parser.cc b/lib/Parser/Parser.cc
index d17856b..ad2e408 100644
--- a/lib/Parser/Parser.cc
+++ b/lib/Parser/Parser.cc
@@ -30,6 +30,7 @@ Parser::Parser(Lexer &L) : Lex(L) {
 
 void Parser::getNextToken() {
     int PreviousLine = CurTok == tok_eof ? 0 : Lex.getLine();
+    PrevTokLine = PreviousLine;
     CurTok = Lex.gettok();
     StartsLine = Lex.getLine() != PreviousLine;
 }
diff --git a/lib/Parser/ParserControlFlow.cc b/lib/Parser/ParserControlFlow.cc
index 7caf555..542ef0e 100644
--- a/lib/Parser/ParserControlFlow.cc
+++ b/lib/Parser/ParserControlFlow.cc
@@ -26,10 +26,13 @@ std::unique_ptr<StmtAST> Parser::ParseIfStmt() {
     if (!Cond) return nullptr;
 
     if (CurTok != tok_then) {
-        cps::sourceError(Lex.getLine(), "expected THEN");
-        return nullptr;
+        cps::sourceError(missingLine(), "expected THEN");
+        // A THEN missing at the end of a line is reported and then treated as
+        // present so the rest of the statement still parses.
+        if (!StartsLine) return nullptr;
+    } else {
+        getNextToken();
     }
-    getNextToken();
 
     std::vector<std::unique_ptr<StmtAST>> ThenStmts = ParseBlock({tok_else, tok_endif});
 
@@ -93,7 +96,7 @@ std::unique_ptr<StmtAST> Parser::ParseForStmt() {
     getNextToken();
 
     if (CurTok != tok_assign) {
-        cps::sourceError(Lex.getLine(), "expected '<-' in FOR loop");
+        cps::sourceError(missingLine(), "expected '<-' in FOR loop");
         return nullptr;
     }
     getNextToken();
@@ -102,7 +105,7 @@ std::unique_ptr<StmtAST> Parser::ParseForStmt() {
     if (!Start) return nullptr;
 
     if (CurTok != tok_to) {
-        cps::sourceError(Lex.getLine(), "expected TO in FOR loop");
+        cps::sourceError(missingLine(), "expected TO in FOR loop");
         return nullptr;
     }
     getNextToken();
diff --git a/lib/Parser/ParserExpression.cc b/lib/Parser/ParserExpression.cc
index 8c5e8ce..b75f704 100644
--- a/lib/Parser/ParserExpression.cc
+++ b/lib/Parser/ParserExpression.cc
@@ -211,7 +211,7 @@ std::unique_ptr<ExprAST> Parser::ParseParenExpr() {
     auto V = ParseExpression();
     if (!V) return nullptr;
     if (CurTok != ')') {
-        cps::sourceError(Lex.getLine(), "expected ')'");
+        cps::sourceError(missingLine(), "expected ')'");
         return nullptr;
     }
     getNextToken();
diff --git a/tests/iii/missing-parenthesis/case.json b/tests/iii/missing-parenthesis/case.json
index 3ccd080..5421130 100644
--- a/tests/iii/missing-parenthesis/case.json
+++ b/tests/iii/missing-parenthesis/case.json
@@ -2,7 +2,7 @@
   "errors": [
     {
       "reason": "expected\\ '\\)'",
-      "line": 3
+      "line": 2
     }
   ]
 }