Use two-stage parsing to disambiguate generic app and comparison. (#6281)

* Use two-stage parsing to disambiguate generic app and comparison.

* Typo fix.

* Update doc.

1 files changed, 30 insertions, 1 deletions

diff --git a/source/slang/slang-check-stmt.cpp b/source/slang/slang-check-stmt.cpp
index 550993957..151c9324a 100644
--- a/source/slang/slang-check-stmt.cpp
+++ b/source/slang/slang-check-stmt.cpp
@@ -54,6 +54,8 @@ void SemanticsStmtVisitor::visitDeclStmt(DeclStmt* stmt)
     // that need to be recursively checked.
     //
     ensureDeclBase(stmt->decl, DeclCheckState::DefinitionChecked, this);
+    if (auto decl = as<Decl>(stmt->decl))
+        decl->hiddenFromLookup = false;
 }
 
 void SemanticsStmtVisitor::visitBlockStmt(BlockStmt* stmt)
@@ -66,14 +68,41 @@ void SemanticsStmtVisitor::visitBlockStmt(BlockStmt* stmt)
             if (as<AggTypeDeclBase>(decl))
                 ensureAllDeclsRec(decl, DeclCheckState::DefinitionChecked);
         }
+
+        // Consider this code:
+        // ```
+        // {
+        //       int a = 5 + b; // should error.
+        //       int b = 3;
+        // }
+        //
+        // ```
+        // In order to detect the error trying to use `b` before it's declared within
+        // a block, our lookup logic contains a condition that ignores a decl if its
+        // `hiddenFromLookup` field is set to `true`.
+        // See _lookUpDirectAndTransparentMembers().
+        // This field will be set to false when we reach the decl through the DeclStmt.
+        //
+        if (auto seqStmt = as<SeqStmt>(stmt->body))
+        {
+            for (auto subStmt : seqStmt->stmts)
+            {
+                if (auto declStmt = as<DeclStmt>(subStmt))
+                {
+                    if (auto decl = as<Decl>(declStmt->decl))
+                        decl->hiddenFromLookup = true;
+                }
+            }
+        }
     }
     checkStmt(stmt->body);
 }
 
 void SemanticsStmtVisitor::visitSeqStmt(SeqStmt* stmt)
 {
-    for (auto ss : stmt->stmts)
+    for (auto& ss : stmt->stmts)
     {
+        ss = maybeParseStmt(ss, *this);
         checkStmt(ss);
     }
 }