Warn on semicolon after `if`. (#2948)

* Warn on semicolon after `if`.

* add test result

---------

Co-authored-by: Yong He <yhe@nvidia.com>

4 files changed, 32 insertions, 4 deletions

diff --git a/source/slang/slang-diagnostic-defs.h b/source/slang/slang-diagnostic-defs.h
index 29ce30323..c01a6ddf1 100644
--- a/source/slang/slang-diagnostic-defs.h
+++ b/source/slang/slang-diagnostic-defs.h
@@ -234,6 +234,8 @@ DIAGNOSTIC(20013, Error, invalidCUDASMVersion, "Expecting CUDA SM version as eit
 
 DIAGNOSTIC(20014, Error, classIsReservedKeyword, "'class' is a reserved keyword in this context; use 'struct' instead.")
 
+DIAGNOSTIC(20101, Warning, unintendedEmptyStatement, "potentially unintended empty statement at this location; use {} instead.")
+
 //
 // 3xxxx - Semantic analysis
 //
diff --git a/source/slang/slang-parser.cpp b/source/slang/slang-parser.cpp
index d606698a1..dcf835234 100644
--- a/source/slang/slang-parser.cpp
+++ b/source/slang/slang-parser.cpp
@@ -184,7 +184,7 @@ namespace Slang
         void parseSourceFile(ModuleDecl* program);
         Decl* ParseStruct();
         ClassDecl* ParseClass();
-        Stmt* ParseStatement();
+        Stmt* ParseStatement(Stmt* parentStmt = nullptr);
         Stmt* parseBlockStatement();
         Stmt* parseLabelStatement();
         DeclStmt* parseVarDeclrStatement(Modifiers modifiers);
@@ -4438,7 +4438,7 @@ namespace Slang
         }
     }
 
-    Stmt* Parser::ParseStatement()
+    Stmt* Parser::ParseStatement(Stmt* parentStmt)
     {
         auto modifiers = ParseModifiers(this);
 
@@ -4575,6 +4575,13 @@ namespace Slang
         }
         else if (LookAheadToken(TokenType::Semicolon))
         {
+            if (as<IfStmt>(parentStmt))
+            {
+                // An empty statement after an `if` is probably a mistake,
+                // so we will diagnose it as such.
+                //
+                sink->diagnose(tokenReader.peekLoc(), Diagnostics::unintendedEmptyStatement);
+            }
             statement = astBuilder->create<EmptyStmt>();
             FillPosition(statement);
             ReadToken(TokenType::Semicolon);
@@ -4690,11 +4697,11 @@ namespace Slang
         ReadToken(TokenType::LParent);
         ifStatement->predicate = ParseExpression();
         ReadToken(TokenType::RParent);
-        ifStatement->positiveStatement = ParseStatement();
+        ifStatement->positiveStatement = ParseStatement(ifStatement);
         if (LookAheadToken("else"))
         {
             ReadToken("else");
-            ifStatement->negativeStatement = ParseStatement();
+            ifStatement->negativeStatement = ParseStatement(ifStatement);
         }
         return ifStatement;
     }
diff --git a/tests/diagnostics/if-empty-body.slang b/tests/diagnostics/if-empty-body.slang
new file mode 100644
index 000000000..de8983352
--- /dev/null
+++ b/tests/diagnostics/if-empty-body.slang
@@ -0,0 +1,11 @@
+//DIAGNOSTIC_TEST(windows):SIMPLE:
+
+// Test that use of empty statement after an `if` leads to an warning.
+
+struct S {}
+
+void test()
+{
+	if (1+1==2);
+		return;
+}
\ No newline at end of file
diff --git a/tests/diagnostics/if-empty-body.slang.expected b/tests/diagnostics/if-empty-body.slang.expected
new file mode 100644
index 000000000..30915e39d
--- /dev/null
+++ b/tests/diagnostics/if-empty-body.slang.expected
@@ -0,0 +1,8 @@
+result code = 0
+standard error = {
+tests/diagnostics/if-empty-body.slang(9): warning 20101: potentially unintended empty statement at this location; use {} instead.
+    if (1+1==2);
+               ^
+}
+standard output = {
+}