Fix assertions due to malformed switch statements (#3858)

* Fix assertions due to malformed switch statements

Fixes the issue #2955

* Checks for multiple case statements with same values
* Checks for multiple default cases
* Constant-folds case exprs into an Integer value

* fix the comments, and updated error code

* one-line comment on diagnostic code

4 files changed, 58 insertions, 5 deletions

diff --git a/source/slang/slang-ast-stmt.h b/source/slang/slang-ast-stmt.h
index 055785333..afa606456 100644
--- a/source/slang/slang-ast-stmt.h
+++ b/source/slang/slang-ast-stmt.h
@@ -143,6 +143,8 @@ class CaseStmt : public CaseStmtBase
     SLANG_AST_CLASS(CaseStmt)
 
     Expr* expr = nullptr;
+
+    Val* exprVal = nullptr;
 };
 
 // a `default` statement inside a `switch`
diff --git a/source/slang/slang-check-impl.h b/source/slang/slang-check-impl.h
index 48bd2093b..55edba6b9 100644
--- a/source/slang/slang-check-impl.h
+++ b/source/slang/slang-check-impl.h
@@ -2741,6 +2741,9 @@ namespace Slang
         void tryInferLoopMaxIterations(ForStmt* stmt);
 
         void checkLoopInDifferentiableFunc(Stmt* stmt);
+
+    private:
+        void validateCaseStmts(SwitchStmt* stmt, DiagnosticSink* sink);
     };
 
     struct SemanticsDeclVisitorBase
diff --git a/source/slang/slang-check-stmt.cpp b/source/slang/slang-check-stmt.cpp
index b68465087..729b24d35 100644
--- a/source/slang/slang-check-stmt.cpp
+++ b/source/slang/slang-check-stmt.cpp
@@ -239,6 +239,47 @@ namespace Slang
         subContext.checkStmt(stmt->body);
     }
 
+    void SemanticsStmtVisitor::validateCaseStmts(SwitchStmt* stmt, DiagnosticSink* sink)
+    {
+        auto blockStmt = as<BlockStmt>(stmt->body);
+        if (!blockStmt)
+            return;
+
+        auto seqStmt = as<SeqStmt>(blockStmt->body);
+        if (!seqStmt)
+            return;
+
+        bool hasDefaultStmt = false;
+        HashSet<Val*> caseStmtVals;
+        for (auto& sStmt : seqStmt->stmts)
+        {
+            if (auto caseStmt = as<CaseStmt>(sStmt))
+            {
+                // check that all case tags are unique
+                if (caseStmt->exprVal)
+                {
+                    // exprVal contains the constant folded expr, that is checked for
+                    // uniqueness within the scope of the switch statement.
+                    if (!caseStmtVals.add(caseStmt->exprVal))
+                    {
+                        sink->diagnose(sStmt, Diagnostics::switchDuplicateCases);
+                        return;
+                    }
+                }
+            }
+            else if (auto defaultStmt = as<DefaultStmt>(sStmt))
+            {
+                // check that there is at most one `default` clause
+                if (hasDefaultStmt)
+                {
+                    sink->diagnose(sStmt, Diagnostics::switchMultipleDefault);
+                    return;
+                }
+                hasDefaultStmt = true;
+            }
+        }
+    }
+
     void SemanticsStmtVisitor::visitSwitchStmt(SwitchStmt* stmt)
     {
         WithOuterStmt subContext(this, stmt);
@@ -247,16 +288,18 @@ namespace Slang
         stmt->condition = CheckExpr(stmt->condition);
         subContext.checkStmt(stmt->body);
 
-        // TODO(tfoley): need to check that all case tags are unique
-
-        // TODO(tfoley): check that there is at most one `default` clause
+        // check the case value exits within the switch
+        validateCaseStmts(stmt, getSink());
     }
 
     void SemanticsStmtVisitor::visitCaseStmt(CaseStmt* stmt)
     {
-        // TODO(tfoley): Need to coerce to type being switch on,
-        // and ensure that value is a compile-time constant
         auto expr = CheckExpr(stmt->expr);
+
+        // coerce to type being switch on, and ensure that value is a compile-time constant
+        // The Vals in the AST are pointer-unique, making them easy to check for duplicates
+        // by addeing them to a HashSet.
+        auto exprVal = tryConstantFoldExpr(expr, ConstantFoldingKind::CompileTime, nullptr);
         auto switchStmt = FindOuterStmt<SwitchStmt>();
 
         if (!switchStmt)
@@ -270,6 +313,7 @@ namespace Slang
         }
 
         stmt->expr = expr;
+        stmt->exprVal = exprVal;
         stmt->parentStmt = switchStmt;
     }
 
diff --git a/source/slang/slang-diagnostic-defs.h b/source/slang/slang-diagnostic-defs.h
index 752bcf03b..882a4c314 100644
--- a/source/slang/slang-diagnostic-defs.h
+++ b/source/slang/slang-diagnostic-defs.h
@@ -476,6 +476,10 @@ DIAGNOSTIC(30504, Warning, forLoopTerminatesInFewerIterationsThanMaxIters, "the
 DIAGNOSTIC(30505, Warning, loopRunsForZeroIterations, "the loop runs for 0 iterations and will be removed.")
 DIAGNOSTIC(30510, Error, loopInDiffFuncRequireUnrollOrMaxIters, "loops inside a differentiable function need to provide either '[MaxIters(n)]' or '[ForceUnroll]' attribute.")
 
+// Switch
+DIAGNOSTIC(30600, Error, switchMultipleDefault, "multiple 'default' cases not allowed within a 'switch' statement")
+DIAGNOSTIC(30601, Error, switchDuplicateCases, "duplicate cases not allowed within a 'switch' statement")
+
 // TODO: need to assign numbers to all these extra diagnostics...
 DIAGNOSTIC(39999, Fatal, cyclicReference, "cyclic reference '$0'.")
 DIAGNOSTIC(39999, Error, cyclicReferenceInInheritance, "cyclic reference in inheritance graph '$0'.")