Add Wcase-dup warning

scripts/diagnostics.txt

@@ -776,10 +776,7 @@ warning bitwise-op-mismatch BitwiseOpMismatch "bitwise operation between operand
 warning comparison-mismatch ComparisonMismatch "comparison between operands of different types ({} and {})"
 warning sign-compare SignCompare "comparison of differently signed types ({} and {})"
 warning case-type CaseTypeMismatch "comparison of different types in '{}' statement ({} and {})"
-warning case-default CaseDefault "'{}' missing 'default' label"
 warning case-outside-range CaseOutsideRange "'{}' item with {} bits can never match the {} bit case expression"
-warning case-enum CaseEnum "enumeration {} not handled in case statement"
-warning case-enum-explicit CaseEnumExplicit "enumeration {} not explicitly handled in case statement"
 warning bitwise-rel-precedence BitwiseRelPrecedence "'{}' has lower precedence than '{}'; '{}' will be evaluated first"
 note NotePrecedenceBitwiseFirst "place parentheses around the '{}' expression to evaluate it first"
 note NotePrecedenceSilence "place parentheses around the '{}' expression to silence this warning"
@@ -872,6 +869,10 @@ error BlockingInAlwaysFF "always_ff procedures cannot have blocking timing contr
 error AlwaysFFEventControl "always_ff procedure must have one and only one event control"
 error SeqEmptyMatch "sequence must not admit an empty match"
 error SeqOnlyEmpty "sequence admits only empty matches"
+warning case-default CaseDefault "'{}' missing 'default' label"
+warning case-enum CaseEnum "enumeration {} not handled in case statement"
+warning case-enum-explicit CaseEnumExplicit "enumeration {} not explicitly handled in case statement"
+warning case-dup CaseDup "'{}' statement contains duplicate items for value '{}' (the second one will never be matched)"
 warning seq-no-match SeqNoMatch "sequence can never be matched"
 warning event-const EventExpressionConstant "edge expression is constant"
 warning empty-stmt EmptyStatement "extra ';' has no effect"
@@ -1174,7 +1175,7 @@ group extra = { empty-member empty-stmt dup-import pointless-void-cast case-gen-
                 ineffective-sign port-width-trunc constant-conversion float-bool-conv int-bool-conv
                 useless-cast unsigned-arith-shift static-init-order static-init-value float-int-conv
                 float-narrow bitwise-rel-precedence arith-in-shift logical-not-parentheses
-                consecutive-comparison case-outside-range case-enum }
+                consecutive-comparison case-outside-range case-enum case-dup }
 
 group pedantic = { empty-pattern implicit-net-port nonstandard-escape-code nonstandard-generate format-multibit-strength
                    nonstandard-sys-func nonstandard-foreach nonstandard-dist isunbounded-param-arg udp-coverage }

scripts/warning_docs.txt

@@ -1561,3 +1561,20 @@ module m;
     end
 endmodule
 ```
+
+-Wcase-dup
+A case statement contains more than one item with the same value.
+Case items are evaluated and selected in order and so the second
+one will never be matched.
+```
+module m;
+    int i;
+    initial begin
+        case (i)
+            1, 2:;
+            3, 1:;
+            default;
+        endcase
+    end
+endmodule
+```

source/ast/Statements.cpp

@@ -1201,14 +1201,21 @@ Statement& CaseStatement::fromSyntax(Compilation& compilation, const CaseStateme
             << LexerFacts::getTokenKindText(keyword);
     }
 
-    if (expr->type->isEnum()) {
-        SmallSet<ConstantValue, 4> elems;
-        for (auto& g : result->items) {
-            for (auto item : g.expressions) {
-                if (auto cv = context.tryEval(*item))
-                    elems.emplace(std::move(cv));
+    SmallMap<ConstantValue, const Expression*, 4> elems;
+    for (auto& g : result->items) {
+        for (auto item : g.expressions) {
+            if (auto cv = context.tryEval(*item)) {
+                auto [it, inserted] = elems.emplace(std::move(cv), item);
+                if (!inserted) {
+                    auto& diag = context.addDiag(diag::CaseDup, item->sourceRange);
+                    diag << LexerFacts::getTokenKindText(keyword) << it->first;
+                    diag.addNote(diag::NotePreviousUsage, it->second->sourceRange);
+                }
             }
         }
+    }
+
+    if (expr->type->isEnum()) {
 
         SmallVector<std::string_view> missing;
         for (auto& ev : expr->type->getCanonicalType().as<EnumType>().values()) {

tests/unittests/ast/WarningTests.cpp

@@ -1181,3 +1181,32 @@ endmodule
     CHECK(diags[10].code == diag::CaseDefault);
     CHECK(diags[11].code == diag::CaseEnum);
 }
+
+TEST_CASE("Case statement dups") {
+    auto tree = SyntaxTree::fromText(R"(
+module m;
+    int i;
+    event e;
+    initial begin
+        case (i)
+            1, 2:;
+            3, 1:;
+            default;
+        endcase
+        case (e)
+            null:;
+            null:;
+            default;
+        endcase
+    end
+endmodule
+)");
+
+    Compilation compilation;
+    compilation.addSyntaxTree(tree);
+
+    auto& diags = compilation.getAllDiagnostics();
+    REQUIRE(diags.size() == 2);
+    CHECK(diags[0].code == diag::CaseDup);
+    CHECK(diags[1].code == diag::CaseDup);
+}