Fix duplicate switch label for general switch statement

frontends/p4/typeChecking/typeChecker.cpp

@@ -3782,9 +3782,17 @@ const IR::Node* TypeInference::postorder(IR::SwitchStatement* stat) {
         if (isCompileTimeConstant(stat->expression))
             warn(ErrorType::WARN_MISMATCH, "%1%: constant expression in switch", stat->expression);
 
+        std::map<big_int, const IR::Node*> caselabels;
         for (auto& c : stat->cases) {
             if (!isCompileTimeConstant(c->label))
                 typeError("%1%: must be a compile-time constant", c->label);
+            if (auto cst = c->label->to<IR::Constant>()) {
+                auto it = caselabels.find(cst->value);
+                if (it != caselabels.end()) {
+                    typeError("%1%: 'switch' label duplicates %2%", c->label, it->second);
+                }
+                caselabels.emplace(cst->value, c->label);
+            }
             auto lt = getType(c->label);
             if (lt == nullptr) continue;
             if (lt->is<IR::Type_InfInt>() && type->is<IR::Type_Bits>()) {

testdata/p4_16_errors/duplicate-switch-case.p4

@@ -0,0 +1,17 @@
+// https://github.com/p4lang/p4c/issues/3612
+#include <core.p4>
+
+control c(in bit<4> a)
+{
+  apply {
+    switch (a) {
+      4w0: {}
+      4w1: {} // { dg-note "" }
+      4w1: {} // { dg-error "" }
+    }
+  }
+}
+
+control ct(in bit<4> a);
+package pt(ct t);
+pt(c()) main;

testdata/p4_16_errors_outputs/duplicate-switch-case.p4

@@ -0,0 +1,18 @@
+#include <core.p4>
+
+control c(in bit<4> a) {
+    apply {
+        switch (a) {
+            4w0: {
+            }
+            4w1: {
+            }
+            4w1: {
+            }
+        }
+    }
+}
+
+control ct(in bit<4> a);
+package pt(ct t);
+pt(c()) main;

testdata/p4_16_errors_outputs/duplicate-switch-case.p4-stderr

@@ -0,0 +1,6 @@
+duplicate-switch-case.p4(10): [--Werror=type-error] error: 4w1: 'switch' label duplicates 4w1
+      4w1: {} // { dg-error "" }
+      ^^^
+duplicate-switch-case.p4(9)
+      4w1: {} // { dg-note "" }
+      ^^^