diff --git a/flang/lib/Semantics/check-declarations.cpp b/flang/lib/Semantics/check-declarations.cpp
--- a/flang/lib/Semantics/check-declarations.cpp
+++ b/flang/lib/Semantics/check-declarations.cpp
@@ -1455,7 +1455,10 @@
                        "Dummy argument '%s' is a procedure; the corresponding"
                        " argument in the interface body is not"_err_en_US);
                  },
-                 [&](const auto &, const auto &) { DIE("can't happen"); },
+                 [&](const auto &, const auto &) {
+                   llvm_unreachable("Dummy arguments are not data objects or"
+                                    "procedures");
+                 },
              },
       arg1.u, arg2.u);
 }
diff --git a/flang/lib/Semantics/resolve-names.cpp b/flang/lib/Semantics/resolve-names.cpp
--- a/flang/lib/Semantics/resolve-names.cpp
+++ b/flang/lib/Semantics/resolve-names.cpp
@@ -287,7 +287,7 @@
     case parser::AccessSpec::Kind::Private:
       return Attr::PRIVATE;
     }
-    common::die("unreachable"); // suppress g++ warning
+    llvm_unreachable("Switch covers all cases"); // suppress g++ warning
   }
   Attr IntentSpecToAttr(const parser::IntentSpec &x) {
     switch (x.v) {
@@ -298,7 +298,7 @@
     case parser::IntentSpec::Intent::InOut:
       return Attr::INTENT_INOUT;
     }
-    common::die("unreachable"); // suppress g++ warning
+    llvm_unreachable("Switch covers all cases"); // suppress g++ warning
   }
 
 private:
@@ -1398,13 +1398,27 @@
   void Post(const parser::AssignedGotoStmt &);
 
   // These nodes should never be reached: they are handled in ProgramUnit
-  bool Pre(const parser::MainProgram &) { DIE("unreachable"); }
-  bool Pre(const parser::FunctionSubprogram &) { DIE("unreachable"); }
-  bool Pre(const parser::SubroutineSubprogram &) { DIE("unreachable"); }
-  bool Pre(const parser::SeparateModuleSubprogram &) { DIE("unreachable"); }
-  bool Pre(const parser::Module &) { DIE("unreachable"); }
-  bool Pre(const parser::Submodule &) { DIE("unreachable"); }
-  bool Pre(const parser::BlockData &) { DIE("unreachable"); }
+  bool Pre(const parser::MainProgram &) {
+    llvm_unreachable("This node is handled in ProgramUnit");
+  }
+  bool Pre(const parser::FunctionSubprogram &) {
+    llvm_unreachable("This node is handled in ProgramUnit");
+  }
+  bool Pre(const parser::SubroutineSubprogram &) {
+    llvm_unreachable("This node is handled in ProgramUnit");
+  }
+  bool Pre(const parser::SeparateModuleSubprogram &) {
+    llvm_unreachable("This node is handled in ProgramUnit");
+  }
+  bool Pre(const parser::Module &) {
+    llvm_unreachable("This node is handled in ProgramUnit");
+  }
+  bool Pre(const parser::Submodule &) {
+    llvm_unreachable("This node is handled in ProgramUnit");
+  }
+  bool Pre(const parser::BlockData &) {
+    llvm_unreachable("This node is handled in ProgramUnit");
+  }
 
   void NoteExecutablePartCall(Symbol::Flag, const parser::Call &);