[SILGen] Insert an unreachable if a function declaration's parameter is uninhabited

include/swift/AST/DiagnosticsSIL.def

@@ -229,6 +229,8 @@ ERROR(guard_body_must_not_fallthrough,none,
       "'guard' body must not fall through, consider using a 'return' or 'throw'"
       " to exit the scope", ())
 WARNING(unreachable_code,none, "will never be executed", ())
+NOTE(unreachable_code_uninhabited_param_note,none,
+	"'%0' is uninhabited, so this function body can never be executed", (StringRef))
 NOTE(unreachable_code_branch,none,
      "condition always evaluates to %select{false|true}0", (bool))
 NOTE(call_to_noreturn_note,none,

lib/SILGen/SILGenProlog.cpp

@@ -423,6 +423,19 @@ void SILGenFunction::emitProlog(AnyFunctionRef TheClosure,
                                 Type resultType, bool throws) {
   uint16_t ArgNo = emitProlog(paramList, selfParam, resultType,
                               TheClosure.getAsDeclContext(), throws);
+
+  // Emit an unreachable instruction if a parameter type is
+  // uninhabited
+  if (paramList) {
+    for (auto *param : *paramList) {
+      if (param->getType()->isStructurallyUninhabited()) {
+        SILLocation unreachableLoc(param);
+        unreachableLoc.markAsPrologue();
+        B.createUnreachable(unreachableLoc);
+        break;
+      }
+    }
+  }
 
   // Emit the capture argument variables. These are placed last because they
   // become the first curry level of the SIL function.

lib/SILGen/SILGenStmt.cpp

@@ -318,6 +318,16 @@ void StmtEmitter::visitBraceStmt(BraceStmt *S) {
       } else {
         diagnose(getASTContext(), ESD.getStartLoc(),
                  diag::unreachable_code);
+        if (!S->getElements().empty()) {
+          for (auto *arg : SGF.getFunction().getArguments()) {
+            if (arg->getType().getASTType()->isStructurallyUninhabited()) {
+              diagnose(getASTContext(), S->getStartLoc(),
+                       diag::unreachable_code_uninhabited_param_note,
+                       arg->getDecl()->getBaseName().userFacingName());
+              break;
+            }
+          }
+        }
       }
       return;
     }

test/SILGen/functions_uninhabited_param.swift

@@ -0,0 +1,10 @@
+// RUN: %target-swift-emit-sil -enable-sil-ownership %s -o /dev/null -verify
+
+//===--- Function declaration with uninhabited parameter type
+
+func foo(baz: Never) -> Int { // expected-note {{'baz' is uninhabited, so this function body can never be executed}}
+  print("I can't be called!") // expected-warning{{will never be executed}}
+  return 0
+}
+
+func bar(baz: Never) -> Int {} // ok