3 files changed, 31 insertions, 2 deletions
diff --git a/src/support/command-line.cpp b/src/support/command-line.cpp
index 355d704bb..d9b7170f5 100644
--- a/src/support/command-line.cpp
+++ b/src/support/command-line.cpp
@@ -124,16 +124,18 @@ void Options::parse(int argc, const char* argv[]) {
   assert(argc > 0 && "expect at least program name as an argument");
   size_t positionalsSeen = 0;
   auto dashes = [](const std::string& s) {
-    for (size_t i = 0;; ++i) {
+    for (size_t i = 0; i < s.size(); ++i) {
       if (s[i] != '-') {
         return i;
       }
     }
+    return s.size();
   };
   for (size_t i = 1, e = argc; i != e; ++i) {
     std::string currentOption = argv[i];
 
-    if (dashes(currentOption) == 0) {
+    // "-" alone is a positional option
+    if (dashes(currentOption) == 0 || currentOption == "-") {
       // Positional.
       switch (positional) {
         case Arguments::Zero:
diff --git a/src/support/file.cpp b/src/support/file.cpp
index d17e3a338..dc9707a1b 100644
--- a/src/support/file.cpp
+++ b/src/support/file.cpp
@@ -34,8 +34,22 @@ std::vector<char> wasm::read_stdin() {
   return input;
 }
 
+template<typename T> struct do_read_stdin { T operator()(); };
+
+template<> std::vector<char> do_read_stdin<std::vector<char>>::operator()() {
+  return wasm::read_stdin();
+}
+
+template<> std::string do_read_stdin<std::string>::operator()() {
+  auto vec = wasm::read_stdin();
+  return std::string(vec.begin(), vec.end());
+}
+
 template<typename T>
 T wasm::read_file(const std::string& filename, Flags::BinaryOption binary) {
+  if (filename == "-") {
+    return do_read_stdin<T>{}();
+  }
   BYN_TRACE("Loading '" << filename << "'...\n");
   std::ifstream infile;
   std::ios_base::openmode flags = std::ifstream::in;
diff --git a/test/lit/stdin-stdout.wast b/test/lit/stdin-stdout.wast
new file mode 100644
index 000000000..8aa9fe89a
--- /dev/null
+++ b/test/lit/stdin-stdout.wast
@@ -0,0 +1,13 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
+;; RUN: wasm-opt < %s - -S -o - | filecheck %s
+
+;; Test that file `-` is interpreted as stdin and stdout.
+
+(module
+  ;; CHECK:      (func $foo
+  ;; CHECK-NEXT:  (nop)
+  ;; CHECK-NEXT: )
+  (func $foo
+    (nop)
+  )
+)