diff --git a/.github/workflows/lint-build.yml b/.github/workflows/lint-build.yml
index f3e52d5..eb9ade3 100644
--- a/.github/workflows/lint-build.yml
+++ b/.github/workflows/lint-build.yml
@@ -21,8 +21,8 @@ jobs:
       - name: Invoke clang
         shell: bash
         run: >+
-          clang++ -DVERSION=0.0.0 -DNDEBUG -WCL4 -Wnon-gcc -Wno-nullability-completeness -Werror -std=c++14 -o
-          /dev/null src/*.cpp
+          clang++ -DVERSION=0.0.0 -DNDEBUG -WCL4 -Wnon-gcc -Wimplicit-fallthrough -Werror -std=c++14 -o /dev/null
+          src/*.cpp
   lint-npm:
     if: ${{ !contains(github.event.head_commit.message, '[skip ci]') }}
     runs-on: ubuntu-latest
diff --git a/src/compat.hh b/src/compat.hh
index bc586f3..574aed1 100644
--- a/src/compat.hh
+++ b/src/compat.hh
@@ -4,7 +4,7 @@
 #include <utility>
 
 #ifdef __has_include
-#if __has_include(<sal.h>)
+#if __has_include(<sal.h>) && !defined(__clang__)
 #include <sal.h>
 #endif
 
@@ -35,6 +35,15 @@ constexpr int EX_NOINPUT = 66;
 #endif
 
 
+#if !defined(__GNUC__) && defined(_MSC_VER)
+// 1 if the compiler is really MSVC, and not clang pretending to be MSVC. 0 for clang and GCC.
+#define REALLY_MSVC 1
+#else
+// 1 if the compiler is really MSVC, and not clang pretending to be MSVC. 0 for clang and GCC.
+#define REALLY_MSVC 0
+#endif
+
+
 #ifdef __cpp_constinit
 // constinit, constexpr, or just const, depending on where lambdas are allowed
 #define CONSTINIT_LAMBDA constinit const
@@ -108,7 +117,7 @@ constexpr int EX_NOINPUT = 66;
 #define FALLTHROUGH [[fallthrough]];
 #endif
 
-#if !defined(__GNUC__) && defined(_MSC_VER)
+#if REALLY_MSVC
 #if _MSC_VER >= 1929
 // mark that the padding of this member can be used for other members
 #define NO_UNIQUE_ADDRESS [[msvc::no_unique_address]]
@@ -160,7 +169,7 @@ constexpr int EX_NOINPUT = 66;
 #endif
 
 
-#if !defined(__GNUC__) && defined(_MSC_VER)
+#if REALLY_MSVC
 #define __builtin_unreachable() __assume(0)
 #endif
 
@@ -172,6 +181,7 @@ constexpr int EX_NOINPUT = 66;
 
 
 #ifdef __clang__
+#pragma clang diagnostic ignored "-Wnullability-completeness"
 // A non-nullable pointer to the specified type.
 #define NONNULL_PTR(...) __VA_ARGS__* _Nonnull
 #elif defined(_Notnull_)
@@ -184,7 +194,7 @@ constexpr int EX_NOINPUT = 66;
 #endif
 
 
-#if (defined(__x86_64__) || defined(_M_X64)) && (defined(__GNUC__) || defined(__clang__))
+#if (defined(__x86_64__) || defined(_M_X64)) && !REALLY_MSVC
 // 1 on gcc or clang for x86-64, where asm blocks are used. 0 otherwise.
 #define ASM_ALLOWED 1
 #else
diff --git a/src/disassembler.cpp b/src/disassembler.cpp
index 317153b..eec038b 100644
--- a/src/disassembler.cpp
+++ b/src/disassembler.cpp
@@ -114,6 +114,7 @@ void disassembler::build_state() {
                     case PSI:
                     case PSC:
                         program_walker::advance(ip, m_program->side_length());
+                        break;
                     default:
                         break;
                 }
diff --git a/src/program_walker.hh b/src/program_walker.hh
index d05ee4e..42609c7 100644
--- a/src/program_walker.hh
+++ b/src/program_walker.hh
@@ -97,7 +97,7 @@ public:
         }
     }
 
-#ifdef _MSC_VER
+#if REALLY_MSVC
 #pragma warning(push)
 #pragma warning(disable : 4702)
     // C4702 is unreachable code, which it complains about when constant-folding the template instantiation
@@ -290,7 +290,7 @@ public:
         }
     }
 
-#ifdef _MSC_VER
+#if REALLY_MSVC
 #pragma warning(pop)
 #endif
 protected:
diff --git a/wasm/build_wasm.sh b/wasm/build_wasm.sh
index 3b37658..4f74ce0 100755
--- a/wasm/build_wasm.sh
+++ b/wasm/build_wasm.sh
@@ -2,7 +2,7 @@
 
 set -e
 
-common_emcc_args=(-WCL4 -Wnon-gcc -Wno-nullability-completeness
+common_emcc_args=(-WCL4 -Wnon-gcc -Wimplicit-fallthrough
     -fno-rtti -fno-exceptions
     -sEXPORTED_RUNTIME_METHODS=ccall -sASYNCIFY -sASYNCIFY_IMPORTS='send_debug_info'
     -o worker.js