[clang] Do not serialize function definitions without a body

clang/lib/Serialization/ASTWriter.cpp

@@ -6227,8 +6227,10 @@ void ASTWriter::WriteDeclUpdatesBlocks(ASTContext &Context,
     // Add a trailing update record, if any. These must go last because we
     // lazily load their attached statement.
     if (!GeneratingReducedBMI || !CanElideDeclDef(D)) {
-      if (HasUpdatedBody) {
-        const auto *Def = cast<FunctionDecl>(D);
+      assert(!(HasUpdatedBody && HasAddedVarDefinition) &&
+             "Declaration can not be both a FunctionDecl and a VarDecl");
+      if (const auto *Def = dyn_cast<FunctionDecl>(D);
+          HasUpdatedBody && Def->doesThisDeclarationHaveABody()) {
         Record.push_back(UPD_CXX_ADDED_FUNCTION_DEFINITION);
         Record.push_back(Def->isInlined());
         Record.AddSourceLocation(Def->getInnerLocStart());

clang/test/Modules/missing-body-in-import.cpp

@@ -0,0 +1,42 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: cd %t
+
+// RUN: %clang_cc1 -std=c++23 mod1.cppm -emit-module-interface -o mod1.pcm -fallow-pcm-with-compiler-errors -verify
+// RUN: %clang_cc1 -std=c++23 mod2.cppm -emit-module-interface -o mod2.pcm -fmodule-file=mod1=mod1.pcm -verify -fallow-pcm-with-compiler-errors
+// RUN: %clang_cc1 -std=c++23 mod3.cppm -emit-module-interface -o mod3.pcm -fmodule-file=mod1=mod1.pcm -fmodule-file=mod2=mod2.pcm -verify -fallow-pcm-with-compiler-errors
+// RUN: %clang_cc1 -std=c++23 main.cpp -fmodule-file=mod1=mod1.pcm -fmodule-file=mod2=mod2.pcm -fmodule-file=mod3=mod3.pcm -verify -fallow-pcm-with-compiler-errors -ast-dump-all
+
+//--- mod1.cppm
+export module mod1;
+
+export template <unsigned N, unsigned M>
+class A {
+public:
+  constexpr A(const char[], const char[]) {
+    auto x = BrokenExpr; // expected-error {{use of undeclared identifier 'BrokenExpr'}}
+  }
+};
+
+export template<A<1,1> NTTP>
+struct B {};
+
+template < unsigned N, unsigned M >
+A(const char (&)[N], const char (&)[M]) -> A< 1, 1 >;
+
+//--- mod2.cppm
+export module mod2;
+import mod1;
+
+struct C: B <A{"a", "b"}> { // expected-error {{non-type template argument is not a constant expression}}
+  constexpr C(int a) { }
+};
+
+//--- mod3.cppm
+// expected-no-diagnostics
+export module mod3;
+export import mod2;
+
+//--- main.cpp
+// expected-no-diagnostics
+import mod3; // no crash