[clang] Do not serialize function definitions without a body

clang/lib/Serialization/ASTWriter.cpp

@@ -6092,12 +6092,14 @@ void ASTWriter::WriteDeclUpdatesBlocks(ASTContext &Context,
 
       // An updated body is emitted last, so that the reader doesn't need
       // to skip over the lazy body to reach statements for other records.
-      if (Kind == UPD_CXX_ADDED_FUNCTION_DEFINITION)
-        HasUpdatedBody = true;
-      else if (Kind == UPD_CXX_ADDED_VAR_DEFINITION)
+      if (Kind == UPD_CXX_ADDED_FUNCTION_DEFINITION) {
+        assert(isa<FunctionDecl>(D) && "expected FunctionDecl");
+        HasUpdatedBody = cast<FunctionDecl>(D)->hasBody();
+      } else if (Kind == UPD_CXX_ADDED_VAR_DEFINITION) {
         HasAddedVarDefinition = true;
-      else
+      } else {
         Record.push_back(Kind);
+      }
 
       switch (Kind) {
       case UPD_CXX_ADDED_IMPLICIT_MEMBER:

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