[clang] Do not serialize function definitions without a body (#121550)

An instantiated templated function definition may not have a body due to
parsing errors inside the templated function. When serializing, an
assert is triggered inside `ASTRecordWriter::AddFunctionDefinition`.

The instantiation may happen on an intermediate module.

The test case was reduced from `mp-units`.

Author: alejandro-alvarez-sonarsource

clang/lib/Serialization/ASTWriter.cpp

@@ -7230,6 +7230,10 @@ void ASTWriter::CompletedImplicitDefinition(const FunctionDecl *D) {
   if (!D->isFromASTFile())
     return; // Declaration not imported from PCH.
 
+  // The function definition may not have a body due to parsing errors.
+  if (!D->doesThisDeclarationHaveABody())
+    return;
+
   // Implicit function decl from a PCH was defined.
   DeclUpdates[D].push_back(DeclUpdate(UPD_CXX_ADDED_FUNCTION_DEFINITION));
 }
@@ -7249,6 +7253,10 @@ void ASTWriter::FunctionDefinitionInstantiated(const FunctionDecl *D) {
   if (!D->isFromASTFile())
     return;
 
+  // The function definition may not have a body due to parsing errors.
+  if (!D->doesThisDeclarationHaveABody())
+    return;
+
   DeclUpdates[D].push_back(DeclUpdate(UPD_CXX_ADDED_FUNCTION_DEFINITION));
 }
 

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