Re: [PATCH] c++/modules: local class merging [PR99426]

[Patrick Palka <ppalka@redhat.com>]

On Tue, 5 Mar 2024, Patrick Palka wrote:

> On Tue, 27 Feb 2024, Patrick Palka wrote:
> 
> > On Mon, 26 Feb 2024, Patrick Palka wrote:
> > 
> > > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this approach
> > > look reasonable?
> > > 
> > > -- >8 --
> > > 
> > > One known missing piece in the modules implementation is merging of a
> > > streamed-in local class with the corresponding in-TU version of the
> > > local class.  This missing piece turns out to cause a hard-to-reduce
> > > use-after-free GC issue due to the entity_ary not being marked as a GC
> > > root (deliberately), and manifests as a serialization error on stream-in
> > > as in PR99426 (see comment #6 for a reduction).  It's also reproducible
> > > on trunk when running the xtreme-header tests without -fno-module-lazy.
> > > 
> > > This patch makes us merge such local classes according to their position
> > > within the containing function's definition, similar to how we merge
> > > FIELD_DECLs of a class according to their index in the TYPE_FIELDS
> > > list.
> > > 
> > > 	PR c++/99426
> > > 
> > > gcc/cp/ChangeLog:
> > > 
> > > 	* module.cc (merge_kind::MK_local_class): New enumerator.
> > > 	(merge_kind_name): Update.
> > > 	(trees_out::chained_decls): Move BLOCK-specific handling
> > > 	of DECL_LOCAL_DECL_P decls to ...
> > > 	(trees_out::core_vals) <case BLOCK>: ... here.  Stream
> > > 	BLOCK_VARS manually.
> > > 	(trees_in::core_vals) <case BLOCK>: Stream BLOCK_VARS
> > > 	manually.  Handle deduplicated local classes.
> > > 	(trees_out::key_local_class): Define.
> > > 	(trees_in::key_local_class): Define.
> > > 	(trees_out::get_merge_kind) <case FUNCTION_DECL>: Return
> > > 	MK_local_class for a local class.
> > > 	(trees_out::key_mergeable) <case FUNCTION_DECL>: Use
> > > 	key_local_class.
> > > 	(trees_in::key_mergeable) <case FUNCTION_DECL>: Likewise.
> > > 	(trees_in::is_matching_decl): Be flexible with type mismatches
> > > 	for local entities.
> > > 
> > > gcc/testsuite/ChangeLog:
> > > 
> > > 	* g++.dg/modules/xtreme-header-7_a.H: New test.
> > > 	* g++.dg/modules/xtreme-header-7_b.C: New test.
> > 
> > > ---
> > >  gcc/cp/module.cc                              | 167 +++++++++++++++---
> > >  .../g++.dg/modules/xtreme-header-7_a.H        |   4 +
> > >  .../g++.dg/modules/xtreme-header-7_b.C        |   6 +
> > >  3 files changed, 149 insertions(+), 28 deletions(-)
> > >  create mode 100644 gcc/testsuite/g++.dg/modules/xtreme-header-7_a.H
> > >  create mode 100644 gcc/testsuite/g++.dg/modules/xtreme-header-7_b.C
> > > 
> > > diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
> > > index fa91c6ff9cb..f77f73a59ed 100644
> > > --- a/gcc/cp/module.cc
> > > +++ b/gcc/cp/module.cc
> > > @@ -2771,6 +2771,7 @@ enum merge_kind
> > >  
> > >    MK_enum,	/* Found by CTX, & 1stMemberNAME.  */
> > >    MK_keyed,     /* Found by key & index.  */
> > > +  MK_local_class, /* Found by CTX, index.  */
> > >  
> > >    MK_friend_spec,  /* Like named, but has a tmpl & args too.  */
> > >    MK_local_friend, /* Found by CTX, index.  */
> > > @@ -2799,7 +2800,7 @@ static char const *const merge_kind_name[MK_hwm] =
> > >      "unique", "named", "field", "vtable",	/* 0...3  */
> > >      "asbase", "partial", "enum", "attached",	/* 4...7  */
> > >  
> > > -    "friend spec", "local friend", NULL, NULL,  /* 8...11 */
> > > +    "local class", "friend spec", "local friend", NULL,  /* 8...11 */
> > >      NULL, NULL, NULL, NULL,
> > >  
> > >      "type spec", "type tmpl spec",	/* 16,17 type (template).  */
> > > @@ -2928,6 +2929,7 @@ public:
> > >    unsigned binfo_mergeable (tree *);
> > >  
> > >  private:
> > > +  tree key_local_class (const merge_key&, tree);
> > >    uintptr_t *find_duplicate (tree existing);
> > >    void register_duplicate (tree decl, tree existing);
> > >    /* Mark as an already diagnosed bad duplicate.  */
> > > @@ -3086,6 +3088,7 @@ public:
> > >    void binfo_mergeable (tree binfo);
> > >  
> > >  private:
> > > +  void key_local_class (merge_key&, tree, tree);
> > >    bool decl_node (tree, walk_kind ref);
> > >    void type_node (tree);
> > >    void tree_value (tree);
> > > @@ -4952,18 +4955,7 @@ void
> > >  trees_out::chained_decls (tree decls)
> > >  {
> > >    for (; decls; decls = DECL_CHAIN (decls))
> > > -    {
> > > -      if (VAR_OR_FUNCTION_DECL_P (decls)
> > > -	  && DECL_LOCAL_DECL_P (decls))
> > > -	{
> > > -	  /* Make sure this is the first encounter, and mark for
> > > -	     walk-by-value.  */
> > > -	  gcc_checking_assert (!TREE_VISITED (decls)
> > > -			       && !DECL_TEMPLATE_INFO (decls));
> > > -	  mark_by_value (decls);
> > > -	}
> > > -      tree_node (decls);
> > > -    }
> > > +    tree_node (decls);
> > >    tree_node (NULL_TREE);
> > >  }
> > >  
> > > @@ -6204,7 +6196,21 @@ trees_out::core_vals (tree t)
> > >  
> > >        /* DECL_LOCAL_DECL_P decls are first encountered here and
> > >           streamed by value.  */
> > > -      chained_decls (t->block.vars);
> > > +      for (tree decls = t->block.vars; decls; decls = DECL_CHAIN (decls))
> > > +	{
> > > +	  if (VAR_OR_FUNCTION_DECL_P (decls)
> > > +	      && DECL_LOCAL_DECL_P (decls))
> > > +	    {
> > > +	      /* Make sure this is the first encounter, and mark for
> > > +		 walk-by-value.  */
> > > +	      gcc_checking_assert (!TREE_VISITED (decls)
> > > +				   && !DECL_TEMPLATE_INFO (decls));
> > > +	      mark_by_value (decls);
> > > +	    }
> > > +	  tree_node (decls);
> > > +	}
> > > +      tree_node (NULL_TREE);
> > > +
> > >        /* nonlocalized_vars is a middle-end thing.  */
> > >        WT (t->block.subblocks);
> > >        WT (t->block.supercontext);
> > > @@ -6717,7 +6723,34 @@ trees_in::core_vals (tree t)
> > >      case BLOCK:
> > >        t->block.locus = state->read_location (*this);
> > >        t->block.end_locus = state->read_location (*this);
> > > -      t->block.vars = chained_decls ();
> > > +
> > > +      for (tree *chain = &t->block.vars;;)
> > > +	if (tree decl = tree_node ())
> > > +	  {
> > > +	    /* For a deduplicated local class, chain the to-be-discarded
> > > +	       decl not the in-TU decl (which is already chained to in-TU
> > > +	       entities).  */
> > > +	    if (is_duplicate (decl))
> > > +	      decl = maybe_duplicate (decl);
> > > +	    else if (DECL_IMPLICIT_TYPEDEF_P (decl)
> > > +		     && TYPE_TEMPLATE_INFO (TREE_TYPE (decl)))
> > > +	      {
> > > +		tree tmpl = TYPE_TI_TEMPLATE (TREE_TYPE (decl));
> > > +		if (DECL_TEMPLATE_RESULT (tmpl) == decl && is_duplicate (tmpl))
> > > +		  decl = DECL_TEMPLATE_RESULT (maybe_duplicate (tmpl));
> > > +	      }
> > > +
> > > +	    if (!DECL_P (decl) || DECL_CHAIN (decl))
> > > +	      {
> > > +		set_overrun ();
> > > +		break;
> > > +	      }
> > > +	    *chain = decl;
> > > +	    chain = &DECL_CHAIN (decl);
> > > +	  }
> > > +	else
> > > +	  break;
> > > +
> > >        /* nonlocalized_vars is middle-end.  */
> > >        RT (t->block.subblocks);
> > >        RT (t->block.supercontext);
> > > @@ -10335,6 +10368,83 @@ trees_in::fn_parms_fini (int tag, tree fn, tree existing, bool is_defn)
> > >      }
> > >  }
> > >  
> > > +/* Encode into KEY the position of the local class declaration DECL
> > > +   within FN.  The position is encoded as the index of the innermost
> > > +   BLOCK (numbered in BFS order) along with the index within its
> > > +   BLOCK_VARS list.  */
> > > +
> > > +void
> > > +trees_out::key_local_class (merge_key& key, tree decl, tree fn)
> > > +{
> > > +  auto_vec<tree, 4> blocks;
> > > +  blocks.quick_push (DECL_INITIAL (fn));
> > > +  unsigned block_ix = 0;
> > > +  while (block_ix != blocks.length ())
> > > +    {
> > > +      tree block = blocks[block_ix];
> > > +      unsigned decl_ix = 0;
> > > +      for (tree var = BLOCK_VARS (block); var; var = DECL_CHAIN (var))
> > > +	{
> > > +	  if (TREE_CODE (var) != TYPE_DECL)
> > > +	    continue;
> > > +	  if (var == decl)
> > > +	    {
> > > +	      key.index = (block_ix << 10) | decl_ix;
> > > +	      return;
> > > +	    }
> > > +	  ++decl_ix;
> > > +	}
> > > +      for (tree sub = BLOCK_SUBBLOCKS (block); sub; sub = BLOCK_CHAIN (sub))
> > > +	blocks.safe_push (sub);
> > > +      ++block_ix;
> > > +    }
> > > +
> > > +  /* Not-found value.  */
> > > +  key.index = 1023;
> > > +}
> > > +
> > > +/* Look up the local class corresponding at the position encoded by
> > > +   KEY within FN.  */
> > > +
> > > +tree
> > > +trees_in::key_local_class (const merge_key& key, tree fn)
> > > +{
> > > +  if (!DECL_INITIAL (fn))
> > > +    return NULL_TREE;
> > > +
> > > +  const unsigned block_pos = key.index >> 10;
> > > +  const unsigned decl_pos = key.index & 1023;
> > > +
> > > +  if (decl_pos == 1023)
> > > +    return NULL_TREE;
> > > +
> > > +  auto_vec<tree, 4> blocks;
> > > +  blocks.quick_push (DECL_INITIAL (fn));
> > > +  unsigned block_ix = 0;
> > > +  while (block_ix != blocks.length ())
> > > +    {
> > > +      tree block = blocks[block_ix];
> > > +      if (block_ix == block_pos)
> > > +	{
> > > +	  unsigned decl_ix = 0;
> > > +	  for (tree var = BLOCK_VARS (block); var; var = DECL_CHAIN (var))
> > > +	    {
> > > +	      if (TREE_CODE (var) != TYPE_DECL)
> > > +		continue;
> > > +	      if (decl_ix == decl_pos)
> > > +		return var;
> > > +	      ++decl_ix;
> > > +	    }
> > > +	  return NULL_TREE;
> > > +	}
> > > +      for (tree sub = BLOCK_SUBBLOCKS (block); sub; sub = BLOCK_CHAIN (sub))
> > > +	blocks.safe_push (sub);
> > > +      ++block_ix;
> > > +    }
> > > +
> > > +  return NULL_TREE;
> > > +}
> > > +
> > >  /* DEP is the depset of some decl we're streaming by value.  Determine
> > >     the merging behaviour.  */
> > >  
> > > @@ -10454,17 +10564,10 @@ trees_out::get_merge_kind (tree decl, depset *dep)
> > >  	    gcc_unreachable ();
> > >  
> > >  	  case FUNCTION_DECL:
> > > -	    // FIXME: This can occur for (a) voldemorty TYPE_DECLS
> > > -	    // (which are returned from a function), or (b)
> > > -	    // block-scope class definitions in template functions.
> > > -	    // These are as unique as the containing function.  While
> > > -	    // on read-back we can discover if the CTX was a
> > > -	    // duplicate, we don't have a mechanism to get from the
> > > -	    // existing CTX to the existing version of this decl.
> > >  	    gcc_checking_assert
> > >  	      (DECL_IMPLICIT_TYPEDEF_P (STRIP_TEMPLATE (decl)));
> > >  
> > > -	    mk = MK_unique;
> > > +	    mk = MK_local_class;
> > >  	    break;
> > >  
> > >  	  case RECORD_TYPE:
> > > @@ -10768,6 +10871,10 @@ trees_out::key_mergeable (int tag, merge_kind mk, tree decl, tree inner,
> > >  	  }
> > >  	  break;
> > >  
> > > +	case MK_local_class:
> > > +	  key_local_class (key, STRIP_TEMPLATE (decl), container);
> > > +	  break;
> > > +
> > >  	case MK_enum:
> > >  	  {
> > >  	    /* Anonymous enums are located by their first identifier,
> > > @@ -11117,11 +11224,10 @@ trees_in::key_mergeable (int tag, merge_kind mk, tree decl, tree inner,
> > >  	    break;
> > >  
> > >  	  case FUNCTION_DECL:
> > > -	    // FIXME: What about a voldemort? how do we find what it
> > > -	    // duplicates? Do we have to number vmorts relative to
> > > -	    // their containing function?  But how would that work
> > > -	    // when matching an in-TU declaration?
> > > -	    kind = "unique";
> > > +	    gcc_checking_assert (mk == MK_local_class);
> > > +	    existing = key_local_class (key, container);
> > > +	    if (existing && inner != decl)
> > > +	      existing = TYPE_TI_TEMPLATE (TREE_TYPE (existing));
> > >  	    break;
> > >  
> > >  	  case TYPE_DECL:
> > > @@ -11374,6 +11480,11 @@ trees_in::is_matching_decl (tree existing, tree decl, bool is_typedef)
> > >  	/* Just like duplicate_decls, presum the user knows what
> > >  	   they're doing in overriding a builtin.   */
> > >  	TREE_TYPE (existing) = TREE_TYPE (decl);
> > > +      else if (decl_function_context (decl))
> > > +	/* The type of a mergeable local entity (such as a function scope
> > > +	   capturing lambda's closure type fields) can depend on an
> > > +	   unmergeable local entity (such as a local variable), so type
> > > +	   equality isn't feasible in general for local entities.  */;
> > >        else
> > >  	{
> > >  	  // FIXME:QOI Might be template specialization from a module,
> > > diff --git a/gcc/testsuite/g++.dg/modules/xtreme-header-7_a.H b/gcc/testsuite/g++.dg/modules/xtreme-header-7_a.H
> > > new file mode 100644
> > > index 00000000000..bf7859fba99
> > > --- /dev/null
> > > +++ b/gcc/testsuite/g++.dg/modules/xtreme-header-7_a.H
> > > @@ -0,0 +1,4 @@
> > > +// { dg-additional-options -fmodule-header }
> > > +
> > > +// { dg-module-cmi {} }
> > > +#include "xtreme-header.h"
> > > diff --git a/gcc/testsuite/g++.dg/modules/xtreme-header-7_b.C b/gcc/testsuite/g++.dg/modules/xtreme-header-7_b.C
> > > new file mode 100644
> > > index 00000000000..03f3dc1bae6
> > > --- /dev/null
> > > +++ b/gcc/testsuite/g++.dg/modules/xtreme-header-7_b.C
> > > @@ -0,0 +1,6 @@
> > > +// A version of xtreme-header_{a.H,b.C} that doesn't pass
> > > +// -fno-module-lazy.
> > > +// { dg-additional-options -fmodules-ts }
> > > +
> > > +#include "xtreme-header.h"
> > > +import "xtreme-header-7_a.H";
> > > -- 
> > 
> > Consider the following minimal testcase added that verifies we now
> > properly merge local classes (and local enums):
> 
> Ping.  I opted to rename "local class" to "local type" throughout to
> capture that enums are included too.

Ping.

> 
> -- >8 --
> 
> Subject: [PATCH] c++/modules: local type merging [PR99426]
> 
> One known missing piece in the modules implementation is merging of a
> streamed-in local type (class or enum) with the corresponding in-TU
> version of the local type.  This missing piece turns out to cause a
> hard-to-reduce use-after-free GC issue due to the entity_ary not being
> marked as a GC root (deliberately), and manifests as a serialization
> error on stream-in as in PR99426 (see comment #6 for a reduction).  It's
> also reproducible on trunk when running the xtreme-header tests without
> -fno-module-lazy.
> 
> This patch makes us merge such local types according to their position
> within the containing function's definition, analogous to how we merge
> FIELD_DECLs of a class according to their index in the TYPE_FIELDS
> list.
> 
> 	PR c++/99426
> 
> gcc/cp/ChangeLog:
> 
> 	* module.cc (merge_kind::MK_local_type): New enumerator.
> 	(merge_kind_name): Update.
> 	(trees_out::chained_decls): Move BLOCK-specific handling
> 	of DECL_LOCAL_DECL_P decls to ...
> 	(trees_out::core_vals) <case BLOCK>: ... here.  Stream
> 	BLOCK_VARS manually.
> 	(trees_in::core_vals) <case BLOCK>: Stream BLOCK_VARS
> 	manually.  Handle deduplicated local types..
> 	(trees_out::key_local_type): Define.
> 	(trees_in::key_local_type): Define.
> 	(trees_out::get_merge_kind) <case FUNCTION_DECL>: Return
> 	MK_local_type for a local type.
> 	(trees_out::key_mergeable) <case FUNCTION_DECL>: Use
> 	key_local_type.
> 	(trees_in::key_mergeable) <case FUNCTION_DECL>: Likewise.
> 	(trees_in::is_matching_decl): Be flexible with type mismatches
> 	for local entities.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/modules/merge-17.h: New test.
> 	* g++.dg/modules/merge-17_a.H: New test.
> 	* g++.dg/modules/merge-17_b.C: New test.
> 	* g++.dg/modules/xtreme-header-7_a.H: New test.
> 	* g++.dg/modules/xtreme-header-7_b.C: New test.
> ---
>  gcc/cp/module.cc                              | 170 +++++++++++++++---
>  gcc/testsuite/g++.dg/modules/merge-17.h       |  28 +++
>  gcc/testsuite/g++.dg/modules/merge-17_a.H     |   3 +
>  gcc/testsuite/g++.dg/modules/merge-17_b.C     |   3 +
>  .../g++.dg/modules/xtreme-header-7_a.H        |   4 +
>  .../g++.dg/modules/xtreme-header-7_b.C        |   6 +
>  6 files changed, 186 insertions(+), 28 deletions(-)
>  create mode 100644 gcc/testsuite/g++.dg/modules/merge-17.h
>  create mode 100644 gcc/testsuite/g++.dg/modules/merge-17_a.H
>  create mode 100644 gcc/testsuite/g++.dg/modules/merge-17_b.C
>  create mode 100644 gcc/testsuite/g++.dg/modules/xtreme-header-7_a.H
>  create mode 100644 gcc/testsuite/g++.dg/modules/xtreme-header-7_b.C
> 
> diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
> index 80b63a70a62..d9e34e9a4b9 100644
> --- a/gcc/cp/module.cc
> +++ b/gcc/cp/module.cc
> @@ -2770,6 +2770,7 @@ enum merge_kind
>  
>    MK_enum,	/* Found by CTX, & 1stMemberNAME.  */
>    MK_keyed,     /* Found by key & index.  */
> +  MK_local_type, /* Found by CTX, index.  */
>  
>    MK_friend_spec,  /* Like named, but has a tmpl & args too.  */
>    MK_local_friend, /* Found by CTX, index.  */
> @@ -2798,7 +2799,7 @@ static char const *const merge_kind_name[MK_hwm] =
>      "unique", "named", "field", "vtable",	/* 0...3  */
>      "asbase", "partial", "enum", "attached",	/* 4...7  */
>  
> -    "friend spec", "local friend", NULL, NULL,  /* 8...11 */
> +    "local type", "friend spec", "local friend", NULL,  /* 8...11 */
>      NULL, NULL, NULL, NULL,
>  
>      "type spec", "type tmpl spec",	/* 16,17 type (template).  */
> @@ -2927,6 +2928,7 @@ public:
>    unsigned binfo_mergeable (tree *);
>  
>  private:
> +  tree key_local_type (const merge_key&, tree);
>    uintptr_t *find_duplicate (tree existing);
>    void register_duplicate (tree decl, tree existing);
>    /* Mark as an already diagnosed bad duplicate.  */
> @@ -3085,6 +3087,7 @@ public:
>    void binfo_mergeable (tree binfo);
>  
>  private:
> +  void key_local_type (merge_key&, tree, tree);
>    bool decl_node (tree, walk_kind ref);
>    void type_node (tree);
>    void tree_value (tree);
> @@ -4951,18 +4954,7 @@ void
>  trees_out::chained_decls (tree decls)
>  {
>    for (; decls; decls = DECL_CHAIN (decls))
> -    {
> -      if (VAR_OR_FUNCTION_DECL_P (decls)
> -	  && DECL_LOCAL_DECL_P (decls))
> -	{
> -	  /* Make sure this is the first encounter, and mark for
> -	     walk-by-value.  */
> -	  gcc_checking_assert (!TREE_VISITED (decls)
> -			       && !DECL_TEMPLATE_INFO (decls));
> -	  mark_by_value (decls);
> -	}
> -      tree_node (decls);
> -    }
> +    tree_node (decls);
>    tree_node (NULL_TREE);
>  }
>  
> @@ -6201,7 +6193,21 @@ trees_out::core_vals (tree t)
>  
>        /* DECL_LOCAL_DECL_P decls are first encountered here and
>           streamed by value.  */
> -      chained_decls (t->block.vars);
> +      for (tree decls = t->block.vars; decls; decls = DECL_CHAIN (decls))
> +	{
> +	  if (VAR_OR_FUNCTION_DECL_P (decls)
> +	      && DECL_LOCAL_DECL_P (decls))
> +	    {
> +	      /* Make sure this is the first encounter, and mark for
> +		 walk-by-value.  */
> +	      gcc_checking_assert (!TREE_VISITED (decls)
> +				   && !DECL_TEMPLATE_INFO (decls));
> +	      mark_by_value (decls);
> +	    }
> +	  tree_node (decls);
> +	}
> +      tree_node (NULL_TREE);
> +
>        /* nonlocalized_vars is a middle-end thing.  */
>        WT (t->block.subblocks);
>        WT (t->block.supercontext);
> @@ -6714,7 +6720,37 @@ trees_in::core_vals (tree t)
>      case BLOCK:
>        t->block.locus = state->read_location (*this);
>        t->block.end_locus = state->read_location (*this);
> -      t->block.vars = chained_decls ();
> +
> +      for (tree *chain = &t->block.vars;;)
> +	if (tree decl = tree_node ())
> +	  {
> +	    /* For a deduplicated local type or enumerator, chain the
> +	       duplicate decl instead of the canonical in-TU decl.  Seeing
> +	       a duplicate here means the containing function whose body
> +	       we're streaming in is a duplicate too, so we'll end up
> +	       discarding this BLOCK (and the rest of the duplicate function
> +	       body) anyway.  */
> +	    if (is_duplicate (decl))
> +	      decl = maybe_duplicate (decl);
> +	    else if (DECL_IMPLICIT_TYPEDEF_P (decl)
> +		     && TYPE_TEMPLATE_INFO (TREE_TYPE (decl)))
> +	      {
> +		tree tmpl = TYPE_TI_TEMPLATE (TREE_TYPE (decl));
> +		if (DECL_TEMPLATE_RESULT (tmpl) == decl && is_duplicate (tmpl))
> +		  decl = DECL_TEMPLATE_RESULT (maybe_duplicate (tmpl));
> +	      }
> +
> +	    if (!DECL_P (decl) || DECL_CHAIN (decl))
> +	      {
> +		set_overrun ();
> +		break;
> +	      }
> +	    *chain = decl;
> +	    chain = &DECL_CHAIN (decl);
> +	  }
> +	else
> +	  break;
> +
>        /* nonlocalized_vars is middle-end.  */
>        RT (t->block.subblocks);
>        RT (t->block.supercontext);
> @@ -10337,6 +10373,83 @@ trees_in::fn_parms_fini (int tag, tree fn, tree existing, bool is_defn)
>      }
>  }
>  
> +/* Encode into KEY the position of the local type (class or enum)
> +   declaration DECL within FN.  The position is encoded as the
> +   index of the innermost BLOCK (numbered in BFS order) along with
> +   the index within its BLOCK_VARS list.  */
> +
> +void
> +trees_out::key_local_type (merge_key& key, tree decl, tree fn)
> +{
> +  auto_vec<tree, 4> blocks;
> +  blocks.quick_push (DECL_INITIAL (fn));
> +  unsigned block_ix = 0;
> +  while (block_ix != blocks.length ())
> +    {
> +      tree block = blocks[block_ix];
> +      unsigned decl_ix = 0;
> +      for (tree var = BLOCK_VARS (block); var; var = DECL_CHAIN (var))
> +	{
> +	  if (TREE_CODE (var) != TYPE_DECL)
> +	    continue;
> +	  if (var == decl)
> +	    {
> +	      key.index = (block_ix << 10) | decl_ix;
> +	      return;
> +	    }
> +	  ++decl_ix;
> +	}
> +      for (tree sub = BLOCK_SUBBLOCKS (block); sub; sub = BLOCK_CHAIN (sub))
> +	blocks.safe_push (sub);
> +      ++block_ix;
> +    }
> +
> +  /* Not-found value.  */
> +  key.index = 1023;
> +}
> +
> +/* Look up the local type corresponding at the position encoded by
> +   KEY within FN.  */
> +
> +tree
> +trees_in::key_local_type (const merge_key& key, tree fn)
> +{
> +  if (!DECL_INITIAL (fn))
> +    return NULL_TREE;
> +
> +  const unsigned block_pos = key.index >> 10;
> +  const unsigned decl_pos = key.index & 1023;
> +
> +  if (decl_pos == 1023)
> +    return NULL_TREE;
> +
> +  auto_vec<tree, 4> blocks;
> +  blocks.quick_push (DECL_INITIAL (fn));
> +  unsigned block_ix = 0;
> +  while (block_ix != blocks.length ())
> +    {
> +      tree block = blocks[block_ix];
> +      if (block_ix == block_pos)
> +	{
> +	  unsigned decl_ix = 0;
> +	  for (tree var = BLOCK_VARS (block); var; var = DECL_CHAIN (var))
> +	    {
> +	      if (TREE_CODE (var) != TYPE_DECL)
> +		continue;
> +	      if (decl_ix == decl_pos)
> +		return var;
> +	      ++decl_ix;
> +	    }
> +	  return NULL_TREE;
> +	}
> +      for (tree sub = BLOCK_SUBBLOCKS (block); sub; sub = BLOCK_CHAIN (sub))
> +	blocks.safe_push (sub);
> +      ++block_ix;
> +    }
> +
> +  return NULL_TREE;
> +}
> +
>  /* DEP is the depset of some decl we're streaming by value.  Determine
>     the merging behaviour.  */
>  
> @@ -10456,17 +10569,10 @@ trees_out::get_merge_kind (tree decl, depset *dep)
>  	    gcc_unreachable ();
>  
>  	  case FUNCTION_DECL:
> -	    // FIXME: This can occur for (a) voldemorty TYPE_DECLS
> -	    // (which are returned from a function), or (b)
> -	    // block-scope class definitions in template functions.
> -	    // These are as unique as the containing function.  While
> -	    // on read-back we can discover if the CTX was a
> -	    // duplicate, we don't have a mechanism to get from the
> -	    // existing CTX to the existing version of this decl.
>  	    gcc_checking_assert
>  	      (DECL_IMPLICIT_TYPEDEF_P (STRIP_TEMPLATE (decl)));
>  
> -	    mk = MK_unique;
> +	    mk = MK_local_type;
>  	    break;
>  
>  	  case RECORD_TYPE:
> @@ -10775,6 +10881,10 @@ trees_out::key_mergeable (int tag, merge_kind mk, tree decl, tree inner,
>  	  }
>  	  break;
>  
> +	case MK_local_type:
> +	  key_local_type (key, STRIP_TEMPLATE (decl), container);
> +	  break;
> +
>  	case MK_enum:
>  	  {
>  	    /* Anonymous enums are located by their first identifier,
> @@ -11131,11 +11241,10 @@ trees_in::key_mergeable (int tag, merge_kind mk, tree decl, tree inner,
>  	    break;
>  
>  	  case FUNCTION_DECL:
> -	    // FIXME: What about a voldemort? how do we find what it
> -	    // duplicates? Do we have to number vmorts relative to
> -	    // their containing function?  But how would that work
> -	    // when matching an in-TU declaration?
> -	    kind = "unique";
> +	    gcc_checking_assert (mk == MK_local_type);
> +	    existing = key_local_type (key, container);
> +	    if (existing && inner != decl)
> +	      existing = TYPE_TI_TEMPLATE (TREE_TYPE (existing));
>  	    break;
>  
>  	  case TYPE_DECL:
> @@ -11388,6 +11497,11 @@ trees_in::is_matching_decl (tree existing, tree decl, bool is_typedef)
>  	/* Just like duplicate_decls, presum the user knows what
>  	   they're doing in overriding a builtin.   */
>  	TREE_TYPE (existing) = TREE_TYPE (decl);
> +      else if (decl_function_context (decl))
> +	/* The type of a mergeable local entity (such as a function scope
> +	   capturing lambda's closure type fields) can depend on an
> +	   unmergeable local entity (such as a local variable), so type
> +	   equality isn't feasible in general for local entities.  */;
>        else
>  	{
>  	  // FIXME:QOI Might be template specialization from a module,
> diff --git a/gcc/testsuite/g++.dg/modules/merge-17.h b/gcc/testsuite/g++.dg/modules/merge-17.h
> new file mode 100644
> index 00000000000..246ccd8011d
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/merge-17.h
> @@ -0,0 +1,28 @@
> +// PR c++/99426
> +
> +inline auto f() {
> +  struct A { int m = 42; };
> +  return A{};
> +}
> +
> +template<class T>
> +inline auto ft() {
> +  decltype(+T()) x;
> +  return [&x] { };
> +}
> +
> +inline auto g() {
> +  enum E { e };
> +  return e;
> +}
> +
> +template<class T>
> +inline auto gt() {
> +  enum E : T { e };
> +  return e;
> +}
> +
> +using ty1 = decltype(f());
> +using ty2 = decltype(ft<int>());
> +using ty3 = decltype(g());
> +using ty4 = decltype(gt<int>());
> diff --git a/gcc/testsuite/g++.dg/modules/merge-17_a.H b/gcc/testsuite/g++.dg/modules/merge-17_a.H
> new file mode 100644
> index 00000000000..0440cd765e9
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/merge-17_a.H
> @@ -0,0 +1,3 @@
> +// { dg-additional-options "-fmodule-header" }
> +// { dg-module-cmi {} }
> +#include "merge-17.h"
> diff --git a/gcc/testsuite/g++.dg/modules/merge-17_b.C b/gcc/testsuite/g++.dg/modules/merge-17_b.C
> new file mode 100644
> index 00000000000..4315b99f172
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/merge-17_b.C
> @@ -0,0 +1,3 @@
> +// { dg-additional-options "-fmodules-ts -fno-module-lazy" }
> +#include "merge-17.h"
> +import "merge-17_a.H";
> diff --git a/gcc/testsuite/g++.dg/modules/xtreme-header-7_a.H b/gcc/testsuite/g++.dg/modules/xtreme-header-7_a.H
> new file mode 100644
> index 00000000000..bf7859fba99
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/xtreme-header-7_a.H
> @@ -0,0 +1,4 @@
> +// { dg-additional-options -fmodule-header }
> +
> +// { dg-module-cmi {} }
> +#include "xtreme-header.h"
> diff --git a/gcc/testsuite/g++.dg/modules/xtreme-header-7_b.C b/gcc/testsuite/g++.dg/modules/xtreme-header-7_b.C
> new file mode 100644
> index 00000000000..201d092e883
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/xtreme-header-7_b.C
> @@ -0,0 +1,6 @@
> +// A version of xtreme-header_{a.H,b.C} that doesn't use
> +// -fno-module-lazy.
> +// { dg-additional-options -fmodules-ts }
> +
> +#include "xtreme-header.h"
> +import "xtreme-header-7_a.H";
> -- 
> 2.44.0.84.gb387623c12
> 
>