[clang][Sema] fix incorrect ambiguous function call which use designated-initializer as template argument

[Backl1ght]

Fixes #166784

After performing initialization of a B using {.b = 42} successfully, we will set FieldDecl of designator b to B::b, and then use the saved FieldDecl in later actions. When it comes to performing initialization of a A using {.b = 42}, we do not clear the saved FieldDecl, use it without any check, which is wrong as there is no B::b inside A.

As a result, we perform the initialization successfully, this makes both f<A> and f<B> viable and finally makes the call to f<{.b = 42}> ambiguous.

This pr add a check to make sure we use correct FieldDecl of designators. I also tried some other methods, but all of them do not work:

• I tried clearing saved FieldDecl after we finish initialization, but I can not find a proper place.
• I tried looking up FieldDecl of designators each time, but anonymous structs/unions set FieldDecl in other place and require us to use the saved FieldDecl.

llvm-project/clang/lib/Sema/SemaInit.cpp

Lines 2939 to 2957 in 545c302

	FieldDecl *KnownField = D->getFieldDecl();
	if (!KnownField) {
	const IdentifierInfo *FieldName = D->getFieldName();
	ValueDecl *VD = SemaRef.tryLookupUnambiguousFieldDecl(RD, FieldName);
	if (auto *FD = dyn_cast_if_present<FieldDecl>(VD)) {
	KnownField = FD;
	} else if (auto *IFD = dyn_cast_if_present<IndirectFieldDecl>(VD)) {
	// In verify mode, don't modify the original.
	if (VerifyOnly)
	DIE = CloneDesignatedInitExpr(SemaRef, DIE);
	ExpandAnonymousFieldDesignator(SemaRef, DIE, DesigIdx, IFD);
	D = DIE->getDesignator(DesigIdx);
	KnownField = cast<FieldDecl>(*IFD->chain_begin());
	}
	if (!KnownField) {
	if (VerifyOnly) {
	++Index;
	return true; // No typo correction when just trying this out.
	}

[llvmbot]

@llvm/pr-subscribers-clang

Author: Zhikai Zeng (Backl1ght)

Changes

Fixes #166784

The cause is that we will cache KnownField after check of B, and use the cache incorrectly when performing check of type A. This pr add a check to make sure we use correct cache.

llvm-project/clang/lib/Sema/SemaInit.cpp

Lines 2939 to 2957 in 545c302

	FieldDecl *KnownField = D->getFieldDecl();
	if (!KnownField) {
	const IdentifierInfo *FieldName = D->getFieldName();
	ValueDecl *VD = SemaRef.tryLookupUnambiguousFieldDecl(RD, FieldName);
	if (auto *FD = dyn_cast_if_present<FieldDecl>(VD)) {
	KnownField = FD;
	} else if (auto *IFD = dyn_cast_if_present<IndirectFieldDecl>(VD)) {
	// In verify mode, don't modify the original.
	if (VerifyOnly)
	DIE = CloneDesignatedInitExpr(SemaRef, DIE);
	ExpandAnonymousFieldDesignator(SemaRef, DIE, DesigIdx, IFD);
	D = DIE->getDesignator(DesigIdx);
	KnownField = cast<FieldDecl>(*IFD->chain_begin());
	}
	if (!KnownField) {
	if (VerifyOnly) {
	++Index;
	return true; // No typo correction when just trying this out.
	}

---

Full diff: https://github.com/llvm/llvm-project/pull/167159.diff

4 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+2)
• (modified) clang/lib/Sema/SemaInit.cpp (+2)
• (modified) clang/lib/Sema/SemaTemplateInstantiateDecl.cpp (+8-1)
• (modified) clang/test/SemaTemplate/temp_arg_nontype_cxx2c.cpp (+23)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 6b396e7ba63f3..869223512ab29 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -459,6 +459,8 @@ Bug Fixes in This Version
 - Fixed a crash triggered by unterminated ``__has_embed``. (#GH162953)
 - Accept empty enumerations in MSVC-compatible C mode. (#GH114402)
 - Fixed false-positive shadow diagnostics for lambdas in explicit object member functions. (#GH163731)
+- Fixed a incorrect diagnostic for ambiguous function call that use a 
+  designated-initializer as template argument. (#GH166784)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index cc6ddf568d346..58c1c01c9b3ba 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -2937,6 +2937,8 @@ InitListChecker::CheckDesignatedInitializer(const InitializedEntity &Entity,
     }
 
     FieldDecl *KnownField = D->getFieldDecl();
+    if (KnownField && KnownField->getParent() != RD)
+      KnownField = nullptr;
     if (!KnownField) {
       const IdentifierInfo *FieldName = D->getFieldName();
       ValueDecl *VD = SemaRef.tryLookupUnambiguousFieldDecl(RD, FieldName);
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 4d58f00168298..2d3d8ef5279ce 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -7060,8 +7060,15 @@ NamedDecl *Sema::FindInstantiatedDecl(SourceLocation Loc, NamedDecl *D,
     // anonymous unions in class templates).
   }
 
-  if (!ParentDependsOnArgs)
+  if (!ParentDependsOnArgs) {
+    if (auto Found =
+            CurrentInstantiationScope
+                ? CurrentInstantiationScope->getInstantiationOfIfExists(D)
+                : nullptr) {
+        return cast<NamedDecl>(Found->dyn_cast<Decl *>());
+    }
     return D;
+  }
 
   ParentDC = FindInstantiatedContext(Loc, ParentDC, TemplateArgs);
   if (!ParentDC)
diff --git a/clang/test/SemaTemplate/temp_arg_nontype_cxx2c.cpp b/clang/test/SemaTemplate/temp_arg_nontype_cxx2c.cpp
index c4ac36e263bc8..613956324cb35 100644
--- a/clang/test/SemaTemplate/temp_arg_nontype_cxx2c.cpp
+++ b/clang/test/SemaTemplate/temp_arg_nontype_cxx2c.cpp
@@ -134,3 +134,26 @@ namespace error_on_type_instantiation {
   template void g<int>();
   // expected-note@-1 {{in instantiation of function template specialization}}
 }
+
+namespace GH166784 {
+
+struct A {
+  int a;
+};
+struct B {
+  int b;
+};
+template <A a> void f() {
+  static_assert(a.a == 42);
+}
+template <B b> void f() {
+  static_assert(b.b == 42);
+}
+
+using T1 = decltype(f<{.a = 42}>());
+using T2 = decltype(f<A{.a = 42}>());
+
+using T3 = decltype(f<{.b = 42}>());
+using T4 = decltype(f<B{.b = 42}>());
+
+} // namespace GH166784

[zyn0217]

Suggested change

	- Fixed an incorrect diagnostic for ambiguous function call that use a
	designated-initializer as template argument. (#GH166784)
	- Fixed an incorrect diagnostic for ambiguous function call that uses a
	designated-initializer as template argument. (#GH166784)

[Backl1ght]

done.

[zyn0217]

We need some comments/explanation for this workaround.

If this is related to cache, is it possible for us to fix that instead?

[Backl1ght]

We need some comments/explanation for this workaround.

I have update a more detailed pr description.

If this is related to cache, is it possible for us to fix that instead?

No I think, the saved FieldDecl will be used in later actions while anonymous structs/unions require us to use saved FieldDecl. Another possible way is clear the saved FieldDecl somewhere, but I can not find a proper place.