[Clang] Skip past code generation for unevaluated lambdas

As suggested by Corentin, this is taken from llvm#122423 (comment)

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -966,6 +966,7 @@ Bug Fixes to C++ Support
   constraints are applied. (#GH122134)
 - Fixed canonicalization of pack indexing types - Clang did not always recognized identical pack indexing. (#GH123033)
 - Fixed a nested lambda substitution issue for constraint evaluation. (#GH123441)
+- Fixed some crashes involving unevaluated lambdas during code generation. (#GH82926)
 
 
 Bug Fixes to AST Handling

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -5297,8 +5297,24 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
     savedContext.pop();
   }
 
-  DeclGroupRef DG(Function);
-  Consumer.HandleTopLevelDecl(DG);
+  // We never need to emit the code for a lambda in unevaluated context.
+  // We also can't mangle a lambda in the require clause of a function template
+  // during constraint checking as the MSI ABI would need to mangle the (not yet
+  // specialized) enclosing declaration
+  // FIXME: Should we try to skip this for non-lambda functions too?
+  bool ShouldSkipCG = [&] {
+    auto *RD = dyn_cast<CXXRecordDecl>(Function->getParent());
+    if (!RD || !RD->isLambda())
+      return false;
+
+    return llvm::any_of(ExprEvalContexts, [](auto &Context) {
+      return Context.isUnevaluated() || Context.isImmediateFunctionContext();
+    });
+  }();
+  if (!ShouldSkipCG) {
+    DeclGroupRef DG(Function);
+    Consumer.HandleTopLevelDecl(DG);
+  }
 
   // This class may have local implicit instantiations that need to be
   // instantiation within this scope.

clang/test/CodeGenCXX/unevaluated-lambdas.cpp

@@ -0,0 +1,37 @@
+// RUN: %clang_cc1 -std=c++2b -emit-llvm %s -o - | FileCheck %s -dump-input=always
+
+namespace GH82926 {
+
+template<class Tp>
+using simd_vector = Tp;
+
+template<class VecT>
+using simd_vector_underlying_type_t
+    = decltype([]<class Tp>(simd_vector<Tp>) {}(VecT {}), 1);
+
+template<class VecT>
+void temp() {
+  // CHECK: call void @_ZZN7GH829264tempIcEEvvENKUliE_clEi
+  [](simd_vector_underlying_type_t<VecT>) {}(42);
+}
+
+void call() {
+  temp<simd_vector<char>>(); 
+}
+
+} // namespace GH82926
+
+namespace GH111058 {
+
+// FIXME: This still crashes because the unevaluated lambda as an argument
+// is also supposed to skipping codegen in Sema::InstantiateFunctionDefinition().
+// auto eat(auto) {}
+
+void foo() {
+	// [] -> decltype(eat([] {})) {};
+
+  // CHECK: call void @"_ZZN8GH1110583fooEvENK3$_0clEv"
+  [] -> decltype([](auto){}(1)) {}();
+}
+
+} // namespace GH111058