[clang] Suppress printing lambda body for constexpr diagnostics (PR #185800)

[Sherman Pay via cfe-commits]

https://github.com/shermpay updated https://github.com/llvm/llvm-project/pull/185800

>From 274e898ca4abfdb8bef6df4bc093cb1617e91e3a Mon Sep 17 00:00:00 2001
From: Sherman Pay <shermanpay at google.com>
Date: Tue, 10 Mar 2026 20:54:59 -0700
Subject: [PATCH] Suppress printing lambda body for constexpr diagnostics

closes: 125914

Introduce `SupressLambdaBody` `PrintingPolicy` that is used only for
constexpr diagnostics. This ensures `--print-ast` still works the same.

Add two tests:
1. To ast-printer-lambda to ensure `--print-ast` works the same.
2. Ensure lambda body is not printed for constexpr diagnostics.
---
 clang/include/clang/AST/PrettyPrinter.h        |  4 ++++
 clang/lib/AST/ExprConstant.cpp                 | 12 +++++++-----
 clang/lib/AST/StmtPrinter.cpp                  |  2 +-
 clang/test/AST/ast-printer-lambda.cpp          | 10 +++++++++-
 clang/test/AST/constexpr-lambda-diagnostic.cpp |  8 ++++++++
 5 files changed, 29 insertions(+), 7 deletions(-)
 create mode 100644 clang/test/AST/constexpr-lambda-diagnostic.cpp

diff --git a/clang/include/clang/AST/PrettyPrinter.h b/clang/include/clang/AST/PrettyPrinter.h
index a937d020b7277..534424fac606f 100644
--- a/clang/include/clang/AST/PrettyPrinter.h
+++ b/clang/include/clang/AST/PrettyPrinter.h
@@ -371,6 +371,10 @@ struct PrintingPolicy {
   LLVM_PREFERRED_TYPE(bool)
   unsigned SuppressDeclAttributes : 1;
 
+  /// Whether to suppress printing the body of a lambda.
+  LLVM_PREFERRED_TYPE(bool)
+  unsigned SuppressLambdaBody : 1;
+
   /// Callbacks to use to allow the behavior of printing to be customized.
   const PrintingCallbacks *Callbacks = nullptr;
 };
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 429fef0a1afa8..76cd7dfbc4542 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -1862,19 +1862,22 @@ APValue *EvalInfo::createHeapAlloc(const Expr *E, QualType T, LValue &LV) {
 void CallStackFrame::describe(raw_ostream &Out) const {
   bool IsMemberCall = false;
   bool ExplicitInstanceParam = false;
+  auto PrintingPolicy = Info.Ctx.getPrintingPolicy();
+  PrintingPolicy.SuppressLambdaBody = true;
+
   if (const auto *MD = dyn_cast<CXXMethodDecl>(Callee)) {
     IsMemberCall = !isa<CXXConstructorDecl>(MD) && !MD->isStatic();
     ExplicitInstanceParam = MD->isExplicitObjectMemberFunction();
   }
 
   if (!IsMemberCall)
-    Callee->getNameForDiagnostic(Out, Info.Ctx.getPrintingPolicy(),
+    Callee->getNameForDiagnostic(Out, PrintingPolicy,
                                  /*Qualified=*/false);
 
   if (This && IsMemberCall) {
     if (const auto *MCE = dyn_cast_if_present<CXXMemberCallExpr>(CallExpr)) {
       const Expr *Object = MCE->getImplicitObjectArgument();
-      Object->printPretty(Out, /*Helper=*/nullptr, Info.Ctx.getPrintingPolicy(),
+      Object->printPretty(Out, /*Helper=*/nullptr, PrintingPolicy,
                           /*Indentation=*/0);
       if (Object->getType()->isPointerType())
           Out << "->";
@@ -1882,8 +1885,7 @@ void CallStackFrame::describe(raw_ostream &Out) const {
           Out << ".";
     } else if (const auto *OCE =
                    dyn_cast_if_present<CXXOperatorCallExpr>(CallExpr)) {
-      OCE->getArg(0)->printPretty(Out, /*Helper=*/nullptr,
-                                  Info.Ctx.getPrintingPolicy(),
+      OCE->getArg(0)->printPretty(Out, /*Helper=*/nullptr, PrintingPolicy,
                                   /*Indentation=*/0);
       Out << ".";
     } else {
@@ -1894,7 +1896,7 @@ void CallStackFrame::describe(raw_ostream &Out) const {
           Info.Ctx.getLValueReferenceType(This->Designator.MostDerivedType));
       Out << ".";
     }
-    Callee->getNameForDiagnostic(Out, Info.Ctx.getPrintingPolicy(),
+    Callee->getNameForDiagnostic(Out, PrintingPolicy,
                                  /*Qualified=*/false);
   }
 
diff --git a/clang/lib/AST/StmtPrinter.cpp b/clang/lib/AST/StmtPrinter.cpp
index 4d364fdcd5502..86adf6626ed15 100644
--- a/clang/lib/AST/StmtPrinter.cpp
+++ b/clang/lib/AST/StmtPrinter.cpp
@@ -2463,7 +2463,7 @@ void StmtPrinter::VisitLambdaExpr(LambdaExpr *Node) {
 
   // Print the body.
   OS << ' ';
-  if (Policy.TerseOutput)
+  if (Policy.TerseOutput || Policy.SuppressLambdaBody)
     OS << "{}";
   else
     PrintRawCompoundStmt(Node->getCompoundStmtBody());
diff --git a/clang/test/AST/ast-printer-lambda.cpp b/clang/test/AST/ast-printer-lambda.cpp
index 08f1ff555b0b1..015aa7e791205 100644
--- a/clang/test/AST/ast-printer-lambda.cpp
+++ b/clang/test/AST/ast-printer-lambda.cpp
@@ -51,6 +51,14 @@ void test1(int i, T... t) {
   auto lambda = [k{t...}] {};
   //CHECK: [k{t...}] {
 }
+{
+  auto lambda = [i]{
+    (void)i;
+  };
+  //CHECK: [i] {
+  //CHECK-NEXT: (void)i;
+  //CHECK-NEXT: };
+}
 }
 
-};
\ No newline at end of file
+};
diff --git a/clang/test/AST/constexpr-lambda-diagnostic.cpp b/clang/test/AST/constexpr-lambda-diagnostic.cpp
new file mode 100644
index 0000000000000..1edc873718967
--- /dev/null
+++ b/clang/test/AST/constexpr-lambda-diagnostic.cpp
@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -fsyntax-only -std=c++17 -verify=expected %s 
+constexpr void undefined();  // expected-note {{declared here}}
+static constexpr int r = [] { // expected-error {{constexpr variable 'r' must be initialized by a constant expression}} \
+                                 expected-note {{in call to '[] {}.operator()()'}}}
+  undefined();  // expected-note {{undefined function 'undefined' cannot be used in a constant expression}}
+  return 0;
+ }();
+