[clang-format] Allow default values for template parameters in lambda

[rymiel]

Previously, upon encountering an equals sign while parsing a lambda in the UnwrappedLineParser, it would fall through and fail. This caused any lambda template with a default argument for a template parameter to be annotated as an ArraySubscriptLSquare.

This patch allows equals signs in the UnwrappedLineParser if we're currently in a template parameter list. This resolved a FIXME that was in the lambda parsing function.

This patch seems deceptively easy, it's likely it doesn't solve the FIXME entirely, or causes other issues (the FIXME itself mentions something about Objective-C, which I cannot comment about). However this patch is sufficient to fix the below issue.

Fixes #68913

[llvmbot]

@llvm/pr-subscribers-clang-format

Author: Emilia Kond (rymiel)

Changes

Previously, upon encountering an equals sign while parsing a lambda in the UnwrappedLineParser, it would fall through and fail. This caused any lambda template with a default argument for a template parameter to be annotated as an ArraySubscriptLSquare.

This patch allows equals signs in the UnwrappedLineParser if we're currently in a template parameter list. This resolved a FIXME that was in the lambda parsing function.

This patch seems deceptively easy, it's likely it doesn't solve the FIXME entirely, or causes other issues (the FIXME itself mentions something about Objective-C, which I cannot comment about). However this patch is sufficient to fix the below issue.

Fixes #68913

---

Full diff: https://github.com/llvm/llvm-project/pull/69052.diff

2 Files Affected:

• (modified) clang/lib/Format/UnwrappedLineParser.cpp (+6-3)
• (modified) clang/unittests/Format/TokenAnnotatorTest.cpp (+38)

diff --git a/clang/lib/Format/UnwrappedLineParser.cpp b/clang/lib/Format/UnwrappedLineParser.cpp
index 82a812fc8bcc610..45a3fdbb5ba12e0 100644
--- a/clang/lib/Format/UnwrappedLineParser.cpp
+++ b/clang/lib/Format/UnwrappedLineParser.cpp
@@ -2226,9 +2226,6 @@ bool UnwrappedLineParser::tryToParseLambda() {
     // followed by an `a->b` expression, such as:
     // ([obj func:arg] + a->b)
     // Otherwise the code below would parse as a lambda.
-    //
-    // FIXME: This heuristic is incorrect for C++20 generic lambdas with
-    // explicit template lists: []<bool b = true && false>(U &&u){}
     case tok::plus:
     case tok::minus:
     case tok::exclaim:
@@ -2268,6 +2265,12 @@ bool UnwrappedLineParser::tryToParseLambda() {
       parseRequiresClause(RequiresToken);
       break;
     }
+    case tok::equal:
+      if (InTemplateParameterList) {
+        nextToken();
+        break;
+      }
+      return true;
     default:
       return true;
     }
diff --git a/clang/unittests/Format/TokenAnnotatorTest.cpp b/clang/unittests/Format/TokenAnnotatorTest.cpp
index b6d4cf166de0281..f4f382cfe8f2494 100644
--- a/clang/unittests/Format/TokenAnnotatorTest.cpp
+++ b/clang/unittests/Format/TokenAnnotatorTest.cpp
@@ -1620,6 +1620,44 @@ TEST_F(TokenAnnotatorTest, UnderstandsLambdas) {
   EXPECT_TOKEN(Tokens[15], tok::kw_requires, TT_RequiresClause);
   EXPECT_TRUE(Tokens[19]->ClosesRequiresClause);
   EXPECT_TOKEN(Tokens[20], tok::l_brace, TT_LambdaLBrace);
+
+  Tokens = annotate("[] <typename T = int> (T t) {}");
+  ASSERT_EQ(Tokens.size(), 15u) << Tokens;
+  EXPECT_TOKEN(Tokens[0], tok::l_square, TT_LambdaLSquare);
+  EXPECT_TOKEN(Tokens[2], tok::less, TT_TemplateOpener);
+  EXPECT_TOKEN(Tokens[7], tok::greater, TT_TemplateCloser);
+  EXPECT_TOKEN(Tokens[12], tok::l_brace, TT_LambdaLBrace);
+
+  Tokens = annotate("[] <int I = 0> (T t) {}");
+  ASSERT_EQ(Tokens.size(), 15u) << Tokens;
+  EXPECT_TOKEN(Tokens[0], tok::l_square, TT_LambdaLSquare);
+  EXPECT_TOKEN(Tokens[2], tok::less, TT_TemplateOpener);
+  EXPECT_TOKEN(Tokens[7], tok::greater, TT_TemplateCloser);
+  EXPECT_TOKEN(Tokens[12], tok::l_brace, TT_LambdaLBrace);
+
+  Tokens = annotate("[] <bool b = false> (T t) {}");
+  ASSERT_EQ(Tokens.size(), 15u) << Tokens;
+  EXPECT_TOKEN(Tokens[0], tok::l_square, TT_LambdaLSquare);
+  EXPECT_TOKEN(Tokens[2], tok::less, TT_TemplateOpener);
+  EXPECT_TOKEN(Tokens[7], tok::greater, TT_TemplateCloser);
+  EXPECT_TOKEN(Tokens[12], tok::l_brace, TT_LambdaLBrace);
+
+  Tokens = annotate("[] <bool b = true && false> (T&& t) {}");
+  ASSERT_EQ(Tokens.size(), 18u) << Tokens;
+  EXPECT_TOKEN(Tokens[0], tok::l_square, TT_LambdaLSquare);
+  EXPECT_TOKEN(Tokens[2], tok::less, TT_TemplateOpener);
+  EXPECT_TOKEN(Tokens[7], tok::ampamp, TT_BinaryOperator);
+  EXPECT_TOKEN(Tokens[9], tok::greater, TT_TemplateCloser);
+  EXPECT_TOKEN(Tokens[12], tok::ampamp, TT_PointerOrReference);
+  EXPECT_TOKEN(Tokens[15], tok::l_brace, TT_LambdaLBrace);
+
+  Tokens = annotate("[] <typename T = int> requires Foo<T> (T t) {}");
+  ASSERT_EQ(Tokens.size(), 20u) << Tokens;
+  EXPECT_TOKEN(Tokens[0], tok::l_square, TT_LambdaLSquare);
+  EXPECT_TOKEN(Tokens[2], tok::less, TT_TemplateOpener);
+  EXPECT_TOKEN(Tokens[7], tok::greater, TT_TemplateCloser);
+  EXPECT_TOKEN(Tokens[8], tok::kw_requires, TT_RequiresClause);
+  EXPECT_TOKEN(Tokens[17], tok::l_brace, TT_LambdaLBrace);
 }
 
 TEST_F(TokenAnnotatorTest, UnderstandsFunctionAnnotations) {

[owenca]

@rymiel, thanks for the co-authorship, but I really didn't deserve it here!