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

…#69052)

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

---------

Co-authored-by: Owen Pan <owenpiano@gmail.com>

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 +2265,11 @@ bool UnwrappedLineParser::tryToParseLambda() {
       parseRequiresClause(RequiresToken);
       break;
     }
+    case tok::equal:
+      if (!InTemplateParameterList)
+        return true;
+      nextToken();
+      break;
     default:
       return true;
     }

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) {