From f3f8fe29e67cd03d63de9bd6e8400c915c361547 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?fn=20=E2=8C=83=20=E2=8C=A5?=
 <70830482+FnControlOption@users.noreply.github.com>
Date: Tue, 16 Jul 2024 14:03:41 -0700
Subject: [PATCH 1/8] Add `Crystal::Call#dot_location`

---
 spec/compiler/parser/parser_spec.cr   | 24 ++++++++++++++++++++++++
 src/compiler/crystal/syntax/ast.cr    |  1 +
 src/compiler/crystal/syntax/parser.cr |  5 +++++
 3 files changed, 30 insertions(+)

diff --git a/spec/compiler/parser/parser_spec.cr b/spec/compiler/parser/parser_spec.cr
index 22e9c5feb385..1dc65f7e0be5 100644
--- a/spec/compiler/parser/parser_spec.cr
+++ b/spec/compiler/parser/parser_spec.cr
@@ -2821,5 +2821,29 @@ module Crystal
       node = Parser.parse(source).as(Annotation).path
       node_source(source, node).should eq("::Foo")
     end
+
+    it "sets correct location of call dot" do
+      parser = Parser.new("a.b")
+      node = parser.parse.as(Call)
+      dot_location = node.dot_location.not_nil!
+      dot_location.line_number.should eq(1)
+      dot_location.column_number.should eq(2)
+    end
+
+    it "sets correct location of call dot in assignment" do
+      parser = Parser.new("a.b = c")
+      node = parser.parse.as(Call)
+      dot_location = node.dot_location.not_nil!
+      dot_location.line_number.should eq(1)
+      dot_location.column_number.should eq(2)
+    end
+
+    it "sets correct location of call dot in operator assignment" do
+      parser = Parser.new("a.b += c")
+      node = parser.parse.as(OpAssign).target.as(Call)
+      dot_location = node.dot_location.not_nil!
+      dot_location.line_number.should eq(1)
+      dot_location.column_number.should eq(2)
+    end
   end
 end
diff --git a/src/compiler/crystal/syntax/ast.cr b/src/compiler/crystal/syntax/ast.cr
index f6d314371034..9641da81f7b6 100644
--- a/src/compiler/crystal/syntax/ast.cr
+++ b/src/compiler/crystal/syntax/ast.cr
@@ -647,6 +647,7 @@ module Crystal
     property block : Block?
     property block_arg : ASTNode?
     property named_args : Array(NamedArgument)?
+    property dot_location : Location?
     property name_location : Location?
     @name_size = -1
     property doc : String?
diff --git a/src/compiler/crystal/syntax/parser.cr b/src/compiler/crystal/syntax/parser.cr
index bd7c67a975b8..2c2e4f0e6383 100644
--- a/src/compiler/crystal/syntax/parser.cr
+++ b/src/compiler/crystal/syntax/parser.cr
@@ -678,6 +678,8 @@ module Crystal
             break
           end
         when .op_period?
+          dot_location = @token.location
+
           check_void_value atomic, location
 
           @wants_regex = false
@@ -761,6 +763,7 @@ module Crystal
               end
 
               atomic = Call.new(atomic, "#{name}=", arg).at(location).at_end(end_location)
+              atomic.dot_location = dot_location
               atomic.name_location = name_location
               next
             when .assignment_operator?
@@ -769,6 +772,7 @@ module Crystal
               next_token_skip_space_or_newline
               value = parse_op_assign
               call = Call.new(atomic, name).at(location)
+              call.dot_location = dot_location
               call.name_location = name_location
               atomic = OpAssign.new(call, method, value).at(location)
               atomic.name_location = op_name_location
@@ -790,6 +794,7 @@ module Crystal
             block = parse_block(block, stop_on_do: @stop_on_do)
             atomic = Call.new atomic, name, (args || [] of ASTNode), block, block_arg, named_args
             atomic.has_parentheses = has_parentheses
+            atomic.dot_location = dot_location
             atomic.name_location = name_location
             atomic.end_location = block.try(&.end_location) || call_args.try(&.end_location) || end_location
             atomic.at(location)

From f05e9b6567921bce0d44484aa51079d84de6cced Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?fn=20=E2=8C=83=20=E2=8C=A5?=
 <70830482+FnControlOption@users.noreply.github.com>
Date: Tue, 16 Jul 2024 14:12:34 -0700
Subject: [PATCH 2/8] Disallow assignments like `a.[] 0 = 1`

---
 spec/compiler/parser/parser_spec.cr   | 3 +++
 spec/support/syntax.cr                | 4 ++--
 src/compiler/crystal/syntax/parser.cr | 4 +++-
 3 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/spec/compiler/parser/parser_spec.cr b/spec/compiler/parser/parser_spec.cr
index 1dc65f7e0be5..a67de2b76188 100644
--- a/spec/compiler/parser/parser_spec.cr
+++ b/spec/compiler/parser/parser_spec.cr
@@ -276,6 +276,9 @@ module Crystal
     assert_syntax_error "a.b() += 1"
     assert_syntax_error "a.[]() += 1"
 
+    assert_syntax_error "a.[] 0 = 1"
+    assert_syntax_error "a.[] 0 += 1"
+
     it_parses "def foo\n1\nend", Def.new("foo", body: 1.int32)
     it_parses "def downto(n)\n1\nend", Def.new("downto", ["n".arg], 1.int32)
     it_parses "def foo ; 1 ; end", Def.new("foo", body: 1.int32)
diff --git a/spec/support/syntax.cr b/spec/support/syntax.cr
index e1fd8f43d951..a6fe6286d11b 100644
--- a/spec/support/syntax.cr
+++ b/spec/support/syntax.cr
@@ -133,8 +133,8 @@ class Crystal::ASTNode
   end
 end
 
-def assert_syntax_error(str, message = nil, line = nil, column = nil, metafile = __FILE__, metaline = __LINE__, metaendline = __END_LINE__)
-  it "says syntax error on #{str.inspect}", metafile, metaline, metaendline do
+def assert_syntax_error(str, message = nil, line = nil, column = nil, metafile = __FILE__, metaline = __LINE__, metaendline = __END_LINE__, *, focus : Bool = false)
+  it "says syntax error on #{str.inspect}", metafile, metaline, metaendline, focus: focus do
     begin
       parse str
       fail "Expected SyntaxException to be raised", metafile, metaline
diff --git a/src/compiler/crystal/syntax/parser.cr b/src/compiler/crystal/syntax/parser.cr
index 2c2e4f0e6383..6c0cf1870f88 100644
--- a/src/compiler/crystal/syntax/parser.cr
+++ b/src/compiler/crystal/syntax/parser.cr
@@ -6190,7 +6190,9 @@ module Crystal
       when Var, InstanceVar, ClassVar, Path, Global, Underscore
         true
       when Call
-        !node.has_parentheses? && ((node.obj.nil? && node.args.empty? && node.block.nil?) || node.name == "[]")
+        return false if node.has_parentheses?
+        return true if node.obj.nil? && node.args.empty? && node.block.nil?
+        node.name == "[]" && node.dot_location.nil?
       else
         false
       end

From 3a2ccfc8200dffbe0408dae7b6f91b0eb70e1f33 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?fn=20=E2=8C=83=20=E2=8C=A5?=
 <70830482+FnControlOption@users.noreply.github.com>
Date: Tue, 16 Jul 2024 14:22:23 -0700
Subject: [PATCH 3/8] Disallow assignments like `a b: 0 = 1`

---
 spec/compiler/parser/parser_spec.cr   | 1 +
 src/compiler/crystal/syntax/parser.cr | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/spec/compiler/parser/parser_spec.cr b/spec/compiler/parser/parser_spec.cr
index a67de2b76188..7e2f45d632dd 100644
--- a/spec/compiler/parser/parser_spec.cr
+++ b/spec/compiler/parser/parser_spec.cr
@@ -278,6 +278,7 @@ module Crystal
 
     assert_syntax_error "a.[] 0 = 1"
     assert_syntax_error "a.[] 0 += 1"
+    assert_syntax_error "a b: 0 = 1"
 
     it_parses "def foo\n1\nend", Def.new("foo", body: 1.int32)
     it_parses "def downto(n)\n1\nend", Def.new("downto", ["n".arg], 1.int32)
diff --git a/src/compiler/crystal/syntax/parser.cr b/src/compiler/crystal/syntax/parser.cr
index 6c0cf1870f88..2882ed7dffb8 100644
--- a/src/compiler/crystal/syntax/parser.cr
+++ b/src/compiler/crystal/syntax/parser.cr
@@ -6191,7 +6191,7 @@ module Crystal
         true
       when Call
         return false if node.has_parentheses?
-        return true if node.obj.nil? && node.args.empty? && node.block.nil?
+        return true if node.obj.nil? && node.args.empty? && node.named_args.nil? && node.block.nil?
         node.name == "[]" && node.dot_location.nil?
       else
         false

From fe6980eac8244de3d0b05e2476d4cad6e6a85fd7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?fn=20=E2=8C=83=20=E2=8C=A5?=
 <70830482+FnControlOption@users.noreply.github.com>
Date: Sat, 20 Jul 2024 10:20:15 -0700
Subject: [PATCH 4/8] Replace `dot_location` with `args_in_brackets`

---
 spec/compiler/parser/parser_spec.cr   | 28 +++++++++++----------------
 src/compiler/crystal/syntax/ast.cr    |  2 +-
 src/compiler/crystal/syntax/parser.cr | 13 +++++--------
 3 files changed, 17 insertions(+), 26 deletions(-)

diff --git a/spec/compiler/parser/parser_spec.cr b/spec/compiler/parser/parser_spec.cr
index 7e2f45d632dd..69ece5f56fab 100644
--- a/spec/compiler/parser/parser_spec.cr
+++ b/spec/compiler/parser/parser_spec.cr
@@ -204,6 +204,8 @@ module Crystal
 
     it_parses "a = 1", Assign.new("a".var, 1.int32)
     it_parses "a = b = 2", Assign.new("a".var, Assign.new("b".var, 2.int32))
+    it_parses "a[] = 1", Call.new("a".call, "[]=", 1.int32)
+    it_parses "a.[] = 1", Call.new("a".call, "[]=", 1.int32)
 
     it_parses "a, b = 1, 2", MultiAssign.new(["a".var, "b".var] of ASTNode, [1.int32, 2.int32] of ASTNode)
     it_parses "a, b = 1", MultiAssign.new(["a".var, "b".var] of ASTNode, [1.int32] of ASTNode)
@@ -528,11 +530,15 @@ module Crystal
     it_parses "foo &.+(2)", Call.new(nil, "foo", block: Block.new([Var.new("__arg0")], Call.new(Var.new("__arg0"), "+", 2.int32)))
     it_parses "foo &.bar.baz", Call.new(nil, "foo", block: Block.new([Var.new("__arg0")], Call.new(Call.new(Var.new("__arg0"), "bar"), "baz")))
     it_parses "foo(&.bar.baz)", Call.new(nil, "foo", block: Block.new([Var.new("__arg0")], Call.new(Call.new(Var.new("__arg0"), "bar"), "baz")))
+    it_parses "foo &.block[]", Call.new(nil, "foo", block: Block.new([Var.new("__arg0")], Call.new(Call.new(Var.new("__arg0"), "block"), "[]")))
     it_parses "foo &.block[0]", Call.new(nil, "foo", block: Block.new([Var.new("__arg0")], Call.new(Call.new(Var.new("__arg0"), "block"), "[]", 0.int32)))
     it_parses "foo &.block=(0)", Call.new(nil, "foo", block: Block.new([Var.new("__arg0")], Call.new(Var.new("__arg0"), "block=", 0.int32)))
     it_parses "foo &.block = 0", Call.new(nil, "foo", block: Block.new([Var.new("__arg0")], Call.new(Var.new("__arg0"), "block=", 0.int32)))
+    it_parses "foo &.block[] = 1", Call.new(nil, "foo", block: Block.new([Var.new("__arg0")], Call.new(Call.new(Var.new("__arg0"), "block"), "[]=", 1.int32)))
     it_parses "foo &.block[0] = 1", Call.new(nil, "foo", block: Block.new([Var.new("__arg0")], Call.new(Call.new(Var.new("__arg0"), "block"), "[]=", 0.int32, 1.int32)))
+    it_parses "foo &.[]", Call.new(nil, "foo", block: Block.new([Var.new("__arg0")], Call.new(Var.new("__arg0"), "[]")))
     it_parses "foo &.[0]", Call.new(nil, "foo", block: Block.new([Var.new("__arg0")], Call.new(Var.new("__arg0"), "[]", 0.int32)))
+    it_parses "foo &.[] = 1", Call.new(nil, "foo", block: Block.new([Var.new("__arg0")], Call.new(Var.new("__arg0"), "[]=", 1.int32)))
     it_parses "foo &.[0] = 1", Call.new(nil, "foo", block: Block.new([Var.new("__arg0")], Call.new(Var.new("__arg0"), "[]=", 0.int32, 1.int32)))
     it_parses "foo(&.is_a?(T))", Call.new(nil, "foo", block: Block.new([Var.new("__arg0")], IsA.new(Var.new("__arg0"), "T".path)))
     it_parses "foo(&.!)", Call.new(nil, "foo", block: Block.new([Var.new("__arg0")], Not.new(Var.new("__arg0"))))
@@ -2826,28 +2832,16 @@ module Crystal
       node_source(source, node).should eq("::Foo")
     end
 
-    it "sets correct location of call dot" do
+    it "sets args_in_brackets to false for `a.b`" do
       parser = Parser.new("a.b")
       node = parser.parse.as(Call)
-      dot_location = node.dot_location.not_nil!
-      dot_location.line_number.should eq(1)
-      dot_location.column_number.should eq(2)
+      node.args_in_brackets?.should be_false
     end
 
-    it "sets correct location of call dot in assignment" do
-      parser = Parser.new("a.b = c")
+    it "sets args_in_brackets to true for `a[b]`" do
+      parser = Parser.new("a[b]")
       node = parser.parse.as(Call)
-      dot_location = node.dot_location.not_nil!
-      dot_location.line_number.should eq(1)
-      dot_location.column_number.should eq(2)
-    end
-
-    it "sets correct location of call dot in operator assignment" do
-      parser = Parser.new("a.b += c")
-      node = parser.parse.as(OpAssign).target.as(Call)
-      dot_location = node.dot_location.not_nil!
-      dot_location.line_number.should eq(1)
-      dot_location.column_number.should eq(2)
+      node.args_in_brackets?.should be_true
     end
   end
 end
diff --git a/src/compiler/crystal/syntax/ast.cr b/src/compiler/crystal/syntax/ast.cr
index 9641da81f7b6..9ccd8dda1f69 100644
--- a/src/compiler/crystal/syntax/ast.cr
+++ b/src/compiler/crystal/syntax/ast.cr
@@ -647,13 +647,13 @@ module Crystal
     property block : Block?
     property block_arg : ASTNode?
     property named_args : Array(NamedArgument)?
-    property dot_location : Location?
     property name_location : Location?
     @name_size = -1
     property doc : String?
     property visibility = Visibility::Public
     property? global : Bool
     property? expansion = false
+    property? args_in_brackets = false
     property? has_parentheses = false
 
     def initialize(@obj, @name, @args = [] of ASTNode, @block = nil, @block_arg = nil, @named_args = nil, @global : Bool = false)
diff --git a/src/compiler/crystal/syntax/parser.cr b/src/compiler/crystal/syntax/parser.cr
index 2882ed7dffb8..98856f2e7692 100644
--- a/src/compiler/crystal/syntax/parser.cr
+++ b/src/compiler/crystal/syntax/parser.cr
@@ -678,8 +678,6 @@ module Crystal
             break
           end
         when .op_period?
-          dot_location = @token.location
-
           check_void_value atomic, location
 
           @wants_regex = false
@@ -763,7 +761,6 @@ module Crystal
               end
 
               atomic = Call.new(atomic, "#{name}=", arg).at(location).at_end(end_location)
-              atomic.dot_location = dot_location
               atomic.name_location = name_location
               next
             when .assignment_operator?
@@ -772,7 +769,6 @@ module Crystal
               next_token_skip_space_or_newline
               value = parse_op_assign
               call = Call.new(atomic, name).at(location)
-              call.dot_location = dot_location
               call.name_location = name_location
               atomic = OpAssign.new(call, method, value).at(location)
               atomic.name_location = op_name_location
@@ -794,7 +790,6 @@ module Crystal
             block = parse_block(block, stop_on_do: @stop_on_do)
             atomic = Call.new atomic, name, (args || [] of ASTNode), block, block_arg, named_args
             atomic.has_parentheses = has_parentheses
-            atomic.dot_location = dot_location
             atomic.name_location = name_location
             atomic.end_location = block.try(&.end_location) || call_args.try(&.end_location) || end_location
             atomic.at(location)
@@ -853,7 +848,8 @@ module Crystal
           atomic = Call.new(atomic, method_name, (args || [] of ASTNode), block, block_arg, named_args).at(location)
           atomic.name_location = name_location
           atomic.end_location = end_location
-          atomic.name_size = 0 if atomic.is_a?(Call)
+          atomic.name_size = 0
+          atomic.args_in_brackets = true
           atomic
         else
           break
@@ -6191,8 +6187,9 @@ module Crystal
         true
       when Call
         return false if node.has_parentheses?
-        return true if node.obj.nil? && node.args.empty? && node.named_args.nil? && node.block.nil?
-        node.name == "[]" && node.dot_location.nil?
+        no_args = node.args.empty? && node.named_args.nil? && node.block.nil?
+        return true if node.obj.nil? && no_args
+        node.name == "[]" && (node.args_in_brackets? || no_args)
       else
         false
       end

From 9911d5244e7fec1a7f905b240e4e832892411eeb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?fn=20=E2=8C=83=20=E2=8C=A5?=
 <70830482+FnControlOption@users.noreply.github.com>
Date: Sat, 20 Jul 2024 10:26:19 -0700
Subject: [PATCH 5/8] Disallow `foo &.[0]? = 1`

---
 spec/compiler/parser/parser_spec.cr   | 7 +++++++
 src/compiler/crystal/syntax/parser.cr | 2 +-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/spec/compiler/parser/parser_spec.cr b/spec/compiler/parser/parser_spec.cr
index 69ece5f56fab..7bdd63a48d75 100644
--- a/spec/compiler/parser/parser_spec.cr
+++ b/spec/compiler/parser/parser_spec.cr
@@ -2236,6 +2236,13 @@ module Crystal
 
     assert_syntax_error "lib Foo%end", %(unexpected token: "%")
 
+    assert_syntax_error "foo[0]? = 1"
+    assert_syntax_error "foo[0]? += 1"
+    assert_syntax_error "foo.[0]? = 1"
+    assert_syntax_error "foo.[0]? += 1"
+    assert_syntax_error "foo &.[0]? = 1"
+    assert_syntax_error "foo &.[0]? += 1"
+
     describe "end locations" do
       assert_end_location "nil"
       assert_end_location "false"
diff --git a/src/compiler/crystal/syntax/parser.cr b/src/compiler/crystal/syntax/parser.cr
index 98856f2e7692..ab5a4423a4e6 100644
--- a/src/compiler/crystal/syntax/parser.cr
+++ b/src/compiler/crystal/syntax/parser.cr
@@ -1623,7 +1623,7 @@ module Crystal
       elsif @token.type.op_lsquare?
         call = parse_atomic_method_suffix obj, location
 
-        if @token.type.op_eq? && call.is_a?(Call)
+        if @token.type.op_eq? && call.is_a?(Call) && call.name == "[]"
           next_token_skip_space
           exp = parse_op_assign
           call.name = "#{call.name}="

From d556c7c0dce46acd8e276e7b143328eb91bedf20 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?fn=20=E2=8C=83=20=E2=8C=A5?=
 <70830482+FnControlOption@users.noreply.github.com>
Date: Sat, 20 Jul 2024 12:12:37 -0700
Subject: [PATCH 6/8] Disallow `foo.[]? = 1` and `foo.[]? += 1`

---
 spec/compiler/parser/parser_spec.cr   |  2 ++
 src/compiler/crystal/syntax/parser.cr | 14 +++++++++++---
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/spec/compiler/parser/parser_spec.cr b/spec/compiler/parser/parser_spec.cr
index 7bdd63a48d75..2f3495debe10 100644
--- a/spec/compiler/parser/parser_spec.cr
+++ b/spec/compiler/parser/parser_spec.cr
@@ -2236,6 +2236,8 @@ module Crystal
 
     assert_syntax_error "lib Foo%end", %(unexpected token: "%")
 
+    assert_syntax_error "foo.[]? = 1"
+    assert_syntax_error "foo.[]? += 1"
     assert_syntax_error "foo[0]? = 1"
     assert_syntax_error "foo[0]? += 1"
     assert_syntax_error "foo.[0]? = 1"
diff --git a/src/compiler/crystal/syntax/parser.cr b/src/compiler/crystal/syntax/parser.cr
index ab5a4423a4e6..9ac03ba45667 100644
--- a/src/compiler/crystal/syntax/parser.cr
+++ b/src/compiler/crystal/syntax/parser.cr
@@ -735,6 +735,9 @@ module Crystal
 
             case @token.type
             when .op_eq?
+              atomic = Call.new(atomic, name)
+              unexpected_token unless can_be_assigned?(atomic)
+
               # Rewrite 'f.x = arg' as f.x=(arg)
               next_token
 
@@ -760,15 +763,20 @@ module Crystal
                 end_location = arg.end_location
               end
 
-              atomic = Call.new(atomic, "#{name}=", arg).at(location).at_end(end_location)
+              atomic.at(location).at_end(end_location)
+              atomic.name = "#{name}="
+              atomic.args = [arg] of ASTNode
               atomic.name_location = name_location
               next
             when .assignment_operator?
+              call = Call.new(atomic, name)
+              unexpected_token unless can_be_assigned?(call)
+
               op_name_location = @token.location
               method = @token.type.to_s.byte_slice(0, @token.type.to_s.size - 1)
               next_token_skip_space_or_newline
               value = parse_op_assign
-              call = Call.new(atomic, name).at(location)
+              call.at(location)
               call.name_location = name_location
               atomic = OpAssign.new(call, method, value).at(location)
               atomic.name_location = op_name_location
@@ -6188,7 +6196,7 @@ module Crystal
       when Call
         return false if node.has_parentheses?
         no_args = node.args.empty? && node.named_args.nil? && node.block.nil?
-        return true if node.obj.nil? && no_args
+        return true if Lexer.ident?(node.name) && no_args
         node.name == "[]" && (node.args_in_brackets? || no_args)
       else
         false

From 903e4b9dc97f9d036b8af3c7081a9b89cefb2114 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?fn=20=E2=8C=83=20=E2=8C=A5?=
 <70830482+FnControlOption@users.noreply.github.com>
Date: Sat, 20 Jul 2024 12:27:32 -0700
Subject: [PATCH 7/8] Disallow `foo &.[](0) = 1`

---
 spec/compiler/parser/parser_spec.cr   | 11 +++++++++++
 src/compiler/crystal/syntax/parser.cr |  2 ++
 2 files changed, 13 insertions(+)

diff --git a/spec/compiler/parser/parser_spec.cr b/spec/compiler/parser/parser_spec.cr
index 2f3495debe10..30ec21e1466f 100644
--- a/spec/compiler/parser/parser_spec.cr
+++ b/spec/compiler/parser/parser_spec.cr
@@ -2245,6 +2245,17 @@ module Crystal
     assert_syntax_error "foo &.[0]? = 1"
     assert_syntax_error "foo &.[0]? += 1"
 
+    assert_syntax_error "foo &.[]?=(1)"
+    assert_syntax_error "foo &.[]? = 1"
+    assert_syntax_error "foo &.[]? 0 =(1)"
+    assert_syntax_error "foo &.[]? 0 = 1"
+    assert_syntax_error "foo &.[]?(0)=(1)"
+    assert_syntax_error "foo &.[]?(0) = 1"
+    assert_syntax_error "foo &.[] 0 =(1)"
+    assert_syntax_error "foo &.[] 0 = 1"
+    assert_syntax_error "foo &.[](0)=(1)"
+    assert_syntax_error "foo &.[](0) = 1"
+
     describe "end locations" do
       assert_end_location "nil"
       assert_end_location "false"
diff --git a/src/compiler/crystal/syntax/parser.cr b/src/compiler/crystal/syntax/parser.cr
index 9ac03ba45667..8dc5b8c1dc14 100644
--- a/src/compiler/crystal/syntax/parser.cr
+++ b/src/compiler/crystal/syntax/parser.cr
@@ -1652,6 +1652,8 @@ module Crystal
         call = call.as(Call)
 
         if @token.type.op_eq?
+          unexpected_token unless can_be_assigned?(call)
+
           next_token_skip_space
           if @token.type.op_lparen?
             next_token_skip_space

From 74ff652c7711c98eaee4894dafb24c4787e69998 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?fn=20=E2=8C=83=20=E2=8C=A5?=
 <70830482+FnControlOption@users.noreply.github.com>
Date: Sat, 20 Jul 2024 12:38:11 -0700
Subject: [PATCH 8/8] Disallow `foo &.bar.[](0) = 1`

---
 spec/compiler/parser/parser_spec.cr   | 5 +++++
 src/compiler/crystal/syntax/parser.cr | 4 ++--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/spec/compiler/parser/parser_spec.cr b/spec/compiler/parser/parser_spec.cr
index 30ec21e1466f..f719e5c41b06 100644
--- a/spec/compiler/parser/parser_spec.cr
+++ b/spec/compiler/parser/parser_spec.cr
@@ -2256,6 +2256,11 @@ module Crystal
     assert_syntax_error "foo &.[](0)=(1)"
     assert_syntax_error "foo &.[](0) = 1"
 
+    assert_syntax_error "foo &.bar.[] 0 =(1)"
+    assert_syntax_error "foo &.bar.[] 0 = 1"
+    assert_syntax_error "foo &.bar.[](0)=(1)"
+    assert_syntax_error "foo &.bar.[](0) = 1"
+
     describe "end locations" do
       assert_end_location "nil"
       assert_end_location "false"
diff --git a/src/compiler/crystal/syntax/parser.cr b/src/compiler/crystal/syntax/parser.cr
index 8dc5b8c1dc14..7153e0ede247 100644
--- a/src/compiler/crystal/syntax/parser.cr
+++ b/src/compiler/crystal/syntax/parser.cr
@@ -1631,7 +1631,7 @@ module Crystal
       elsif @token.type.op_lsquare?
         call = parse_atomic_method_suffix obj, location
 
-        if @token.type.op_eq? && call.is_a?(Call) && call.name == "[]"
+        if @token.type.op_eq? && call.is_a?(Call) && can_be_assigned?(call)
           next_token_skip_space
           exp = parse_op_assign
           call.name = "#{call.name}="
@@ -1671,7 +1671,7 @@ module Crystal
         else
           call = parse_atomic_method_suffix call, location
 
-          if @token.type.op_eq? && call.is_a?(Call) && call.name == "[]"
+          if @token.type.op_eq? && call.is_a?(Call) && can_be_assigned?(call)
             next_token_skip_space
             exp = parse_op_assign
             call.name = "#{call.name}="