A setter type is the argument's type

spec/compiler/codegen/def_spec.cr

@@ -568,4 +568,72 @@ describe "Code gen: def" do
       foo { |a, b| }
       ))
   end
+
+  it "codegens setter (non-inlineable body, non-struct)" do
+    run(%(
+      class Foo
+        def x=(y)
+          'a'
+          'b'
+        end
+      end
+
+      foo = Foo.new
+      z = foo.x = 123
+      z
+      )).to_i.should eq(123)
+  end
+
+  it "codegens setter (non-inlineable body, struct)" do
+    run(%(
+      class Foo
+        def x=(y)
+          'a'
+          'b'
+        end
+      end
+
+      struct Bar
+        @x = 123
+        def x; @x; end
+      end
+
+      foo = Foo.new
+      z = foo.x = Bar.new
+      z.x
+      )).to_i.should eq(123)
+  end
+
+  it "codegens setter (inlineable body, non-struct)" do
+    run(%(
+      class Foo
+        def x=(y)
+          'a'
+        end
+      end
+
+      foo = Foo.new
+      z = foo.x = 123
+      z
+      )).to_i.should eq(123)
+  end
+
+  it "codegens setter (inlineable body, struct)" do
+    run(%(
+      class Foo
+        def x=(y)
+          'a'
+        end
+      end
+
+      struct Bar
+        @x = 123
+        def x; @x; end
+      end
+
+      foo = Foo.new
+      z = foo.x = Bar.new
+      z.x
+      )).to_i.should eq(123)
+  end
 end

spec/compiler/semantic/def_spec.cr

@@ -486,4 +486,18 @@ describe "Semantic: def" do
       foo
     ), "there's no self in this scope"
   end
+
+  it "types setter" do
+    assert_type(%(
+      class Foo
+        def x=(y)
+          'a'
+        end
+      end
+
+      foo = Foo.new
+      z = foo.x = 1
+      z
+    )) { int32 }
+  end
 end

src/compiler/crystal/codegen/call.cr

@@ -393,7 +393,7 @@ class Crystal::CodeGenVisitor
     body = target_def.body
 
     # Try to inline the call
-    if try_inline_call(target_def, body, self_type, call_args)
+    if try_inline_call(node, target_def, body, self_type, call_args)
       return
     end
 
@@ -419,37 +419,44 @@ class Crystal::CodeGenVisitor
   # to read a constant value or the value of an instance variable.
   # Additionally, not inlining instance variable getters changes the semantic
   # a program, so we must always inline these.
-  def try_inline_call(target_def, body, self_type, call_args)
+  def try_inline_call(node, target_def, body, self_type, call_args)
     return false if target_def.is_a?(External)
 
     case body
     when Nop, NilLiteral, BoolLiteral, CharLiteral, StringLiteral, NumberLiteral, SymbolLiteral
       return true unless @needs_value
 
       accept body
-      inline_call_return_value target_def, body
+      inline_call_return_value node, target_def, body, self_type, call_args
       return true
     when Var
       if body.name == "self"
         return true unless @needs_value
 
         @last = self_type.passed_as_self? ? call_args.first : type_id(self_type)
-        inline_call_return_value target_def, body
+        inline_call_return_value node, target_def, body, self_type, call_args
         return true
       end
     when InstanceVar
       return true unless @needs_value
 
       read_instance_var(body.type, self_type, body.name, call_args.first)
-      inline_call_return_value target_def, body
+      inline_call_return_value node, target_def, body, self_type, call_args
       return true
     end
 
     false
   end
 
-  def inline_call_return_value(target_def, body)
-    if target_def.type.nil_type?
+  def inline_call_return_value(node, target_def, body, self_type, call_args)
+    if node.setter?
+      # For a setter value (`foo.x = y`) the last value is `y`, which, depending
+      # on whether `foo` is passed as self or not, is the first or second codegen argument
+      @last = call_args[self_type.try(&.passed_as_self?) ? 1 : 0]
+
+      first_arg_type = node.args.first.type
+      @last = box_value(@last, first_arg_type) if first_arg_type.passed_by_value?
+    elsif target_def.type.nil_type?
       @last = llvm_nil
     else
       @last = upcast(@last, target_def.type, body.type)
@@ -510,23 +517,30 @@ class Crystal::CodeGenVisitor
         end
       end
     else
+      # For a setter value (`foo.x = y`) the last value is `y`, which, depending
+      # on whether `foo` is passed as self or not, is the first or second codegen argument
+      if node.is_a?(Call) && node.setter?
+        @last = call_args[self_type.try(&.passed_as_self?) ? 1 : 0]
+        type = node.args.first.type
+      end
+
       case type
       when .no_return?
         unreachable
       when .passed_by_value?
-        if @needs_value
-          union = alloca llvm_type(type)
-          store @last, union
-          @last = union
-        else
-          @last = llvm_nil
-        end
+        @last = @needs_value ? box_value(@last, type) : llvm_nil
       end
     end
 
     @last
   end
 
+  def box_value(value, type)
+    union = alloca llvm_type(type)
+    store value, union
+    union
+  end
+
   def set_call_attributes(node : Call, target_def, self_type, is_closure, fun_type)
     if external = target_def.c_calling_convention?
       set_call_attributes_external(node, external)

src/compiler/crystal/semantic/call.cr

@@ -77,8 +77,13 @@ class Crystal::Call
 
     @target_defs = matches
 
-    bind_to matches if matches
-    bind_to block.break if block
+    # For a setter like `foo.x = y` we'd like to use `y`'s type
+    if self.setter?
+      bind_to args.first
+    else
+      bind_to matches if matches
+      bind_to block.break if block
+    end
 
     if (parent_visitor = @parent_visitor) && matches
       if parent_visitor.typed_def? && matches.any?(&.raises?)

src/compiler/crystal/syntax/ast.cr

@@ -545,6 +545,7 @@ module Crystal
     property? global : Bool
     property? expansion = false
     property? has_parentheses : Bool
+    property? setter = false
 
     def initialize(@obj, @name, @args = [] of ASTNode, @block = nil, @block_arg = nil, @named_args = nil, global = false, @name_column_number = 0, has_parentheses = false)
       @global = !!global
@@ -589,6 +590,7 @@ module Crystal
       clone = Call.new(@obj.clone, @name, @args.clone, @block.clone, @block_arg.clone, @named_args.clone, @global, @name_column_number, @has_parentheses)
       clone.name_size = name_size
       clone.expansion = expansion?
+      clone.setter = setter?
       clone
     end
 
@@ -602,6 +604,12 @@ module Crystal
       Location.new(loc.filename, loc.line_number, name_column_number + name_size)
     end
 
+    def setter?
+      name.ends_with?('=') &&
+        !(33 <= name.byte_at(0) <= 64) && # first letter must not be a symbol like `=` or `!`
+        args.size == 1 && !named_args && !block && !block_arg
+    end
+
     def_equals_and_hash obj, name, args, block, block_arg, named_args, global?
   end