Give proper error when doing sizeof uninstantiated generic type

spec/compiler/codegen/sizeof_spec.cr

@@ -68,14 +68,6 @@ describe "Code gen: sizeof" do
       ").to_i.should eq(16)
   end
 
-  it "gives error if using instance_sizeof on something that's not a class" do
-    assert_error "instance_sizeof(Int32)", "Int32 is not a class, it's a struct"
-  end
-
-  it "gives error if using instance_sizeof on a generic type without type vars" do
-    assert_error "instance_sizeof(Array)", "can't calculate instance_sizeof of generic class"
-  end
-
   it "gets instance_sizeof a generic type with type vars" do
     run(%(
       class Foo(T)

spec/compiler/semantic/primitives_spec.cr

@@ -61,22 +61,6 @@ describe "Semantic: primitives" do
     assert_type("1 + 2") { int32 }
   end
 
-  it "types sizeof" do
-    assert_type("sizeof(Float64)") { int32 }
-  end
-
-  it "types sizeof NoReturn (missing type) (#5717)" do
-    assert_type("x = nil; x ? sizeof(typeof(x)) : 1") { int32 }
-  end
-
-  it "types instance_sizeof" do
-    assert_type("instance_sizeof(Reference)") { int32 }
-  end
-
-  it "types instance_sizeof NoReturn (missing type) (#5717)" do
-    assert_type("x = nil; x ? instance_sizeof(typeof(x)) : 1") { int32 }
-  end
-
   it "errors when comparing void (#225)" do
     assert_error %(
       lib LibFoo

spec/compiler/semantic/sizeof_spec.cr

@@ -0,0 +1,31 @@
+require "../../spec_helper"
+
+describe "Semantic: sizeof" do
+  it "types sizeof" do
+    assert_type("sizeof(Float64)") { int32 }
+  end
+
+  it "types sizeof NoReturn (missing type) (#5717)" do
+    assert_type("x = nil; x ? sizeof(typeof(x)) : 1") { int32 }
+  end
+
+  it "types instance_sizeof" do
+    assert_type("instance_sizeof(Reference)") { int32 }
+  end
+
+  it "types instance_sizeof NoReturn (missing type) (#5717)" do
+    assert_type("x = nil; x ? instance_sizeof(typeof(x)) : 1") { int32 }
+  end
+
+  it "errors on sizeof uninstantiated generic type (#6415)" do
+    assert_error "sizeof(Array)", "can't take sizeof uninstantiated generic type Array(T)"
+  end
+
+  it "gives error if using instance_sizeof on something that's not a class" do
+    assert_error "instance_sizeof(Int32)", "Int32 is not a class, it's a struct"
+  end
+
+  it "gives error if using instance_sizeof on a generic type without type vars" do
+    assert_error "instance_sizeof(Array)", "can't take instance_sizeof uninstantiated generic type Array(T)"
+  end
+end

src/compiler/crystal/semantic/main_visitor.cr

@@ -2552,6 +2552,10 @@ module Crystal
 
       type = node.exp.type?
 
+      if type.is_a?(GenericType)
+        node.exp.raise "can't take sizeof uninstantiated generic type #{type}"
+      end
+
       # Try to resolve the sizeof right now to a number literal
       # (useful for sizeof inside as a generic type argument, but also
       # to make it easier for LLVM to optimize things)
@@ -2573,8 +2577,8 @@ module Crystal
 
       type = node.exp.type?
 
-      if type.is_a? GenericType
-        node.raise "can't calculate instance_sizeof of generic class #{type} without specifying its type vars"
+      if type.is_a?(GenericType)
+        node.exp.raise "can't take instance_sizeof uninstantiated generic type #{type}"
       end
 
       # Try to resolve the instance_sizeof right now to a number literal