diff --git a/crates/ra_hir/src/code_model.rs b/crates/ra_hir/src/code_model.rs
index fda31d5177c..817c27410a1 100644
--- a/crates/ra_hir/src/code_model.rs
+++ b/crates/ra_hir/src/code_model.rs
@@ -755,7 +755,7 @@ pub struct TypeParam {
 impl TypeParam {
     pub fn name(self, db: &impl HirDatabase) -> Name {
         let params = db.generic_params(self.id.parent);
-        params.types[self.id.local_id].name.clone()
+        params.types[self.id.local_id].name.clone().unwrap_or_else(Name::missing)
     }
 
     pub fn module(self, db: &impl HirDatabase) -> Module {
diff --git a/crates/ra_hir_def/src/generics.rs b/crates/ra_hir_def/src/generics.rs
index e9c28c7309e..7553d8a8719 100644
--- a/crates/ra_hir_def/src/generics.rs
+++ b/crates/ra_hir_def/src/generics.rs
@@ -27,8 +27,16 @@
 /// Data about a generic parameter (to a function, struct, impl, ...).
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub struct TypeParamData {
-    pub name: Name,
+    pub name: Option<Name>,
     pub default: Option<TypeRef>,
+    pub provenance: TypeParamProvenance,
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub enum TypeParamProvenance {
+    TypeParamList,
+    TraitSelf,
+    ArgumentImplTrait,
 }
 
 /// Data about the generic parameters of a function, struct, impl, etc.
@@ -68,6 +76,11 @@ fn new(db: &impl DefDatabase, def: GenericDefId) -> (GenericParams, InFile<Sourc
             GenericDefId::FunctionId(it) => {
                 let src = it.lookup(db).source(db);
                 generics.fill(&mut sm, &src.value);
+                // lower `impl Trait` in arguments
+                let data = db.function_data(it);
+                for param in &data.params {
+                    generics.fill_implicit_impl_trait_args(param);
+                }
                 src.file_id
             }
             GenericDefId::AdtId(AdtId::StructId(it)) => {
@@ -89,8 +102,11 @@ fn new(db: &impl DefDatabase, def: GenericDefId) -> (GenericParams, InFile<Sourc
                 let src = it.lookup(db).source(db);
 
                 // traits get the Self type as an implicit first type parameter
-                let self_param_id =
-                    generics.types.alloc(TypeParamData { name: name![Self], default: None });
+                let self_param_id = generics.types.alloc(TypeParamData {
+                    name: Some(name![Self]),
+                    default: None,
+                    provenance: TypeParamProvenance::TraitSelf,
+                });
                 sm.insert(self_param_id, Either::Left(src.value.clone()));
                 // add super traits as bounds on Self
                 // i.e., trait Foo: Bar is equivalent to trait Foo where Self: Bar
@@ -142,7 +158,11 @@ fn fill_params(&mut self, sm: &mut SourceMap, params: ast::TypeParamList) {
             let name = type_param.name().map_or_else(Name::missing, |it| it.as_name());
             // FIXME: Use `Path::from_src`
             let default = type_param.default_type().map(TypeRef::from_ast);
-            let param = TypeParamData { name: name.clone(), default };
+            let param = TypeParamData {
+                name: Some(name.clone()),
+                default,
+                provenance: TypeParamProvenance::TypeParamList,
+            };
             let param_id = self.types.alloc(param);
             sm.insert(param_id, Either::Right(type_param.clone()));
 
@@ -173,8 +193,23 @@ fn add_where_predicate_from_bound(&mut self, bound: ast::TypeBound, type_ref: Ty
         self.where_predicates.push(WherePredicate { type_ref, bound });
     }
 
+    fn fill_implicit_impl_trait_args(&mut self, type_ref: &TypeRef) {
+        type_ref.walk(&mut |type_ref| {
+            if let TypeRef::ImplTrait(_) = type_ref {
+                let param = TypeParamData {
+                    name: None,
+                    default: None,
+                    provenance: TypeParamProvenance::ArgumentImplTrait,
+                };
+                let _param_id = self.types.alloc(param);
+            }
+        });
+    }
+
     pub fn find_by_name(&self, name: &Name) -> Option<LocalTypeParamId> {
-        self.types.iter().find_map(|(id, p)| if &p.name == name { Some(id) } else { None })
+        self.types
+            .iter()
+            .find_map(|(id, p)| if p.name.as_ref() == Some(name) { Some(id) } else { None })
     }
 }
 
diff --git a/crates/ra_hir_def/src/resolver.rs b/crates/ra_hir_def/src/resolver.rs
index f7bac580112..05cf4646a89 100644
--- a/crates/ra_hir_def/src/resolver.rs
+++ b/crates/ra_hir_def/src/resolver.rs
@@ -490,10 +490,12 @@ fn process_names(&self, db: &impl DefDatabase, f: &mut dyn FnMut(Name, ScopeDef)
             }
             Scope::GenericParams { params, def } => {
                 for (local_id, param) in params.types.iter() {
-                    f(
-                        param.name.clone(),
-                        ScopeDef::GenericParam(TypeParamId { local_id, parent: *def }),
-                    )
+                    if let Some(name) = &param.name {
+                        f(
+                            name.clone(),
+                            ScopeDef::GenericParam(TypeParamId { local_id, parent: *def }),
+                        )
+                    }
                 }
             }
             Scope::ImplBlockScope(i) => {
diff --git a/crates/ra_hir_def/src/type_ref.rs b/crates/ra_hir_def/src/type_ref.rs
index 5f10e9a88ba..10941477053 100644
--- a/crates/ra_hir_def/src/type_ref.rs
+++ b/crates/ra_hir_def/src/type_ref.rs
@@ -124,6 +124,50 @@ pub(crate) fn from_ast_opt(node: Option<ast::TypeRef>) -> Self {
     pub(crate) fn unit() -> TypeRef {
         TypeRef::Tuple(Vec::new())
     }
+
+    pub fn walk(&self, f: &mut impl FnMut(&TypeRef)) {
+        go(self, f);
+
+        fn go(type_ref: &TypeRef, f: &mut impl FnMut(&TypeRef)) {
+            f(type_ref);
+            match type_ref {
+                TypeRef::Fn(types) | TypeRef::Tuple(types) => {
+                    types.iter().for_each(|t| go(t, f))
+                }
+                TypeRef::RawPtr(type_ref, _)
+                | TypeRef::Reference(type_ref, _)
+                | TypeRef::Array(type_ref)
+                | TypeRef::Slice(type_ref) => go(&type_ref, f),
+                TypeRef::ImplTrait(bounds) | TypeRef::DynTrait(bounds) => {
+                    for bound in bounds {
+                        match bound {
+                            TypeBound::Path(path) => go_path(path, f),
+                            TypeBound::Error => (),
+                        }
+                    }
+                }
+                TypeRef::Path(path) => go_path(path, f),
+                TypeRef::Never | TypeRef::Placeholder | TypeRef::Error => {}
+            };
+        }
+
+        fn go_path(path: &Path, f: &mut impl FnMut(&TypeRef)) {
+            if let Some(type_ref) = path.type_anchor() {
+                go(type_ref, f);
+            }
+            for segment in path.segments().iter() {
+                if let Some(args_and_bindings) = segment.args_and_bindings {
+                    for arg in &args_and_bindings.args {
+                        let crate::path::GenericArg::Type(type_ref) = arg;
+                        go(type_ref, f);
+                    }
+                    for (_, type_ref) in &args_and_bindings.bindings {
+                        go(type_ref, f);
+                    }
+                }
+            }
+        }
+    }
 }
 
 pub(crate) fn type_bounds_from_ast(type_bounds_opt: Option<ast::TypeBoundList>) -> Vec<TypeBound> {
diff --git a/crates/ra_hir_ty/src/infer/expr.rs b/crates/ra_hir_ty/src/infer/expr.rs
index 31259a01de7..97cb20cea40 100644
--- a/crates/ra_hir_ty/src/infer/expr.rs
+++ b/crates/ra_hir_ty/src/infer/expr.rs
@@ -10,7 +10,7 @@
     resolver::resolver_for_expr,
     AdtId, AssocContainerId, Lookup, StructFieldId,
 };
-use hir_expand::name::{name, Name};
+use hir_expand::name::Name;
 use ra_syntax::ast::RangeOp;
 
 use crate::{
@@ -654,7 +654,7 @@ fn substs_for_method_call(
         // Parent arguments are unknown, except for the receiver type
         if let Some(parent_generics) = def_generics.as_ref().map(|p| p.iter_parent()) {
             for (_id, param) in parent_generics {
-                if param.name == name![Self] {
+                if param.provenance == hir_def::generics::TypeParamProvenance::TraitSelf {
                     substs.push(receiver_ty.clone());
                 } else {
                     substs.push(Ty::Unknown);
diff --git a/crates/ra_hir_ty/src/lib.rs b/crates/ra_hir_ty/src/lib.rs
index c64b81f9848..cb7a60352fd 100644
--- a/crates/ra_hir_ty/src/lib.rs
+++ b/crates/ra_hir_ty/src/lib.rs
@@ -368,7 +368,7 @@ pub fn as_single(&self) -> &Ty {
     /// Return Substs that replace each parameter by itself (i.e. `Ty::Param`).
     pub(crate) fn identity(generic_params: &Generics) -> Substs {
         Substs(
-            generic_params.iter().map(|(idx, p)| Ty::Param { idx, name: p.name.clone() }).collect(),
+            generic_params.iter().map(|(idx, p)| Ty::Param { idx, name: p.name.clone().unwrap_or_else(Name::missing) }).collect(),
         )
     }
 
diff --git a/crates/ra_hir_ty/src/lower.rs b/crates/ra_hir_ty/src/lower.rs
index 39406d8cec5..6f768147520 100644
--- a/crates/ra_hir_ty/src/lower.rs
+++ b/crates/ra_hir_ty/src/lower.rs
@@ -341,6 +341,7 @@ pub(super) fn substs_from_path_segment(
         // Self type as an implicit first type parameter, but it can't be
         // actually provided in the type arguments
         // (well, actually sometimes it can, in the form of type-relative paths: `<Foo as Default>::default()`)
+        // TODO handle this using type param provenance
         substs.push(Ty::Unknown);
     }
     if let Some(generic_args) = &segment.args_and_bindings {
diff --git a/crates/ra_hir_ty/src/utils.rs b/crates/ra_hir_ty/src/utils.rs
index 0b1806a84fd..314a3241f9d 100644
--- a/crates/ra_hir_ty/src/utils.rs
+++ b/crates/ra_hir_ty/src/utils.rs
@@ -127,7 +127,8 @@ pub(crate) fn param_idx(&self, param: TypeParamId) -> u32 {
         self.find_param(param).0
     }
     pub(crate) fn param_name(&self, param: TypeParamId) -> Name {
-        self.find_param(param).1.name.clone()
+        // FIXME make this return Option
+        self.find_param(param).1.name.clone().unwrap_or_else(Name::missing)
     }
     fn find_param(&self, param: TypeParamId) -> (u32, &TypeParamData) {
         if param.parent == self.def {