diff --git a/chalk-parse/src/ast.rs b/chalk-parse/src/ast.rs
index 624ca3f3f58..c05be1b470a 100644
--- a/chalk-parse/src/ast.rs
+++ b/chalk-parse/src/ast.rs
@@ -55,6 +55,7 @@ pub struct TraitFlags {
     pub auto: bool,
     pub marker: bool,
     pub upstream: bool,
+    pub fundamental: bool,
     pub deref: bool,
 }
 
diff --git a/chalk-parse/src/parser.lalrpop b/chalk-parse/src/parser.lalrpop
index 2cd22e29696..e0a531cb58a 100644
--- a/chalk-parse/src/parser.lalrpop
+++ b/chalk-parse/src/parser.lalrpop
@@ -58,7 +58,7 @@ StructDefn: StructDefn = {
 };
 
 TraitDefn: TraitDefn = {
-    <auto:AutoKeyword?> <marker:MarkerKeyword?> <upstream:UpstreamKeyword?> <deref:DerefLangItem?> "trait" <n:Id><p:Angle<ParameterKind>>
+    <auto:AutoKeyword?> <marker:MarkerKeyword?> <upstream:UpstreamKeyword?> <fundamental:FundamentalKeyword?> <deref:DerefLangItem?> "trait" <n:Id><p:Angle<ParameterKind>>
         <w:QuantifiedWhereClauses> "{" <a:AssocTyDefn*> "}" => TraitDefn
     {
         name: n,
@@ -69,6 +69,7 @@ TraitDefn: TraitDefn = {
             auto: auto.is_some(),
             marker: marker.is_some(),
             upstream: upstream.is_some(),
+            fundamental: fundamental.is_some(),
             deref: deref.is_some(),
         },
     }
diff --git a/src/coherence/test.rs b/src/coherence/test.rs
index 24669871262..10c3686507f 100644
--- a/src/coherence/test.rs
+++ b/src/coherence/test.rs
@@ -257,6 +257,39 @@ fn downstream_impl_of_fundamental_43355() {
     }
 }
 
+#[test]
+fn fundamental_traits() {
+    // We want to enable negative reasoning about some traits. For example, consider the str type.
+    // We know that str is never going to be Sized and we have made a decision to allow people to
+    // depend on that. The following two impls are rejected as overlapping despite the fact that we
+    // know that str will never be Sized.
+    lowering_error! {
+        program {
+            #[upstream] trait Sized { }
+            #[upstream] struct str { }
+            trait Bar { }
+            impl Bar for str { }
+            impl<T> Bar for T where T: Sized { }
+        } error_msg {
+            "overlapping impls of trait \"Bar\""
+        }
+    }
+
+    // If we make Sized fundamental, we're telling the Rust compiler that it can reason negatively
+    // about it. That means that `not { str: Sized }` is provable. With that change, these two
+    // impls are now valid.
+    lowering_success! {
+        program {
+            #[upstream] #[fundamental] trait Sized { }
+            #[upstream] struct str { }
+            trait Bar { }
+            impl Bar for str { }
+            impl<T> Bar for T where T: Sized { }
+        }
+    }
+
+}
+
 #[test]
 fn orphan_check() {
     // These tests are largely adapted from the compile-fail coherence-*.rs tests from rustc
diff --git a/src/ir.rs b/src/ir.rs
index 65e5b9afe24..75088faaa61 100644
--- a/src/ir.rs
+++ b/src/ir.rs
@@ -272,6 +272,7 @@ pub struct TraitFlags {
     crate auto: bool,
     crate marker: bool,
     crate upstream: bool,
+    crate fundamental: bool,
     pub deref: bool,
 }
 
diff --git a/src/ir/lowering.rs b/src/ir/lowering.rs
index dc9758ae18f..e2749576233 100644
--- a/src/ir/lowering.rs
+++ b/src/ir/lowering.rs
@@ -1056,6 +1056,7 @@ impl LowerTrait for TraitDefn {
                     auto: self.flags.auto,
                     marker: self.flags.marker,
                     upstream: self.flags.upstream,
+                    fundamental: self.flags.fundamental,
                     deref: self.flags.deref,
                 },
             })
diff --git a/src/rules.rs b/src/rules.rs
index c8c7b8db6b3..07604d2b609 100644
--- a/src/rules.rs
+++ b/src/rules.rs
@@ -503,6 +503,32 @@ impl TraitDatum {
         //             IsUpstream(V),
         //             CannotProve,              // returns ambiguous
         //     }
+        //
+        // In certain situations, this is too restrictive. Consider the following code:
+        //
+        //    // In crate std:
+        //    trait Sized { }
+        //    struct str { }
+        //
+        //    // In crate bar: (depends on std)
+        //    trait Bar { }
+        //    impl Bar for str { }
+        //    impl<T> Bar for T where T: Sized { }
+        //
+        // Here, because of the rules we've defined, these two impls overlap. The std crate is
+        // upstream to bar, and thus it is allowed to compatibly implement Sized for str. If str
+        // can implement Sized in a compatible future, these two impls definitely overlap since the
+        // second impl covers all types that implement Sized.
+        //
+        // The solution we've got right now is to mark Sized as "fundamental" when it is defined.
+        // This signals to the Rust compiler that it can rely on the fact that str does not
+        // implement Sized in all contexts. A consequence of this is that we can no longer add an
+        // implementation of Sized compatibly for str. This is the trade off you make when defining
+        // a fundamental trait.
+        //
+        // To implement fundamental traits, we simply just do not add the rule above that allows
+        // upstream types to implement upstream traits. Fundamental traits are not allowed to
+        // compatibly do that.
 
         let mut clauses = Vec::new();
 
@@ -578,9 +604,12 @@ impl TraitDatum {
                 clauses.push(impl_maybe_allowed);
             }
 
-            let impl_may_exist = self.binders.map_ref(|bound_datum| ProgramClauseImplication {
-                consequence: DomainGoal::Holds(WhereClause::Implemented(bound_datum.trait_ref.clone())),
-                conditions: bound_datum.where_clauses
+            // Fundamental traits can be reasoned about negatively without any ambiguity, so no
+            // need for this rule if the trait is fundamental.
+            if !self.binders.value.flags.fundamental {
+                let impl_may_exist = self.binders.map_ref(|bound_datum| ProgramClauseImplication {
+                    consequence: DomainGoal::Holds(WhereClause::Implemented(bound_datum.trait_ref.clone())),
+                    conditions: bound_datum.where_clauses
                     .iter()
                     .cloned()
                     .casted()
@@ -588,9 +617,10 @@ impl TraitDatum {
                     .chain(bound_datum.trait_ref.type_parameters().map(|ty| DomainGoal::IsUpstream(ty).cast()))
                     .chain(iter::once(Goal::CannotProve(())))
                     .collect(),
-            }).cast();
+                }).cast();
 
-            clauses.push(impl_may_exist);
+                clauses.push(impl_may_exist);
+            }
         }
 
         let condition = DomainGoal::FromEnv(FromEnv::Trait(trait_ref.clone()));