diff --git a/crates/rustc_codegen_spirv/src/builder/spirv_asm.rs b/crates/rustc_codegen_spirv/src/builder/spirv_asm.rs
index 1238156613..3b26f4a318 100644
--- a/crates/rustc_codegen_spirv/src/builder/spirv_asm.rs
+++ b/crates/rustc_codegen_spirv/src/builder/spirv_asm.rs
@@ -15,7 +15,7 @@ use rustc_codegen_ssa::traits::{AsmBuilderMethods, InlineAsmOperandRef};
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_hir::LlvmInlineAsmInner;
 use rustc_middle::bug;
-use rustc_span::source_map::Span;
+use rustc_span::{Span, DUMMY_SP};
 use rustc_target::asm::{InlineAsmRegClass, InlineAsmRegOrRegClass, SpirVInlineAsmRegClass};
 
 pub struct InstructionTable {
@@ -546,24 +546,24 @@ impl<'cx, 'tcx> Builder<'cx, 'tcx> {
         struct Unapplicable;
 
         /// Recursively match `ty` against `pat`, returning one of:
-        /// * `Ok(None)`: `pat` matched but contained no type variables
-        /// * `Ok(Some(var))`: `pat` matched and `var` is the type variable
+        /// * `Ok([None])`: `pat` matched but contained no type variables
+        /// * `Ok([Some(var)])`: `pat` matched and `var` is the type variable
         /// * `Err(Mismatch)`: `pat` didn't match or isn't supported right now
         fn match_ty_pat(
             cx: &CodegenCx<'_>,
             pat: &TyPat<'_>,
             ty: Word,
-        ) -> Result<Option<Word>, Unapplicable> {
+        ) -> Result<[Option<Word>; 1], Unapplicable> {
             match pat {
-                TyPat::Any => Ok(None),
-                &TyPat::T => Ok(Some(ty)),
+                TyPat::Any => Ok([None]),
+                &TyPat::T => Ok([Some(ty)]),
                 TyPat::Either(a, b) => {
                     match_ty_pat(cx, a, ty).or_else(|Unapplicable| match_ty_pat(cx, b, ty))
                 }
                 _ => match (pat, cx.lookup_type(ty)) {
                     (TyPat::Any, _) | (&TyPat::T, _) | (TyPat::Either(..), _) => unreachable!(),
 
-                    (TyPat::Void, SpirvType::Void) => Ok(None),
+                    (TyPat::Void, SpirvType::Void) => Ok([None]),
                     (TyPat::Pointer(_, pat), SpirvType::Pointer { pointee: ty, .. })
                     | (TyPat::Vector(pat), SpirvType::Vector { element: ty, .. })
                     | (
@@ -587,6 +587,38 @@ impl<'cx, 'tcx> Builder<'cx, 'tcx> {
             }
         }
 
+        #[derive(Debug)]
+        struct Ambiguous;
+
+        /// Construct a type from `pat`, replacing `TyPat::Var(i)` with `ty_vars[i]`.
+        /// If the pattern isn't constraining enough to determine an unique type,
+        /// `Err(Ambiguous)` is returned instead.
+        fn subst_ty_pat(
+            cx: &CodegenCx<'_>,
+            pat: &TyPat<'_>,
+            ty_vars: &[Option<Word>],
+        ) -> Result<Word, Ambiguous> {
+            Ok(match pat {
+                &TyPat::Var(i) => match ty_vars.get(i) {
+                    Some(&Some(ty)) => ty,
+                    _ => return Err(Ambiguous),
+                },
+
+                TyPat::Vector4(pat) => SpirvType::Vector {
+                    element: subst_ty_pat(cx, pat, ty_vars)?,
+                    count: 4,
+                }
+                .def(DUMMY_SP, cx),
+
+                TyPat::SampledImage(pat) => SpirvType::SampledImage {
+                    image_type: subst_ty_pat(cx, pat, ty_vars)?,
+                }
+                .def(DUMMY_SP, cx),
+
+                _ => return Err(Ambiguous),
+            })
+        }
+
         // FIXME(eddyb) try multiple signatures until one fits.
         let mut sig = match instruction_signatures(instruction.class.opcode)? {
             [sig
@@ -597,7 +629,7 @@ impl<'cx, 'tcx> Builder<'cx, 'tcx> {
             _ => return None,
         };
 
-        let mut combined_var = None;
+        let mut combined_ty_vars = [None];
 
         let mut ids = instruction.operands.iter().filter_map(|o| o.id_ref_any());
         while let TyListPat::Cons { first: pat, suffix } = *sig.input_types {
@@ -609,25 +641,31 @@ impl<'cx, 'tcx> Builder<'cx, 'tcx> {
                 // Non-value ID operand (or value operand of unknown type),
                 // only `TyPat::Any` is valid.
                 None => match pat {
-                    TyPat::Any => Ok(None),
+                    TyPat::Any => Ok([None]),
                     _ => Err(Unapplicable),
                 },
             };
-            match match_result {
-                Ok(Some(var)) => match combined_var {
-                    Some(combined_var) => {
-                        // FIXME(eddyb) this could use some error reporting
-                        // (it's a type mismatch), although we could also
-                        // just use the first type and let validation take
-                        // care of the mismatch
-                        if var != combined_var {
-                            return None;
-                        }
-                    }
-                    None => combined_var = Some(var),
-                },
-                Ok(None) => {}
+
+            let ty_vars = match match_result {
+                Ok(ty_vars) => ty_vars,
                 Err(Unapplicable) => return None,
+            };
+
+            for (&var, combined_var) in ty_vars.iter().zip(&mut combined_ty_vars) {
+                if let Some(var) = var {
+                    match *combined_var {
+                        Some(combined_var) => {
+                            // FIXME(eddyb) this could use some error reporting
+                            // (it's a type mismatch), although we could also
+                            // just use the first type and let validation take
+                            // care of the mismatch
+                            if var != combined_var {
+                                return None;
+                            }
+                        }
+                        None => *combined_var = Some(var),
+                    }
+                }
             }
         }
         match sig.input_types {
@@ -642,20 +680,9 @@ impl<'cx, 'tcx> Builder<'cx, 'tcx> {
             _ => return None,
         }
 
-        let var = combined_var?;
-        match sig.output_type.unwrap() {
-            &TyPat::T => Some(var),
-            TyPat::Vector4(&TyPat::T) => Some(
-                SpirvType::Vector {
-                    element: var,
-                    count: 4,
-                }
-                .def(self.span(), self),
-            ),
-            TyPat::SampledImage(&TyPat::T) => {
-                Some(SpirvType::SampledImage { image_type: var }.def(self.span(), self))
-            }
-            _ => None,
+        match subst_ty_pat(self, sig.output_type.unwrap(), &combined_ty_vars) {
+            Ok(ty) => Some(ty),
+            Err(Ambiguous) => None,
         }
     }