What causes "uninferred constructor made it to codegen " in gleam-lang/gleam?

Fix any type errors reported earlier in the module; unresolved inference is often downstream of them.

uninferred constructor made it to codegen

Error message

uninferred constructor made it to codegen

What it means

An internal compiler assertion raised while generating Erlang code for a constructor pattern: the pattern's `constructor` field holds Inferred::Unknown instead of a known PatternConstructor. Constructor names must be resolved during type inference before codegen, so an uninferred constructor indicates an earlier phase failed to annotate the pattern.

Solutions

Fix any type errors reported earlier in the module; unresolved inference is often downstream of them.
Update gleam to the latest version.
Reduce to a minimal module and file a bug at github.com/gleam-lang/gleam if no prior error was reported.

Defensive patterns

Strategy: validation

Validate before calling

// Ensure inference completed: build and fix all reported type errors before codegen
gleam check || exit 1

Type guard

let Inferred::Known(constructor) = &pattern.constructor else { return Err(...); };

Try / catch

let status = Command::new("gleam").args(["build"]).status()?;
if !status.success() { /* treat as compiler failure */ }

Prevention

Run `gleam check` and clear all type errors before building.
Match only on values of fully resolved custom types.
Report uninferred-constructor panics with a minimal reproducer.

When it happens

Trigger: Generating code for a Pattern::Constructor whose type inference did not resolve its constructor (e.g. inference was interrupted by earlier errors, or a compiler bug left the inference unresolved).

Common situations: Compiling code with pattern matches on custom types where an earlier inference error occurred; usually accompanied by prior error diagnostics; otherwise a compiler bug.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

AI-assisted analysis of gleam-lang/gleam@15b07c7830 (2026-09-14). Data as JSON: /api/errors/692cdef2d5d8f34a. Report an issue: GitHub.

Appendix: source

Thrown at compiler-core/src/erlang/pattern.rs:207

                if let Some(tail) = tail {
                    self.pattern(builder, &tail.pattern);
                } else {
                    builder.empty_list_pattern(SrcSpan {
                        start: location.end,
                        end: location.end,
                    });
                }
            }

            Pattern::Constructor {
                arguments,
                constructor,
                location,
                name_location,
                ..
            } => {
                let Inferred::Known(PatternConstructor { name, .. }) = constructor else {
                    panic!("uninferred constructor made it to codegen ")
                };

                if arguments.is_empty() {
                    builder.atom_pattern(*name_location, &to_snake_case(name));
                } else {
                    let tuple = builder.start_tuple_pattern(*location);
                    builder.atom_pattern(*name_location, &to_snake_case(name));
                    for argument in arguments {
                        self.pattern(builder, &argument.value);
                    }
                    builder.end_tuple_pattern(tuple);
                }
            }

            Pattern::StringPrefix {
                left_side_string,
                left_side_assignment,
                right_side_assignment,

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