test: add compile error cases for error union switch

test/cases/compile_errors/switch_expression-duplicate_error_prong.zig

@@ -0,0 +1,33 @@
+fn f(n: Error!i32) i32 {
+    if (n) |x|
+        _ = x
+    else |e| switch (e) {
+        error.Foo => 1,
+        error.Bar => 2,
+        error.Baz => 3,
+        error.Foo => 2,
+    }
+}
+fn g(n: Error!i32) i32 {
+    n catch |e| switch (e) {
+        error.Foo => 1,
+        error.Bar => 2,
+        error.Baz => 3,
+        error.Foo => 2,
+    };
+}
+
+const Error = error{ Foo, Bar, Baz };
+
+export fn entry() usize {
+    return @sizeOf(@TypeOf(&f)) + @sizeOf(@TypeOf(&g));
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :8:9: error: duplicate switch value
+// :5:9: note: previous value here
+// :16:9: error: duplicate switch value
+// :13:9: note: previous value here

test/cases/compile_errors/switch_expression-duplicate_error_prong_when_else_present.zig

@@ -0,0 +1,35 @@
+fn f(n: Error!i32) i32 {
+    if (n) |x|
+        _ = x
+    else |e| switch (e) {
+        error.Foo => 1,
+        error.Bar => 2,
+        error.Baz => 3,
+        error.Foo => 2,
+        else => 10,
+    }
+}
+fn g(n: Error!i32) i32 {
+    n catch |e| switch (e) {
+        error.Foo => 1,
+        error.Bar => 2,
+        error.Baz => 3,
+        error.Foo => 2,
+        else => 10,
+    };
+}
+
+const Error = error{ Foo, Bar, Baz };
+
+export fn entry() usize {
+    return @sizeOf(@TypeOf(&f)) + @sizeOf(@TypeOf(&g));
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :8:9: error: duplicate switch value
+// :5:9: note: previous value here
+// :17:9: error: duplicate switch value
+// :14:9: note: previous value here

test/cases/compile_errors/switch_expression-missing_error_prong.zig

@@ -0,0 +1,33 @@
+const Error = error {
+    One,
+    Two,
+    Three,
+    Four,
+};
+fn f(n: Error!i32) i32 {
+    if (n) |x| x else |e| switch (e) {
+        error.One => 1,
+        error.Two => 2,
+        error.Three => 3,
+    }
+}
+fn h(n: Error!i32) i32 {
+    n catch |e| switch (e) {
+        error.One => 1,
+        error.Two => 2,
+        error.Three => 3,
+    };
+}
+
+export fn entry() usize {
+    return @sizeOf(@TypeOf(&f)) + @sizeOf(@TypeOf(&h));
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :8:27: error: switch must handle all possibilities
+// :8:27: note: unhandled error value: 'error.Four'
+// :15:17: error: switch must handle all possibilities
+// :15:17: note: unhandled error value: 'error.Four'

test/cases/compile_errors/switch_expression-multiple_else_prongs.zig

@@ -5,8 +5,24 @@ fn f(x: u32) void {
         else => true,
     };
 }
+fn g(x: error{Foo, Bar, Baz}!u32) void {
+    const value: bool = if (x) |_| true else |e| switch (e) {
+        error.Foo => false,
+        else => true,
+        else => true,
+    };
+}
+fn h(x: error{Foo, Bar, Baz}!u32) void {
+    const value: u32 = x catch |e| switch (e) {
+        error.Foo => 1,
+        else => 2,
+        else => 3,
+    };
+}
 export fn entry() void {
     f(1234);
+    g(1234);
+    h(1234);
 }
 
 // error
@@ -15,3 +31,7 @@ export fn entry() void {
 //
 // :5:9: error: multiple else prongs in switch expression
 // :4:9: note: previous else prong here
+// :12:9: error: multiple else prongs in switch expression
+// :11:9: note: previous else prong here
+// :19:9: error: multiple else prongs in switch expression
+// :18:9: note: previous else prong here

test/cases/compile_errors/switch_expression-unreachable_else_prong_error.zig

@@ -0,0 +1,32 @@
+fn foo(x: u2) void {
+    const y: Error!u2 = x;
+    if (y) |_| {} else |e| switch (e) {
+        error.Foo => {},
+        error.Bar => {},
+        error.Baz => {},
+        else => {},
+    }
+}
+
+fn bar(x: u2) void {
+    const y: Error!u2 = x;
+    y catch |e| switch (e) {
+        error.Foo => {},
+        error.Bar => {},
+        error.Baz => {},
+        else => {},
+    };
+}
+
+const Error = error{ Foo, Bar, Baz };
+
+export fn entry() usize {
+    return @sizeOf(@TypeOf(&foo)) + @sizeOf(@TypeOf(&bar));
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :7:14: error: unreachable else prong; all cases already handled
+// :17:14: error: unreachable else prong; all cases already handled

test/cases/compile_errors/switch_on_error_union_discard.zig

@@ -0,0 +1,12 @@
+export fn entry() void {
+    const x: error{}!u32 = 0;
+    if (x) |v| v else |_| switch (_) {
+    }
+}
+
+
+// error
+// backend=stage2
+// target=native
+//
+// :3:24: error: discard of error capture; omit it instead

test/cases/compile_errors/switch_on_error_with_1_field_with_no_prongs.zig

@@ -0,0 +1,20 @@
+const Error = error{M};
+
+export fn entry() void {
+    const f: Error!void = void{};
+    if (f) {} else |e| switch (e) {}
+}
+
+export fn entry2() void {
+    const f: Error!void = void{};
+    f catch |e| switch (e) {};
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :5:24: error: switch must handle all possibilities
+// :5:24: note: unhandled error value: 'error.M'
+// :10:17: error: switch must handle all possibilities
+// :10:17: note: unhandled error value: 'error.M'