feat: scenarios for type mismatch across the value types

evaluator/gherkin/errors.feature

@@ -16,3 +16,36 @@ Feature: Evaluator error handling
     Given a Integer-flag with key "wrong-flag" and a fallback value "13"
     When the flag was evaluated with details
     Then the error-code should be "TYPE_MISMATCH"
+
+  @type-mismatch
+  Scenario Outline: Type mismatch across the value types
+    # Numeric coercion is deliberately absent: whether 0.5 may be narrowed to an integer is
+    # unsettled (https://github.com/open-feature/spec/issues/430) and flagd's own ADR permits
+    # coercion when it is lossless, so an integer/float row would assert a rule that does not
+    # exist yet.
+    Given a <type>-flag with key "<key>" and a fallback value "<default>"
+    When the flag was evaluated with details
+    Then the resolved details value should be "<default>"
+    And the error-code should be "TYPE_MISMATCH"
+
+    Examples: a boolean flag requested as something else
+      # The two numeric rows catch a language where the boolean type is a subtype of the integer
+      # type, as it is in Python: isinstance(True, int) is True, so a naive check admits a
+      # boolean and the caller is handed 1 or 1.0 with no error.
+      | key          | type    | default  |
+      | boolean-flag | String  | fallback |
+      | boolean-flag | Integer | 1        |
+      | boolean-flag | Float   | 0.1      |
+
+    Examples: a string flag requested as something else
+      | key         | type    | default |
+      | string-flag | Boolean | false   |
+      | string-flag | Integer | 1       |
+      | string-flag | Float   | 0.1     |
+
+    Examples: a numeric flag requested as a non-numeric type
+      | key          | type    | default  |
+      | integer-flag | Boolean | false    |
+      | integer-flag | String  | fallback |
+      | float-flag   | Boolean | false    |
+      | float-flag   | String  | fallback |

gherkin/errors.feature

@@ -0,0 +1,46 @@
+@rpc @in-process @file @type-mismatch
+Feature: flagd type mismatch handling
+
+  # Validates that asking for a flag through the wrong accessor returns the caller's default
+  # with error code TYPE_MISMATCH, rather than a coerced value. A coerced value is the worst
+  # failure mode a flag has: the application receives something plausible and no signal that
+  # anything went wrong.
+  #
+  # Numeric coercion is deliberately absent. Whether 0.5 may be narrowed to an integer is
+  # unsettled in the specification (https://github.com/open-feature/spec/issues/430), and
+  # flagd's own ADR permits coercion when it is lossless -- so an integer/float row here would
+  # assert a rule that does not exist yet. "Is a string a boolean?" has no such defence.
+  #
+  # It's associated with the flags configured in flags.
+
+  Scenario Outline: Requesting the wrong type returns the default
+    Given an option "cache" of type "CacheType" with value "disabled"
+    And a stable flagd provider
+    And a <type>-flag with key "<key>" and a default value "<default>"
+    When the flag was evaluated with details
+    Then the resolved details value should be "<default>"
+    And the reason should be "ERROR"
+    And the error-code should be "TYPE_MISMATCH"
+
+    Examples: a string flag requested as something else
+      | key         | type    | default  |
+      | string-flag | Boolean | false    |
+      | string-flag | Integer | 1        |
+      | string-flag | Float   | 0.1      |
+      | wrong-flag  | Boolean | false    |
+
+    Examples: a boolean flag requested as something else
+      # The two numeric rows are the ones that catch a language where the boolean type is a
+      # subtype of the integer type, as it is in Python: isinstance(True, int) is True, so a
+      # naive check admits a boolean and the caller is handed 1 or 1.0 with no error.
+      | key          | type    | default  |
+      | boolean-flag | String  | fallback |
+      | boolean-flag | Integer | 1        |
+      | boolean-flag | Float   | 0.1      |
+
+    Examples: a numeric flag requested as a non-numeric type
+      | key          | type    | default  |
+      | integer-flag | Boolean | false    |
+      | integer-flag | String  | fallback |
+      | float-flag   | Boolean | false    |
+      | float-flag   | String  | fallback |