RULE-6-9-1: fix false positives for redeclarations that use the same type alias

cpp/misra/src/rules/RULE-6-9-1/TypeAliasesDeclaration.ql

@@ -17,14 +17,91 @@
 import cpp
 import codingstandards.cpp.misra
 
+/**
+ * Holds if `decl1` and `decl2` refer to the same source location (same file,
+ * line and column).
+ */
+predicate sameSourceLocation(DeclarationEntry decl1, DeclarationEntry decl2) {
+  decl1.getLocation().getFile() = decl2.getLocation().getFile() and
+  decl1.getLocation().getStartLine() = decl2.getLocation().getStartLine() and
+  decl1.getLocation().getStartColumn() = decl2.getLocation().getStartColumn()
+}
+
+/**
+ * Gets a line, in the file of declaration entry `decl`, that is part of the
+ * source range in which the type used for the declared entity appears.
+ *
+ * This is the range spanned by the declaration entry itself, extended - for a
+ * function definition - up to the start of the function body. The extension is
+ * required because a function definition may use a trailing return type
+ * (`auto f() -> T`), which appears after the function name and hence outside the
+ * declaration entry's own (name-based) location.
+ */
+predicate declTypeUseLine(DeclarationEntry decl, File file, int line) {
+  file = decl.getLocation().getFile() and
+  (
+    line in [decl.getLocation().getStartLine() .. decl.getLocation().getEndLine()]
+    or
+    exists(FunctionDeclarationEntry fde |
+      fde = decl and
+      fde.getBlock().getLocation().getFile() = file and
+      line in [fde.getLocation().getStartLine() .. fde.getBlock().getLocation().getStartLine()]
+    )
+  )
+}
+
+/**
+ * Holds if the type alias `t` is mentioned within the source lines spanned by
+ * the declaration entry `decl`.
+ *
+ * `TypedefType.getATypeNameUse()` is documented to return a conservative
+ * (incomplete) set of type name uses - in particular it omits uses on
+ * prototypes and around template instantiations. As a result it frequently
+ * fails to associate an alias with a redeclaration that genuinely uses it
+ * (e.g. a function prototype in a header whose definition lives in a `.cpp`),
+ * which produces false positives for this rule. `TypeMention` records every
+ * syntactic mention of a type together with its location, so we use it to
+ * confirm whether `decl` really does use the alias `t`.
+ */
+predicate typeAliasMentionedIn(TypedefType t, DeclarationEntry decl) {
+  exists(TypeMention tm |
+    // Match on the qualified name rather than object identity: a mention of a
+    // type alias template (e.g. `Result`) resolves to the generic alias, while
+    // `getATypeNameUse()` reports the instantiated alias (e.g. `Result<X>`).
+    // These are distinct `TypedefType`s but the same alias spelling, which is
+    // what this rule is concerned with.
+    tm.getMentionedType().(TypedefType).getQualifiedName() = t.getQualifiedName() and
+    tm.getLocation().getFile() = decl.getLocation().getFile() and
+    declTypeUseLine(decl, tm.getLocation().getFile(), tm.getLocation().getStartLine())
+  )
+}
+
 from DeclarationEntry decl1, DeclarationEntry decl2, TypedefType t
 where
   not isExcluded(decl1, Declarations5Package::typeAliasesDeclarationQuery()) and
   not isExcluded(decl2, Declarations5Package::typeAliasesDeclarationQuery()) and
   not decl1 = decl2 and
   decl1.getDeclaration() = decl2.getDeclaration() and
+  // Two declaration entries that share the exact same source location are the
+  // same source declaration seen from different translation units, not two
+  // redeclarations that could disagree on a type alias. Comparing them produces
+  // false positives whenever `getATypeNameUse()` happens to associate the alias
+  // with one copy but not the other.
+  not sameSourceLocation(decl1, decl2) and
+  // Declaration entries synthesised for template instantiations are not source
+  // redeclarations and duplicate the entries of the uninstantiated template
+  // (without recording their type name uses), so comparing them yields false
+  // positives.
+  not decl1.getDeclaration().isFromTemplateInstantiation(_) and
+  not decl2.getDeclaration().isFromTemplateInstantiation(_) and
   t.getATypeNameUse() = decl1 and
   not t.getATypeNameUse() = decl2 and
+  // `getATypeNameUse()` is incomplete, so it may report that `t` is not used on
+  // `decl2` even when `decl2` uses exactly the same alias as `decl1`. Confirm via
+  // `TypeMention` that `decl2` really does not mention `t` before reporting a
+  // divergence, otherwise the same alias used on a prototype/definition pair is
+  // wrongly flagged.
+  not typeAliasMentionedIn(t, decl2) and
   //exception cases - we dont want to disallow struct typedef name use
   not t.getBaseType() instanceof Struct and
   not t.getBaseType() instanceof Enum